Mechanisms of reactions and reactivity of organic. Radical and ionic reaction mechanisms

Guidelines for independent work of 1st year students in biological and bio organic chemistry

(module 1)

Approved

Academic Council of the University

Kharkiv KhNMU

Main types and mechanisms of reactions in organic chemistry: Method. decree. for 1st year students / comp. A.O. Syrovaya, L.G. Shapoval, V.N. Petyunina, E.R. Grabovetskaya, V.A. Makarov, S.V. Andreeva, S.A. Nakonechnaya, L.V. Lukyanova, R.O. Bachinsky, S.N. Kozub, T.S. Tishakova, O.L. Levashova, N.V. Kopoteva, N.N. Chalenko. - Kharkov: KhNMU, 2014. - P. 32.

Compiled by: A.O. Syrovaya, L.G. Shapoval, V.N. Petyunina, E.R. Grabovetskaya, V.A. Makarov, S.V. Andreeva, L.V. Lukyanova, S.A. Nakonechnaya, R.O. Bachinsky, S.N. Kozub, T.S. Tishakova, O.L. Levashova, N.V. Kopoteva, N.N. Chalenko

Topic I: classification of chemical reactions.

Reactivity of Alkanes, Alkenes, Arenes, Alcohols, Phenols, Amines, Aldehydes, Ketones, and Carboxylic Acids

Motivational characteristic of the topic

The study of this topic is the basis for understanding some of the biochemical reactions that take place in the process of metabolism in the body (lipid peroxidation, the formation of hydroxy acids from unsaturated ones in the Krebs cycle, etc.), as well as for understanding the mechanism of such reactions in the synthesis of medical preparations and analogues natural compounds.

learning goal

To be able to predict the ability of the main classes of organic compounds to enter into reactions of homolytic and heterolytic interactions according to their electronic structure and electronic effects of substituents.

1. FREE RADICAL AND ELECTROPHILIC REACTIONS (REACTIVITY OF HYDROCARBONS)

Learning-targeted questions

1. Be able to describe the mechanisms of the following reactions:

Radical substitution - R S

Electrophilic addition - A E

Electrophilic substitution - S E

2. Be able to explain the effect of substituents on reactivity in electrophilic interactions based on electronic effects.

Baseline

1. The structure of the carbon atom. Types of hybridization of its electronic orbitals.

2. Structure, length and energy of - and -bonds.

3. Conformations of cyclohexane.

4. Pairing. Open and closed (aromatic) conjugated systems.

5. Electronic effects of substituents.

6. Transition state. Electronic structure of the carbocation. Intermediaries - and  - complexes.

Practical navski

1. Learn to determine the possibility of breaking a covalent bond, the type and mechanism of the reaction.

2. Be able to experimentally perform bromination reactions of compounds with double bonds and aromatic compounds.

test questions

1. Give the mechanism of the ethylene hydrogenation reaction.

2. Describe the mechanism of propenoic acid hydration reaction. Explain the role of acid catalysis.

3. Write the reaction equation for the nitration of toluene (methylbenzene). What is the mechanism of this reaction?

4. Explain the deactivating and orienting effect of the nitro group in the nitrobenzene molecule using the bromination reaction as an example.

Learning tasks and algorithms for their solution

Task number 1. Describe the reaction mechanism of bromination of isobutane and cyclopentane under light irradiation.

Solution algorithm . Molecules of isobutane and cyclopentane consist of sp 3 hybridized carbon atoms. C - C bonds in their molecules are non-polar, and C - H bonds are of low polarity. These bonds are quite easily subjected to homolytic rupture with the formation of free radicals - particles that have unpaired electrons. Thus, in the molecules of these substances, a radical substitution reaction must occur - R S -reaction or chain.

The stages of any R S -reaction are: initiation, growth and chain termination.

Initiation is the process of generating free radicals when high temperature or ultraviolet irradiation:

Chain growth occurs due to the interaction of a highly reactive free radical  Br with a low-polar C - H bond in the cyclopentane molecule with the formation of a new cyclopentyl radical:

The cyclopentyl radical interacts with a new bromine molecule, causing a homolytic bond cleavage in it and forming bromocyclopentane and a new bromine radical:

The free bromine radical attacks the new cyclopentane molecule. Thus, the stage of chain growth is repeated many times, i.e., a chain reaction occurs. Chain termination completes the chain reaction by combining different radicals:

Since all carbon atoms in a cyclopentane molecule are equal, only monocyclobromopentane is formed.

In isobutane, C - H bonds are not equivalent. They differ in the energy of homolytic dissociation and the stability of the formed free radicals. It is known that the breaking energy of the C-H bond increases from the tertiary to the primary carbon atom. The stability of free radicals decreases in the same order. That is why in the isobutane molecule the bromination reaction proceeds regioselectively - at the tertiary carbon atom:

It should be pointed out that for the more active chlorine radical, regioselectivity is not fully adhered to. During chlorination, hydrogen atoms at any carbon atoms can be replaced, but the content of the substitution product at tertiary carbon will be the largest.

Task number 2. Using oleic acid as an example, describe the mechanism of the lipid peroxidation reaction that occurs in radiation sickness as a result of damage to cell membranes. What substances act as antioxidants in our body?

Solution algorithm. An example of a radical reaction is lipid peroxidation, in which unsaturated fatty acids, which are part of cell membranes, are exposed to the action of radicals. With radioactive irradiation, the possible decay of water molecules into radicals. Hydroxyl radicals attack the unsaturated acid molecule at the methylene group adjacent to the double bond. In this case, a radical stabilized due to the participation of an unpaired electron in conjugation with electrons of  bonds is formed. Further, the organic radical interacts with a diradical oxygen molecule to form unstable hydroperoxides, which decompose to form aldehydes, which are oxidized to acids - the final products of the reaction. The consequence of peroxide oxidation is the destruction of cell membranes:

The inhibitory effect of vitamin E (tocopherol) in the body is due to its ability to bind free radicals that are produced in cells

In the phenoxide radical that is formed, the unpaired electron is in conjugation with the -electron cloud of the aromatic ring, which leads to its relative stability.

Task number 3. Give the mechanism of ethylene bromination reaction.

Solution algorithm. For compounds that consist of carbon atoms in the state of sp 2 - or sp-hybridization, there are typical reactions that proceed with the breaking of -bonds, i.e., addition reactions. These reactions can proceed by a radical or ionic mechanism, depending on the nature of the reactant, the polarity of the solvent, temperature, etc. Ionic reactions proceed under the action of either electrophilic reagents, which have an electron affinity, or nucleophilic ones, which donate their electrons. Electrophilic reagents can be cations and compounds that have atoms with unfilled electron shells. The simplest electrophilic reagent is the proton. Nucleophilic reagents are anions, or compounds with atoms that have unshared electron pairs.

For alkenes - compounds that have sp 2 - or sp-hybridized carbon atom, there are typical electrophilic addition reactions - A E reactions. In polar solvents, in the absence of sunlight, the halogenation reaction proceeds according to the ionic mechanism with the formation of carbocations:

Under the action of the π-bond in ethylene, the bromine molecule is polarized with the formation of an unstable π-complex, which turns into a carbocation. In it, bromine is bonded to carbon by a π bond. The process ends with the interaction of the bromine anion with this carbocation to the final reaction product, dibromoethane.

Task #4 . On the example of propene hydration reaction justify Markovnikov's rule.

Solution algorithm. Since the water molecule is a nucleophilic reagent, its addition via a double bond without a catalyst is impossible. The role of catalysts in such reactions is played by acids. The formation of carbocations occurs when a proton of an acid is added when a π-bond is broken:

A water molecule is attached to the carbocation that has been formed due to the paired electrons of the oxygen atom. A stable alkyl derivative of oxonium is formed, which is stabilized with the release of a proton. The reaction product is sec-propanol (propan-2-ol).

In the hydration reaction, the proton joins according to the Markovnikov rule - to a more hydrogenated carbon atom, since, due to the positive inductive effect of the CH 3 group, the electron density is shifted to this atom. In addition, the tertiary carbocation formed as a result of the addition of a proton is more stable than the primary one (the influence of two alkyl groups).

Task number 5. Substantiate the possibility of formation of 1,3-dibromopropane during bromination of cyclopropane.

Solution algorithm. Molecules that are three- or four-membered cycles (cyclopropane and cyclobutane) exhibit the properties of unsaturated compounds, since the electronic state of their "banana" bonds resembles a π-bond. Therefore, like unsaturated compounds, they enter into addition reactions with a ring break:

Task number 6. Describe the reaction of interaction of hydrogen bromide with butadiene-1,3. What is the nature of this reaction?

Solution algorithm. In the interaction of hydrogen bromide with butadiene-1,3, products 1,2 addition (1) and 1,4 addition (2) are formed:

The formation of product (2) is due to the presence in the conjugated system of a π-electron cloud common to the entire molecule, as a result of which it enters into an electrophilic addition reaction (A E - reaction) in the form of a whole block:

Task number 7. Describe the mechanism of the benzene bromination reaction.

Solution algorithm. For aromatic compounds that contain a closed conjugated electron system and which therefore have significant strength, electrophilic substitution reactions are characteristic. The presence of increased electron density on both sides of the ring protects it from attack by nucleophilic reagents and, vice versa, facilitates the possibility of attack by cations and other electrophilic reagents.

The interaction of benzene with halogens occurs in the presence of catalysts - AlCl 3 , FeCl 3 (the so-called Lewis acids). They cause the polarization of the halogen molecule, after which it attacks the π-electrons of the benzene ring:

π-complex σ-complex

At the beginning, a π-complex is formed, which slowly transforms into a σ-complex, in which bromine forms a covalent bond with one of the carbon atoms due to two of the six electrons of the aromatic ring. The four π electrons that remain are evenly distributed among the five atoms of the carbon ring; The σ-complex is a less favorable structure due to the loss of aromaticity, which is restored by the emission of a proton.

Electrophilic substitution reactions in aromatic compounds also include sulfonation and nitration. The role of the nitrating agent is performed by the nitroyl cation - NO 2+, which is formed by the interaction of concentrated sulfuric and nitric acids (nitrating mixture); and the role of the sulfonating agent is the SO 3 H + cation, or sulfur oxide (IV), if sulfonation is carried out with oleum.

Solution algorithm. The activity of compounds in S E reactions depends on the value of the electron density in the aromatic nucleus (direct dependence). In this regard, the reactivity of substances should be considered in conjunction with the electronic effects of substituents and heteroatoms.

The amino group in aniline exhibits the +M effect, as a result of which the electron density in the benzene nucleus increases and its highest concentration is observed in the ortho and para positions. The reaction is facilitated.

The nitro group in nitrobenzene has -I and -M effects, therefore, it deactivates the benzene ring in the ortho and para positions. Since the interaction of the electrophile occurs at the site of the highest electron density, in this case meta-isomers are formed. Thus, electron-donating substituents are ortho- and para-orientants (orientants of the first kind and activators of S E reactions; electron-withdrawing substituents are meta-orientants (orientants of the second kind) deactivators of S E reactions).

In five-membered heterocycles (pyrrole, furan, thiophene), which belong to π-excess systems, S E reactions proceed more easily than in benzene; while the α-position is more reactive.

Heterocyclic systems with a pyridine nitrogen atom are π-insufficient, therefore they are more difficult to enter into electrophilic substitution reactions; while the electrophile occupies the β-position with respect to the nitrogen atom.

Attachment 1
REACTION MECHANISMS IN ORGANIC CHEMISTRY
N.V. Sviridenkova, NUST MISIS, Moscow
WHY STUDY THE MECHANISMS OF CHEMICAL REACTIONS?
What is a mechanism chemical reaction? To answer this question, consider the equation for the butene combustion reaction:

C 4 H 8 + 6O 2 \u003d 4CO 2 + 4H 2 O.

If the reaction actually proceeded as described in the equation, then one butene molecule would have to collide simultaneously with six oxygen molecules at once. However, this is unlikely to happen: it is known that the simultaneous collision of more than three particles is almost improbable. The conclusion suggests itself that this reaction, like the vast majority of chemical reactions, proceeds in several successive stages. The reaction equation shows only the initial substances and the final result of all transformations, and does not explain in any way how products are formed from the starting materials. In order to find out exactly how the reaction proceeds, what stages it includes, what intermediate products are formed, it is necessary to consider the reaction mechanism.

So, reaction mechanism- this is a detailed description of the course of the reaction in stages, which shows in what order and how chemical bonds in the reacting molecules are broken and new bonds and molecules are formed.

Consideration of the mechanism makes it possible to explain why some reactions are accompanied by the formation of several products, while in other reactions only one substance is formed. Knowing the mechanism allows chemists to predict the products of chemical reactions before they are carried out in practice. Finally, knowing the reaction mechanism, one can control the course of the reaction: create conditions for increasing its rate and increasing the yield of the desired product.
BASIC CONCEPTS: ELECTROPHIL, NUCLEOPHIL, CARBOCATION
In organic chemistry, reagents are traditionally divided into three types: nucleophilic, electrophilic and radical. You have already met with radicals earlier in the study of halogenation reactions of alkanes. Let us consider other types of reagents in more detail.

Nucleophilic reagents or simply nucleophiles(translated from Greek as “lovers of nuclei”) are particles with an excess of electron density, most often negatively charged or having an unshared electron pair. Nucleophiles attack low electron density molecules or positively charged reactants. Examples of nucleophiles are ions OH - , Br - , molecules NH 3 .

Electrophilic reagents or electrophiles(translated from Greek as “electron lovers”) are particles with a lack of electron density. Electrophiles often carry a positive charge. Electrophiles attack high electron density molecules or negatively charged reactants. Examples of electrophiles are H +, NO 2 +.

An atom of a polar molecule carrying a partial positive charge can also act as an electrophile. An example is the hydrogen atom in the HBr molecule, on which a partial positive charge arises due to the displacement of the common electron pair of the bond to the bromine atom, which has greater value electronegativity H δ + → Br δ - .

Reactions proceeding according to the ionic mechanism are often accompanied by the formation of carbocations. Carbocation called a charged particle that has a free R-orbital on a carbon atom. One of the carbon atoms in a carbocation carries a positive charge. Examples of carbocations are particles of CH 3 -CH 2 + , CH 3 -CH + -CH 3 . Carbocations are formed at one of the stages in the reactions of addition of halogens and hydrogen halides to alkenes to alkenes, as well as in substitution reactions involving aromatic hydrocarbons.
MECHANISM OF ADDITION TO UNSATURATED HYDROCARBONS

The addition of halogens, hydrogen halides, water to unsaturated hydrocarbons (alkenes, alkynes, diene hydrocarbons) proceeds through ionic mechanism called electrophilic connection.

Let us consider this mechanism using the example of the addition reaction of hydrogen bromide to an ethylene molecule.

Despite the fact that the hydrobromination reaction is described by a very simple equation, its mechanism includes several stages.

Stage 1 In the first stage, the hydrogen halide molecule forms with π -an unstable system by an electron cloud of a double bond - " π -complex" due to partial transfer π -electron density per hydrogen atom carrying a partial positive charge.

Stage 2 The hydrogen-halogen bond is broken with the formation of an electrophilic particle H + and a nucleophilic particle Br - . The released electrophile H + attaches to the alkene due to the electron pair of the double bond, forming σ complex is a carbocation.

Stage 3 At this stage, a negatively charged nucleophile is added to a positively charged carbocation to form the final reaction product.

WHY IS MARKOVNIKOV'S RULE FULFILLED?
The proposed mechanism explains well the formation of predominantly one of the products in the case of the addition of hydrogen halides to unsymmetrical alkenes. Recall that the addition of hydrogen halides obeys the Markovnikov rule, according to which hydrogen is added at the double bond site to the most hydrogenated carbon atom (i.e., associated with largest number hydrogen atoms), and halogen to the least hydrogenated. For example, when hydrogen bromide is added to propene, 2-bromopropane is predominantly formed:

In electrophilic addition reactions to unsymmetrical alkenes, two carbocations can be formed in the second stage of the reaction. Then react with a nucleophile, and hence, the more stable of them will determine the reaction product.

Consider which carbocations are formed in the case of propene and compare their stability. The addition of the proton H + at the site of the double bond can lead to the formation of two carbocations, secondary and primary:

The resulting particles are very unstable, since the positively charged carbon atom in the carbocation has an unstable electronic configuration. Such particles are stabilized when the charge is distributed (delocalized) over as many atoms as possible. Electron donor alkyl groups that donate electron density to an electron-deficient carbon atom contribute to and stabilize carbocations. Let's see how this happens.

Due to the difference in the electronegativity of carbon and hydrogen atoms, a certain excess of electron density appears on the carbon atom of the -CH 3 group, and some of its deficiency C δ- H 3 δ+ appears on the hydrogen atom. The presence of such a group next to a positively charged carbon atom inevitably causes a shift in the electron density towards the positive charge. Thus, the methyl group acts as a donor, donating part of its electron density. Such a group is said to have positive inductive effect (+ I -effect). How large quantity such electron donor (+ I ) - substituents are surrounded by carbon bearing a positive charge, the more stable the corresponding carbocation. Thus, the stability of carbocations increases in the series:

In the case of propene, the most stable is the secondary carbocation, since in it the positively charged carbon atom of the carbocation is stabilized by two + I - effects of neighboring methyl groups. It is he who is predominantly formed and reacts further. An unstable primary carbocation, apparently, exists for a very short time, so that during its "life" it does not have time to attach a nucleophile and form a reaction product.

When the bromide ion is added to the secondary carbocation at the last stage, 2-bromopropane is formed:

IS MARKOVNIKOV'S RULE ALWAYS FULFILLED?

Consideration of the mechanism of propylene hydrobromination reaction allows us to formulate general rule electrophilic addition: "when unsymmetrical alkenes interact with electrophilic reagents, the reaction proceeds through the formation of the most stable carbocation." The same rule makes it possible to explain the formation of addition products in some cases contrary to Markovnikov's rule. Thus, the addition of hydrogen halides to trifluoropropylene formally proceeds against the Markovnikov rule:

How can such a product be obtained, since it was formed as a result of the addition of Br - to the primary, and not to the secondary carbocation? The contradiction is easily eliminated when considering the reaction mechanism and comparing the stability of intermediate particles:

The -CF 3 group contains three electron-withdrawing fluorine atoms, pulling the electron density away from the carbon atom. Therefore, a significant lack of electron density appears on the carbon atom. To compensate for the emerging partial positive charge, the carbon atom pulls on itself the electron density of neighboring carbon atoms. Thus, the -CF 3 group is electron-withdrawing and shows negative inductive effect (- I ) . In this case, the primary carbocation turns out to be more stable, since the destabilizing effect of the -CF 3 group weakens through two σ-bonds. And the secondary carbocation, destabilized by the neighboring electron-withdrawing group CF 3, is practically not formed.

The presence of electron-withdrawing groups at the double bond -NO 2, -COOH, -COH, etc., has a similar effect on addition. In this case, the addition product is also formally formed against the Markovnikov rule. For example, when hydrogen chloride is added to propenoic (acrylic) acid, 3-chloropropanoic acid is predominantly formed:

Thus, the direction of attachment to unsaturated hydrocarbons is easy to establish by analyzing the structure of the hydrocarbon. Briefly, this can be represented by the following diagram:

It should be noted that the Markovnikov rule is satisfied only if the reaction proceeds according to the ionic mechanism. When carrying out radical reactions, Markovnikov's rule is not fulfilled. Thus, the addition of hydrogen bromide HBr in the presence of peroxides (H 2 O 2 or organic peroxides) proceeds against the Markovnikov rule:

The addition of peroxides changes the reaction mechanism, it becomes radical. This example shows how important it is to know the reaction mechanism and the conditions under which it occurs. Then, by choosing the appropriate conditions for carrying out the reaction, it is possible to direct it along the mechanism necessary in this particular case, and to obtain exactly those products that are needed.
THE MECHANISM OF HYDROGEN ATOMS SUBSTITUTION IN AROMATIC HYDROCARBONS
The presence in the benzene molecule of a stable conjugated π -electronic system makes addition reactions almost impossible. For benzene and its derivatives, the most typical reactions are the substitution of hydrogen atoms, which proceed with the preservation of aromaticity. In this case, the benzene core containing π- electrons, interacts with electrophilic particles. Such reactions are called by electrophilic substitution reactions in the aromatic series. These include, for example, halogenation, nitration and alkylation of benzene and its derivatives.

All electrophilic substitution reactions in aromatic hydrocarbons proceed according to the same ionic mechanism, regardless of the nature of the reactant. The mechanism of substitution reactions includes several stages: the formation of an electrophilic agent E +, the formation π -complex, then σ- complex and, finally, the collapse σ- complex to form a substitution product.

The electrophilic particle E + is formed during the interaction of the reagent with the catalyst, for example, when a halogen molecule is exposed to aluminum chloride. The resulting particle E + interacts with the aromatic nucleus, first forming π -, and then σ- complex:

At education σ- complex, the electrophilic particle E + attaches to one of the carbon atoms of the benzene ring through σ- connections. In the resulting carbocation, the positive charge is evenly distributed (delocalized) between the remaining five carbon atoms.

The reaction ends with the elimination of a proton from σ- complex. In this case, two electrons σ -C-H bonds return to the cycle, and a stable six-electron aromatic π the system is being regenerated.

In a benzene molecule, all six carbon atoms are equal. Substitution of a hydrogen atom can occur with equal probability for any of them. And how will the substitution occur in the case of benzene homologues? Consider methylbenzene (toluene) as an example.

It is known from experimental data that electrophilic substitution in the case of toluene always proceeds with the formation of two products. So, the nitration of toluene proceeds with the formation P-nitrotoluene and about-nitrotoluene:

Other reactions of electrophilic substitution (bromination, alkylation) proceed similarly. It was also found that in the case of toluene, the substitution reactions proceed faster and under milder conditions than in the case of benzene.

It is very easy to explain these facts. The methyl group is an electron donor and, as a result, further increases the electron density of the benzene ring. A particularly strong increase in the electron density occurs in about- and P- positions with respect to the -CH 3 group, which facilitates the attachment of a positively charged electrophilic particle precisely to these places. Therefore, the rate of the substitution reaction as a whole increases, and the substituent is directed mainly to ortho- and pair-provisions.

The mechanism of any chemical reaction completely determines the rate of this reaction, its dependence on temperature and the nature of the solvent. Establishing the reaction mechanism allows not only to more or less fully represent the essence of a chemical reaction, to comprehend the nature of the transformation of initial reagents into final ones, but also to get the opportunity to control this reaction by changing the conditions for its occurrence.

Elucidation of the reaction mechanism is a difficult task even in the case of simple reagents. Difficulties increase as the complexity of the reaction and reagents increases.

The first ideas about reaction mechanisms belong to the field of organic chemistry and organic reactions. The works of van't Hoff, Bodenstein, Nernst, Menshutkin, Arrhenius, Shilov, Bach, Melvin-Hughes, Hinshelwood, Ingold, Semenov, Emanuel, and others played a leading role in the study of mechanisms and the development of ideas about them. It is difficult to overestimate the theoretical work of Eyring, Polyany, Glaston, Kimball and others to create a theory of absolute reaction rates, which fully corresponds to the current state of science and the level of chemical knowledge of today. The deepest, clearest and at the same time exhaustive review of the mechanisms of organic reactions is given in the excellent monograph by K. Ingold "Theoretical Foundations of Organic Chemistry" (M., Mir, 1973). Another area of ​​chemical science in which concepts of reaction mechanisms were successfully developed and applied was the chemistry of coordination compounds. This is due to the fact that coordination chemistry deals with organic molecules as ligands and the multistep nature of the complex formation reaction. Since the second half of the 20th century, in both these areas of chemistry, ideas about mechanisms have developed in parallel, significantly enriching each other.

Let us consider in general terms the mechanisms of the most important types of organic reactions.

Reactions S R . The most important of the reactions of free radical substitution of the hydrogen atom in alkanes and their substituted ones are the reactions of halogenation - fluorination, chlorination and bromination. The mechanism of the chlorination reaction has been studied in the most detail. Chlorination of alkanes can proceed either as a photochemical (when irradiated with UV light) or as a thermal transformation:

The reaction begins with the generation of atoms (free radicals) of chlorine when a gas mixture (or solution) is irradiated with light quanta:

This stage of the multi-stage process (6.15) is called the initiation (birth) of the chain. It begins with the appearance of free radicals in the reaction sphere. After the dissociation reaction (photochemical or thermal) (6.16) proceeds, the second stage of the redox reaction begins - the detachment of a hydrogen atom from methane:

In this reaction, SG acts as an oxidizing agent and removes one electron from the bonding MO V F C _ H . As a result, the link order is reduced from 1 to +X> the bond becomes one-electron and unstable. Following this, the proton leaves the cation-

The reaction (6.17) is repeated, as a result of which the active reagent SG appears again.

Thus, the stoichiometric mechanism of the reaction (6.15) and, similarly, the bromination reactions of alkanes consists of three elementary stages:

As the chain reaction of electron transfer proceeds from one part of the SG type (Br *) to another CH 3 (R), chain termination may occur when the free radical leading the reaction (6.15) and similar reactions disappears due to the combination of radicals: SG + SG -\u003e C1 2, CHJ + CH 3 -\u003e C 2 H 6, etc. When methane is chlorinated, one light quantum gives about 10 4 particles of CH 3 C1. This means that the quantum yield of the reaction is 10 4 . Chlorination of alkanes in solution, as well as their bromination in the gas phase, gives shorter chains. The bromination chain reaction can contain only two links (the quantum yield is 2).

Of great interest is the activation mechanism of chain reactions, i.e., energy and structural changes in the reactants (in this case, RH and Г*) in the transition state. On fig. 6.3 shows the energy activation curves for the reactions of chlorination (l) and bromination (b) of methane.

Rice. 6.3.

The energy costs for the formation of the transition state * are only 16 kJ/mol, while for

[CH 3 * H-Br] “they are almost 5 times larger and amount to 75.6 kJ / mol. Due to such differences in activation energies, the rate of formation of CH 3 C1 is 2.5 * 10 5 higher than CH 3 Br.

In the transition state *, the halogen atom is oriented along the C-H line, attacking the C-H bond from the side of the antibonding orbital. He first, probably, pulls an electron from the bonding H * - orbital to Tj, and then removes it. Following the electron, the [C-HH bond loses a proton, the rate of removal of which under these conditions can be about 10~13 , i.e., it takes place during one oscillation of the C-H bond. Thus, the rate-limiting halogenation step is elementary and does not have any intermediates, i.e. intermediate particles.

Other important reactions also have a chain mechanism - the oxidation of hydrocarbons, the nitration of alkanes, polymerization, etc. These mechanisms will be considered later.

SN reactions. This symbol refers to nucleophilic substitution reactions

of the electron-donating (-C1, -Br, -I, OH 2, etc.) functional group. Most often, these are halogen substitution reactions in halogen derivatives

Hydrocarbons (R-G) or -OH 2 in protonated alcohols R-OH 2 to any anionic nucleophile (SG, Br", G, F", NOj, N0 3, CN", NCS", HSO ;, SO]-, H 2 P0 4, HPO ', HCOO, CH 3 SOSG, etc.).

This type of reactions includes the most important reactions of processing halogen derivatives of alkanes into other classes of compounds - alcohols, amines, ethers and esters, alkyl cyanides and alkyl thiocyanates, nitrossid- ion, etc., as well as alcohols into halogen derivatives or esters (including fats).

To SN reactions also include the Menyiutkin reactions discovered in the last century

and the reverse decomposition reactions of tetraalkylammonium cations (according to Hoffman):

To complete the picture of the variety of reactions of nucleophilic substitution at the carbon atom, we should note the alkylation of trialkylammonium with salts of grialkylsulfonium

and deamination of alkylamines under the action of aqueous solutions of nitrous acid

All the various reactions listed above can be written as a general scheme:

Y acts as a nucleophilic reagent, on which the charge is omitted, as well as on the leaving group X.

The question arises of how the C-X bond is broken in the saturated compound RX, what is the role of the particle Y in this process, and how the geometric and electronic nature of R, X, and Y affects the reaction (6.18), in particular, its rate and activation parameters. It is also important to establish how the nature of the liquid medium affects the S N reactions if the reaction proceeds in solution.

Systematic studies of the mechanisms of the reaction (6.18) began in 1927 with the work of K. Ingold and continue to the present. It turned out that, depending on the nature of the reagents RX and Y, as well as the solvent, reactions of the type (6.18) can proceed in one stage, as a synchronous bimolecular process according to the mechanism S N 2 (2 - means a bimolecular elementary act), and in two stages - after a little stable and extremely reactive intermediate - carbonium cation R + (karbkatibn) according to the S N 1 mechanism (1 - means a monomolecular elementary act of the limiting stage of the reaction).

The S N 2 mechanism is simpler and clearer. At present, the energy activation of the reactants and the formation of the transition state are described as follows. Consider the simplest example of the nucleophilic substitution of a halogen for a hydroxyl group (the reaction of the formation of alcohols from RT) under the action of an aqueous alkali solution on an alkyl halide:

The reaction of substitution of Br for OH occurs in this case in one active collision (1(Г 13 s). An active collision is considered to be such a collision in which the reagents OH ”and RBr have not only a sufficient energy reserve equal to or greater than the activation energy, but also having such orientation in space, in which OH - is directed by its electron pair in

center of tetrahedral ~C-bonds from the side opposite to the location

bromine atom. Hence it follows that in the reaction S N 2, not only the energy factor (activation enthalpy A//*) is extremely important, but also the structural one (activation entropy AS*).

In the course of the act of reaction (6.19), electronic transformations must occur. The nucleophile (OH "etc.) attacks the alkyl halide (or any other compound with a leaving group X) from the side of the loosening orbital of the C-Br bond and imposes its electron pair (T^_ Br orbitals) on it. Electron pair OH; begins to destroy the C bond -Vg not only by lowering the order chemical bond C-Br from 1 to 0 in the limit, but also with an electrostatic field acts on the electron pairs of CH 3, RCH 2, R 2 CH or R 3 C in such a way that the alkyl with the attacked C-atom in the center acquires a flat sp 2 -structure. This is facilitated by the stretching of the C-Br bond during the vibration and the removal of Br 5 "from the central C-atom. Thus, the electron density is transferred from the nucleophile (OH" and others) to the leaving - 204

the general group (Br~, etc.). To receive an electron pair, the ions Br" (as well as SG and G) have a vacancy that is not too high in energy 4 d- (3d-, 5d-) an orbital that can be a good reservoir for excess electrons. The ease of replacing X with Y in reaction (6.18) depends on the nucleophilicity (basicity and polarizability) of Y, on the strength of the C-X chemical bond, determined by the properties of the binding and loosening Ch? with " x molecular orbitals, as well as the polarizability of the leaving group X.

The C-F bond has very unfavorable properties for the reaction (6.18). Therefore, the F atom is not replaced by nucleophiles. On the other hand, atom I is in this respect in the most favorable conditions and is replaced by nucleophiles more easily than bromine, and even more so chlorine.

The properties, including symmetry, of bonding and loosening a-MO C-X are such that they allow only one single direction of attack of the nucleophile Y on the object R-X attacks from the side opposite to the replaced atom X. Substitution of X by the bimolecular mechanism is impossible when the carbon atom is attacked by the bromine atom, i.e. between the bonds

C h Y . Since such mechanisms of bimolecular replacement of ear-

the incoming ligand (L x) incoming (L Y) are allowed, proved and, apparently, exist for the complex compounds M - L y, it is possible

conclude about the fundamental differences between the molecular orbitals of the carbon atom and complexed metal atoms. The details of these differences remain to be explored.

Of the experimental evidence of the mechanism of S N 2 substitution of the atom Br "(and other substituents - electrophiles), the main one is the second kinetic order of the substitution reaction (6.19):

Thus, the S N 2 mechanism corresponds to the dependence of the reaction rate on the concentration of both reactants.

Usually the reactant Y (here OH") is taken in a large excess and the pseudo-first-order rate constant, the so-called effective rate constant k. She is equal k^ .

Its division by the current concentration With gives the bimolecular (true) rate constant to b. It should be noted that the current concentration of reagents is included in the kinetic equations With^, with he _, which constantly decreases with time. However, for convenience, the concentration of the substance enclosed in brackets, [OH -], etc. is substituted into the kinetic equations. In thermodynamics, equilibrium concentrations, i.e., concentrations that are independent of time, are enclosed in brackets. It should be borne in mind that in the kinetic equations the same symbol [Y] denotes by no means equilibrium concentrations. For the convenience of writing kinetic equations, the symbol of current concentrations c Y is usually omitted and simply Su is written. If the concentration of one of the reagents, for example, OIT, is tens of times higher than the concentration of the second reagent, usually RX, then the current concentration of the first reagent is identified with the initial concentration c Y and

assume that c Y = .

Another important piece of evidence for the S N 2 mechanism is the reversal of the RX configuration if only the RX and RY molecules are optically active. It can be seen from the reaction scheme (6.21) that after the isotopic substitution of Br for Br *, its optical isomer (II) is formed from the molecule of 2-bromo-butane (I)

by changing the direction of all chemical bonds in the transition state. After the substitution reaction, the sign of the angle of rotation of the plane of polarization of light changes. Configuration inversion in S N 2 reactions is possible only when RX is attacked from the rear, i.e. from the side opposite to X.

In the process of substitution, starting from the initial and to the final states (6.19), the number of chemical bonds remains equal to four. There are also four bonds in the transition state, not five, since three C-H bonds are two-electron two-center s / L bonds, while the other two bonds along the z axis can be considered one-electron, or rather, the Br'--C - Br bond is two-center . An essential element of the nucleophilic substitution mechanism is charge transfer through the central carbon atom. It is believed that the charge on this C-atom in the initial molecule RnH JII C - Br changes little in the elementary act of substitution from the initial state to the transition state. Therefore, the electronic nature of R (any substituent at the carbon atom) has little effect on the stability of the transition state and the reaction rate.

On the other hand, the properties of the transition state, the probability of its formation, the mechanism and rate of the reaction are greatly influenced by the spatial (bulk) properties of the substituents. As hydrogen atoms in CH 3 C1, CH 3 Br and CH 3 1 are replaced by CH 3, other alkyls, phenyl and larger groups, the mechanism, reaction rate and nature of the transition state change. Below are the relative rates of substitution of Br for I in alkyl halides (CH 3 CH 2 Br is taken as the unit of rate):

Thus, the reactivity of haloalkyls depends on whether the halogen is bonded to the primary, secondary, or tertiary carbon atom. As the volume of substituents at the carbon atom increases, the mechanism of substitution of the halogen atom changes, passing from the pure 8k2 mechanism in CH 3 G to pure S N 1 in (CH 3) 3 SG in highly polar solvents.

Mechanism S N 1. Tertiary butyl bromide with hydroxide ion reacts at a low rate according to a first order kinetic equation

Equation (6.22) shows that the reaction rate depends only on the current RBr concentration and does not depend on the OH concentration at all. This means that the mechanism of interaction (6.21) is impossible due to strong steric interference created by three CH 3 groups on the way of OH - attack. Strong shielding of the reaction center - the central C-atom does not allow the transfer of (-)-charge from OH "to the bromine atom in R 3 CBr:

However, the substitution reaction Br occurs due to the difficult and slow stage of the preliminary solvolytic (classical name - electrolytic) dissociation of the initial halide RT:

followed by fast ion recombination:

The S N 1 mechanism is described by two stages of interaction. The first stage of dissociation (6.23) for polar covalent compounds of the alkyl halides type occurs with the breaking of the chemical bond along the ionic type and requires not only a high activation energy, but also a strong solvation of the ions. The first stage of the mechanism can only conditionally be considered monomolecular, since the dissociation of molecules into ions in solution cannot proceed under ordinary conditions as a spontaneous process. For the dissociation of the C-Br bond, the participation of a solvent as a reagent is necessary. The solvent at the stage of activation of the alkyl halide causes polarization of the C-Br bond due to the dipole-dipole interaction of RBr with solvent molecules solv and the interaction by the type of hydrogen bond R-Br--HSolv, if proton-donor molecules HSolv act as the solvent. The transition state of the slow reversible stage (6.23) can be represented as follows:

Alcohols, carboxylic acids and other solvents act as HSolv. The energy of solvation of two counterions, reaching up to 600 kJ/mol, covers the costs of activation of the first stage of the reaction (6.22). At the second stage, the process of ionic association of the cation and anion occurs, which proceeds quickly and does not require a significant activation energy, like all known processes of combining ions of the opposite sign. In general, the rate of the substitution reaction proceeding according to the S N 1 mechanism is determined by the concentration of the carbocation R + . This concentration is so low that the carbocation is not detected even in electronic absorption spectra. This is understandable, since the dissociation reactions of polar C-X bonds in most cases have very low equilibrium constants, for the dissociation reactions of alkyl halides they are so small that they still do not measured.

At the first stage of the reaction, energy is required not only for the ionic dissociation of the CBg bond, but also for the change in the conformation of the hydrocarbon residue (CH 3) 3 C + tetrahedral (hybridization $ /? 3) to flat (hybridization sp 2, fig. 6.4).

Thus, at the first stage, the activation energy consists of two parts:

where is the energy consumption for the heterolytic cleavage of the C-Br bond; D? k0||f -

energy consumption for changing the geometric and electronic configuration.

Rice. 6.4. Change in hybridization of the central carbon atom in S N 1 reactions

The most important proofs of the Sm1 mechanism are the independence of the rate constant of equation (6.22) from the concentration of OH* and the partial racemization of the alcohol, which is formed from the optically pure alkyl halide RiR^R^CBr, since the attack of a planar carbocation is equally probable from both sides of the plane (Fig. 6.5). Therefore, in a chemical reaction, both optical antipodes are formed: (II) and (III). However, it turns out that the optical antipodes (II) and (III) are not formed in equal amounts, so the optical activity of the resulting alcohol is retained. This fact means that the elementary act of interaction (6.23) and (6.24) is much more complicated than we imagine. The current opinion that the leaving group Br" exerts steric interference for the attack of OH" from Br, as a result of which one of the antipodes is formed in a smaller amount than the second, is apparently correct.

In contrast to the S N 2 mechanism, the reactivity of alkyl halides reacting according to the S N 1 mechanism (for example, in a solution of HCOOH) is strongly

Rice. 6.5. The racemization of an optically active alkyl halide (I) as a result of the formation of two enantiomers (II) and (III) depends on the electronic effects of the substituents in R,R 2 R 3 CBr, R,R 2 CHBr, and RCH 2 Br.

If R is an electron donor, then the rate of reaction (6.18) increases, since the carbocation is stabilized and, vice versa, if R is an electron acceptor. For this reason, the rate of substitution of Br ~ for ORT under the action of H 2 0 as a reagent:

catastrophically decrease in the series of halides (velocities are given in relative units):

The speeds in this series are determined by the electron-donating ability of CH 3 -

group (+/- effect) compared to the H-atom. Therefore, the (+)-charge of the central-?

of the carbon atom R 3 C is partially delocalized due to electronic effects, for example, the +/- effect of CH 3 groups:

An important feature of nucleophilic halogen substitution reactions in reactions subject to the S N 1 mechanism is the atomic rearrangement of a complex hydrocarbon residue during the reaction and significant amounts of the resulting alkene due to the departure of one H* ion from the carbocation.

In most cases, however, the reactions of electrophilic substitution of a halogen or another functional group, depending on the chemical nature of the reagents R-X, Y and the solvent Solv, in reaction (6.18) proceed more complicated: simultaneously by S N 2-, and Sn1-mechanisms. These two chemical reaction streams lead to kinetic equations intermediate between reactions (6.20) and (6.22). In these equations, the concentration of ObG has a fractional order.

The following regularities have been established for the reactions of nucleophilic substitution of halogen and similar substituents: 1) the higher the polarity of the solvent, the more likely the S N 1 mechanism; 2) the stronger the nucleophile Y in reaction (6.18), the more likely the 8m 2 mechanism; 3) the more bulky substituents at -X, the more likely the S N 1 mechanism; 4) the more covalent bond -X, the less likely the mechanism S N 1; 5) the higher the concentration of the nucleophile (OH", OR", etc.), the more likely S N 2-MexaHH3M and the lower they are, the more likely the mechanism S N 1.

In this section, only the mechanisms of nucleophilic substitution of the halogen atom in saturated alkyl halides are considered in detail.

Reactions E. This symbol denotes the chemical reactions of functional derivatives of hydrocarbons RX, in which pair elimination occurs (from English, elimination - elimination, symbol E), i.e., the removal of the functional group X together with the hydrogen atom, with the formation of a n-bond, for example :

Elimination can occur not only from neighboring atoms (1,2-elimination), but also from the same atom (1,1-elimination) with the formation of carbenes: R-CC1 2 H -+ RCC1.

Mechanism E2. Most often, HX elimination with the formation of alkenes, alkynes, and denenes proceeds according to the bimolecular mechanism E2. This important conclusion was made in 1927 by K. Ingold. The E2 reaction takes place in one stage:

In the elementary act (10-13 s), a hydrogen atom in the form of a proton and a bromine ion are simultaneously removed. Undoubtedly, to remove Br" a particle of a proton-donor solvent is needed, which is included in the transition state. Such a particle can be an H 2 0 molecule or an alcohol molecule. [In reaction (6.26) denoted by Solv.]

In this case, the kinetic equation of the reaction (6.26) is of the second order, i.e., it contains in the first degree R-G concentrations(or R-X, where

X - not only halogens, but also alkylammonium residues -N (R) 3, H 2 0 -, dial-

kylsulfonium -SR 2 etc.) and base reagent (OH", RO", etc.).

The activation mechanism and structure of the transition states have not been precisely established, however, it is known that the removal of FT and X "occurs synchronously, carbocations and carbanions are not formed as stable intermediates. The isotopic substitution of hydrogen H for deuterium D in the act of formation of the n-bond does not occur. Experimental studies have shown that in the transition state of the n-component of the o-C-C bonds are not formed. Therefore, the st,n-conversion of electrons occurs beyond the top of the potential barrier, in the course of the rapid transformation of the transition state into the final products. The E2 mechanism act as two-center reagents with which a proton acceptor (base) and a proton donor (acid, proton-donor solvent) interact synchronously. Therefore, the reaction can be considered not as a bimolecular, but as at least trimolecular (E3). However, the third reagent is a proton-donor solvent present in the reaction sphere in large excess and is not included in the kinetic equation. Therefore, the kinetic equation for E2-sactions

corresponds formally to the bimolecular elementary act. To reveal the activation mechanism and detail the composition and contours of the transition state of the reaction (6.26), not only the activation energies are needed, but most of all, the activation entropies AS". Detailed information of this kind is not available. The E2 mechanism is typical for derivatives having an X substituent at the primary and, in some cases, secondary carbon atom.

The formation of an n-bond during the dehydration of alcohols occurs in the presence of acids (in catalytic dehydration, oxides with their acid centers act as acids) similarly to dehydrohalogenation (cleavage of NH), dehydroamination (cleavage of NR 3), dehydrosulfidation (cleavage of SR 2):

However, the dehydrogenation of alcohols proceeds according to the monomolecular mechanism E1, while the elimination of HBr (reaction 6.26) and other hydrogen halides, the tertiary amine from trialkyl-p-phenylethylammonium, and dialkyl sulfide from dialkylethylsulfonium can also proceed according to the bimolecular mechanism E2.

Mechanism El. This mechanism corresponds to the monomolecular reaction of elimination of the functional group and is also accompanied by the formation of an n-bond. The reaction takes place in two stages. At the first slow stage, which limits the rate of alkene formation, ionic dissociation of the apkyl halide (usually tertiary R,(R2)C(R3)Br) or protonated alcohol occurs at

bonds -C-G ^ -C + G or -C-OH, - -C + H,0.

For a noticeable speed of the process, a strongly solvating medium is required (polar solvent Solv):

The second stage in the formation of an n-bond (alkene, alkyne, allene, etc.) is accompanied by the removal of the charge from the carbocation in the form of a strongly solvated or chemically bound proton, for example:

It should be noted that the reaction of formation of the carbocation is reversible, while under the experimental conditions the second stage of deprotonation of the carbocation is almost chemically irreversible. The alcohol present in the reaction sphere binds H2SO4 in an acid-base reaction, so the protonation of the alkene, in this example (CH 3) 2 C=CH 2 , necessary for the reversibility of the recorded reaction, is unlikely.

In a real situation, the occurrence of paired cleavage reactions (E) is accompanied by other accompanying reactions, which include either the original functional derivative (R-Г, R-OH, etc.), or an intermediate, which can be carbocation, carbanion, etc.

So, for example, along with the reaction of cleavage of NH, H 2 0 and other stable molecules from functional derivatives, i.e., the formation P- connections. there is a nucleophilic substitution of the halogen G "in R-G for any of the nucleophilic particles present in the solution (OFT, CH 3 SG, RS", G ", etc.), in the reaction S N 1, S N 2, as well as rearrangement reactions carbocations leading to the formation of isomers:

A complicating circumstance in the chemistry of E2 and E1 reactions is the multivariance of proton elimination if a halide ion or another functional group to be eliminated occupies a place at a secondary or tertiary carbon atom. So, in the alkyl halide (I) it is possible to remove Br "together with one of the protons at C, -, C 3 - and C 6 atoms with the formation of 2-ethyl-3-methylbutene-1 (II), 3,4-dimethylpentene ( III), 2,3-dimethyl-pentene-2 ​​(IV). All three alkenes are present in the reaction mixture. However, compound (IV) prevails in large excess, in which the tertiary hydrogen atom protonates, which has a minimum binding energy compared to the secondary and primary (see alkanes). In compound (I), bromine is located at the tertiary carbon atom. The HBr cleavage reaction occurs through the carbocation according to the E1 mechanism, and the reaction rate does not depend on the concentration of the alcoholate ion (KOH alcohol solution), since the latter is not included into the kinetic equation (6.29):

The further fate of the carbocation depends on the composition of the solution and other conditions.

Ad reactions. This symbol denotes the reactions of electrophilic, nucleophilic and free radical addition of reagents according to

C=C, C=C, -C=addition).

Catalytic addition via n-bond (for example, catalytic hydrogenation) has more complex mechanism and is not considered in this section.

Addition of proton molecules to alkenes. These molecules include all inorganic acids, as well as carboxylic acids, hydrides H 2 0, H 2 0 2, H 2 S, etc., which can transfer the proton LG to alkenes (alkynes and etc.). The proton is an electrophilic particle. Therefore, this entire group of reactions is called electrophilic addition reactions (Ad E). Attachment takes place in two stages: the first is slow, limiting, and the second is fast:

At the first stage, an asymmetric polarization of the n-bond occurs with a shift of both n-electrons of ethylene to C |; they are converted from the p-state (n-symmetry of the chemical bond) to the r-state (st-symmetry of the chemical bond) and the protonation of the electron pair. So there is CH 3 - group. Electronic transformations require not only the expenditure of energy, but also the deformation of the initial chemical bonds. Therefore, this stage is slow, limiting, and determines the A(1 E 2-mechanism) as a bimolecular electrophilic addition described by the kinetic equation:

where k v- rate constant; with s. c is the alkene concentration; c HjS0

Polarization of the n-bond can occur to one of the two C-atoms of the n-bond. In ethylene and its unsubstituted symmetry, both C-atoms are the same. Therefore, the addition of a proton to any of the carbon atoms of ethylene (1,2-dimethylethylene, 1,2-diethylethylene) is equally probable. A different situation arises when ethylene is substituted asymmetrically. So, in isobutylene (2-methylpropene), the proton is more easily (faster) added to C, -, and not to the Arrangement:

In this case, the proton is attached to the most hydrogenated carbon atom, C, i.e., according to Markovnikov's rule (1869). The formation of a C-H bond at the primary carbon atom is energetically more favorable than to the secondary and even more favorable than to the tertiary. In addition to the energy factor, when the proton is oriented toward one or another carbon atom, many researchers believe that the electronic factor of stabilization of the carbocation acts. If we are talking about the addition of a proton to isobutylene, then the decrease in the electron density deficit, which is energetically unfavorable, the 2-isobutyl cation CH С CH, is easier (due to +/-

the effect of three adjacent methyl groups) than that of the 1-isobutyl cation +

CH-(pH - CH 2, in which the shift of the a-electron density to the carbocation CH 3

comes from only one isopropyl group. For this reason, isobutylene almost does not form CH 3 -CH-CH 2 X product with HX.

vation (Fig. 6.6) is clearly visible. It can be seen that the transition state of formation of the 2-isobutyl cation occurs with lower energy, the transition state is more probable, and the carbocation is more stable. Therefore, the reaction goes faster along the path (6.31) than along the second path.

The second stage of addition is fast, since it reduces to the ionic interaction of the carbocation with the anion СГ, Br“, CN“, RCOCT, or to

Rice. 6.6. Energies of transition states of H + addition to isobutylene

ion-molecular interaction with molecules H 2 0, ROH, etc. It does not require significant energy costs.

In addition to proton-donor molecules, other electrophilic molecules also enter into the reaction of electrophilic addition to alkenes, in particular, halogens C1 2, Br 2 (1 2 does not add under normal conditions), cyanogen (CN) 2, dithiocyanate (NCS) 2, etc.

Attachment of halogen molecules. Halogen molecules, such as Br 2 , are electrophilic particles (electrophiles). Therefore, in the reaction with alkenes, they polarize the electron cloud of the n-bond so that the r-electron pair passes to the antibonding 'P*-orbital Br 2 . The bond order Br-Br becomes equal to zero and the bond is broken. In this case, one of the atoms of the Br 2 molecule carries away the 71-electron with it, turning into Br. "The second bromine atom in the transition state interacts with the radical cation of the alkene, forming either a bromine carbonium ion

C-C-, or alkenebromonium -C-C-. Therefore, the first limiti- Br Br +

The first stage (it requires an activation energy to break the Br-Br- and 71-bond and is slower than the second stage) can be written in a simplified way as follows:

This bimolecular stage corresponds to the Ab Ё 2 mechanism and is bimolecular, since two molecules are involved in the stage of formation of the transition state (#). The kinetic equation for this stage (and for the entire reaction as a whole) includes the concentrations of the alkene and Br 2 , i.e., it is of the second order.

The second stage of ion-ion interaction, leading to dibromoalkane, passes quickly and does not affect the overall reaction rate. It should be borne in mind that the above ideas about the mechanism are roughly approximate. The true activation mechanism is much more complex and requires further research.

Nucleophilic addition reactions at ^X=0 bonds in

aldehydes, ketones, ethers. This type of reaction, whose mechanism according to Ingold is denoted by (Ad N), where N is the symbol of the nucleophile, includes

addition of a nucleophile (N") to the carbon atom of the ^2C=0 group. If

the reagent is complex and consists of a nucleophilic and an electrophilic part, as in

8- 6+ 6+6- 6+ 6- 6+ 6- 6+ 5-

example CHjMgBr, HBr, HOH, HOR, HCN, etc., then the nucleophile is attached to the carbonyl carbon, and the electrophile is attached to the carbonyl oxygen:

In this case, the main active principle in HX-type reagents with a mobile proton is the nucleophilic part X ", and not the proton. All these reactions are reversible, i.e., the detachment of the attached at

^C=0 reagent.

Among the nucleophilic addition reactions, one can single out the addition reactions of organomagnesium compounds, lithium alkyls, hydrocyanic acid (formation of cyanohydrins), water (hydration of aldehydes and ketones), alcohols (formation of hemiacetals and hemiketals), metal hydrides (LiH, LiAlH 4), hydroxylamine NH 2 OH ( formation of oximes), phenylhydrazine (formation of phenylhydrazones), ammonia and amines (formation of azomethines), aldehydes and ketones (aldol condensation), hydrogen at the moment of isolation (hydrogenation), alkali and aldehyde (self-oxidation-self-healing reaction - Canizzaro reaction. Kinetic studies of these reactions showed that the rate-limiting addition step, as in the case

if the nucleophile carries a negative charge, so

if the nucleophile is a molecule, it is the attack of the reagent on C=0 with the opening of the r-bond. In these reaction equations, X = H, alkyl, aryl, OR, R--O, F, etc.

AdN reactions are bimolecular with as yet unidentified transition states and activation mechanisms ( AN* and AS*- not obtained), but with known kinetic equations, which for non-catalytic reactions (catalysts OH "or H + are absent) have the form:

Only in a few cases (for example, with RMgX in a number of solvents or in Cannizzaro reactions) does the rate have a third or even fourth order.

Electrophilic and nucleophilic substitution reactions in aromatic systems. Aromatic compounds - arenes, aromatic heterocycles, aromatic macrocycles - are capable of electrophilic substitution reactions or a hydrogen atom

or a strongly electron-withdrawing functional group

as well as to reactions of nucleophilic substitution of functional groups

Electrophilic substitution reactions - nitration, sulfonation, halogenation, acylation, alkylation in the aromatic series are among the most important, as they allow you to go from hydrocarbons or their heterocyclic derivatives to any of their functional derivatives.

The role of nucleophilic substitution reactions is less significant in a number of aromatic compounds.

Electrophilic substitution reactions in arenes. to install general mechanism of electrophilic substitution in arenes and non-benzenoid aromatic systems, we will consider step by step all the above reactions of electrophilic substitution, starting with the most studied and widely used in industry and scientific research - the nitration reaction.

The nitration reaction is the replacement of an H-atom in an aromatic molecule with a nitro group - N0 2 as a result of the action of a nitrating reagent (HN0 3 + H 2 S0 4 N0 2 + H 2 0 + HSO *, nitronium salts

N0 2 C10 4, N0 2 BF 4 ", mixed anhydride of nitric and acetic acids - acetyl nitrate CH 3 C0-0-N0 2):

Most often, a mixture of concentrated HNO3 + H 2 S0 4 is used. This mixture easily nitrates benzene, naphthalene, chlorobenzene, extremely easily acetylaniline (acetanilide), alkylphenols, toluene, xylenes, more difficultly nitrobenzene, benzenesulfonic acid, and benzoic acid. Nitration of benzene reaches the stage of m-dinitrobenzene (I)V-N0 2 and only with difficulty and large

losses due to oxidative degradation can be obtained trinitrobenzene 0,N-

^^-no 2

fast reversible stage, an intermediate is formed, the so-called 71-complex due to the shift of the n-electron density of the aromatic ring to the nitronium cation:

This n-complex is essentially a charge-transfer complex (CTC), a fairly stable associate. The formation constants of such complexes have not been measured. In this complex, the hydrogen atoms on the C-H bonds are sufficiently protonized (i.e., the C-H bonds in them are more polar than in C 6 H 6) and can enter into solvation interaction with particles that have at least a weakly basic character (HS0 4 , H 3 0*, etc.). In the n-complex, during active collisions in solution, the sextet of n-electrons can be polarized towards one of the C-H bonds, which is the most solvated. Then the following will happen. A pair of n-electrons with a bonding 4 J Kj orbitals, as the arena most weakly associated with the C 6 backbone,

undergoes n,a-conversion and turns into a-electron pair sp3- type.

As a result of the high cost of activation energy for the destruction of the aromatic r-system of benzene and its transformation into an open conjugated

n-system С-С-С-С-С-, which makes a smaller contribution to the stabilization of the cycle than the aromatic six of n-electrons, the so-called a-complex is formed (a very unfortunate name, which, however, has become established in the world literature):

In fact, no complex is formed here, but a carbocation arises, which has four conjugated π-electrons and one bonding vacant cell (“hole”, (+)-charge). The positive charge is not localized at one carbon atom and pulsates throughout the five-atom conjugated system. This delocalization of the (+)-charge, the mechanism of which is not exactly known, allows us to write the conjugated carbocation in the form

Rice. 6.7.

V+)»|l, ​​i.e. i-bond *.W N0 ;

of non-integer multiplicity covers the skeleton of 5C atoms. There is no cyclic conjugation; unlike benzene, it is broken by an insulating atom C,. Such h>

the system is less stable than the aromatic molecule, and therefore tends to overcome the energy barrier (Fig. 6.7) and again turn into a low-energy (stable) aromatic r-system due to the transfer of a proton to the base (HSO4, NOj, etc.):

There is no doubt that the acts of formation of the n-complex and the conjugated carbocation (a-complex) are reversible processes, when the reacting system, up to reaching the transition state, can again return to the initial state. However, the step of degradation of the transition state into final products C 6 H 5 N0 2 and acid is irreversible. The nitro group cannot be replaced back by a hydrogen atom.

sulfonation reaction. This electrophilic substitution reaction is extremely important, as it makes it possible to obtain water-soluble aromatic compounds - sulfonic acids and their numerous derivatives in the sulfo group. The sulfonation reaction is the replacement of the H-atom in arenes with a sulfonyl, or simply sulfo group, under the action of H 2 S0 4 (conc), oleum (pyrosulfuric acid H 2 S 2 0 7 and polyacids of a higher order H2S3O10, etc.), sulfur oxide (Y1) S0 3 in the liquid or gas phase, its associates with bases, for example Py-S0 3, where Py is pyridine.

In concentrated H 2 S0 4, the strongest electrophilic particle that attacks the arene is protonated sulfur oxide HSO, which appears in negligible amounts in solution by the reaction

Here the usual acid-base reaction occurs, in which one H 2 S0 4 molecule ("acid") protonates another H 2 S0 4 molecule ("base"). It should be recalled that all particles are both acids and bases at the same time. Which property the particle will manifest - acidic or basic, will depend on the conditions for the existence of the particle (concentration, solvent, temperature, nature of other dissolved particles). Dehydration of protonated sulfuric acid under the influence of H 2 S0 4 gives the hydrotrioxide sulfur cation HSOJ.

The mechanism of its interaction with arenas is no different from the +

interactions with N0 2:

The only difference is that the sulfonation process is reversible and the sulfo group in a strongly acidic medium can again turn into arenes (the so-called hydrolysis of sulfonic acids, but in fact the electrophilic substitution of the sulfo group for a hydrogen atom, going through the same transition state as the direct reaction) .

Sulfonation is always more difficult than nitration, so

as HS0 3 is a weaker electrophile than N0 2 .

The reactions of nitration and sulfonation are of the second order - the first with respect to the arene and the first with respect to the electrophile, and at the limiting stage they are monomolecular (conditionally) reactions. In fact, it is not the st-complex that reacts by itself, but in pair with a proton-acceptor particle.

halogenation reactions. All halogens, except for F 2 , are insufficiently strong electrophiles and cannot destroy the aromatic p-system at the activation stage to a conjugated carbocation. Therefore, they most often require activators (catalysts) in the form of halogen acceptors (AlCl, FeCl 3 , BF 3 , etc.). Halogen acceptors belong to the so-called Lewis acids, i.e., they, like H +, have a vacant r r - or rf.-orbital.

Chlorine and bromine without activators only dissolve in benzene and do not give a substitution reaction. To start the reaction, an activator is needed, which is supposed to interact with the polarized chlorine molecule. There is every reason to believe that the halogen first forms an unstable n-complex (or CTC) with the arene:

Its formation constant is unknown, but apparently small. l- the complex exists without an activator for an infinitely long time. In the presence of A1C1 3, its interaction is likely to form a ternary complex

which probably forms the transition state. The decrease in the order of the C1-C1 o-bond occurs due to a decrease in the electron density on the bonding H "-orbital (A1C1 3 acts as an electron acceptor) and a simultaneous increase in the electron density on the loosening "Vg-MO (operating arenas). As a result, both reagents (C 6 H 6 and A1C1 3) in the ternary complex break the C1 2 molecule into ions, an ion pair appears, forming

bath of carbocation + I x Cl and tetrachloraluminate [А1С1

The ion pair easily loses a proton, which escapes in the form of hydrogen tetrachloraluminate H[A1C1 4 ]. Bromine is a weaker electrophile than chlorine; fluorine reacts too violently, while 1 2 is inert in the reaction with benzene.

Acylation reactions. This type of reaction includes the replacement of a hydrogen atom with an acyl R-C=0 when acting on an aromatic system

acid chlorides (formyl chloride HCOS1, acetyl chloride CH 3 COC1, phosgene C1COC1) or their anhydrides (CH 3 C0-0-COCH 3) in the presence of activators (catalysts), for example A!C1 3 .

As in halogenation reactions, the activator enhances the electron-withdrawing properties of acid chlorides due to their ionization:

Acid chlorides do not form p-complexes with arenes, in contrast to C1 2 . One-

it is possible to form an l-complex with one of the ions (CH 3 CO) of the salt:

Nitro and sulfo derivatives of arenes, as well as pyridine, are not acylated, while alkylbenzenes, phenols, amines, pyrrole, furan, and thiophene are more or less readily acylated.

Friedel-Kraft alkylation reactions As even weaker electrophiles than acid chlorides, haloalkyls R-G (G \u003d Cl, Br, I) can act. Arenes weakly interact with alkyl halides by the type of universal solvation, which occurs due to van der Waals forces and dispersion forces.

When catalysts are added, a triple interaction occurs:

А1С1 3 (or BF 3 , FeCl 3) polarizes the bond ~pC-G due to weak donor-

acceptor interaction R ~ r: + AlCl 3 - - R-G: A1C1 3 , where o -

free orbital. Weak complex in polar solvents is negligible small degree can be dissociated

Despite the extremely low concentrations of R in solution, due to the high electron acceptor (electrophilicity) of a simple carbocation, an electrophilic substitution of a proton for a carbocation occurs:

This mechanism corresponds to all other electrophilic substitution reactions, in which the first stage includes the activation of an electrophilic reagent (HN0 3 , H 2 S0 4 , Г 2 , RCOC1, RG) and the appearance of a strong electrophile. The second stage is the rapid reversible formation of the l-complex (CPC). The third, rate-limiting step requires a high activation energy and a complex transition state and leads to arene substitution products; it is monomolecular.

An essential element of this mechanism (Se2) is the change in the electronic nature and reactivity of the aromatic nucleus after the introduction of a functional group into it. As a result of the interaction of two partners of the functional group (X) and the aromatic ring (Ar), a shift occurs in a- and l-electron density to one or another partner. This displacement either activates or deactivates the benzene ring (and the cycles of other aromatic systems) to electrophilic reagents. Groups such as -F, -Br, -Cl, -I, -N0 2, -NO, -NH 3, -S0 2 0H, -SO2 -, -CHO, -CO-, -COOH, -COC1, -conh 2, -CN, etc., cause a shift of the a-electron density from the benzene nucleus towards itself, as a result of which the rates of interaction of these Ar-X with electrophiles are greatly reduced, i.e., this group reduces the reactivity of the aromatic nucleus to electrophilic substitution. As a result of their entry into the arene molecule, the electron density decreases in all positions, but especially strongly in ortho- and lard positions. A special place is occupied by halogens, which will be discussed below. In table. Table 6.2 shows the relative rates of evaporating benzene derivatives with nitric acid, acetylation with acetyl chloride in the presence of AlCl 3, and alkylation with ethyl bromide in the presence of a GaBr 3 catalyst.

From Table. 6.2 it can be seen that the electron-donor groups activate the benzene core in all electrophilic substitution reactions, while all other substituents, which are electron acceptors, more or less strongly deactivate it, lowering the reaction rates from 1 to 1 (Г® (5+ > 56+ ):

Table 6.2. The manifestation of the electron-withdrawing and electron-donating effect of substituents in the benzene ring on the rate of nitration (HN0 3), ethylation (C 2 H s Br + CaBr 3) and acetylation (CH 3 COC1 + A1C1 3)

Compound	Relative speeds
	shaving	ethylation	acetylation
from 6 n 5 ch 3
C 6 H 5 C (CH 3),
S t N t SOOS 2 N 5
c 6 h 5 no 2
c 6 h*ch 2 cn
c 6 h 5 ch 2 no 2
C 6 H ) N(CHj),

The deactivation or activation of the benzene ring by the same dark alkyl or functional groups is not the same for various electrophilic substitution reactions. This reveals an infinitely greater specificity chemical properties connections.

Halogen atoms reduce the electron density in the benzene nucleus, but it decreases most of all in the l / e / la position. This is the result of a complex interaction of arsnes with functional groups of halogens (D). In addition to the st-acceptor action (-/-effect), they exhibit an ll-donor action, as a result of which the electron density on the n-orbitals of halogens decreases, and on the N **-orbitals of benzene it increases, and it is especially strong in ortho- and la /? l-positions.

The total al-shift under the influence of halogens is such that, on the whole, all carbon atoms lose their electron density, weaker ortho- and pair- atoms and stronger meta- carbon atoms of the benzene ring.

All strong n-electron donors, such as -NH 2 , -NHR, -NR 2 , -OH, -OR, -0 ", capable of n-conjugation with the benzene ring or other aromatic nucleus, cause an increase in the electron density in the nucleus and especially strong in ortho- and pair- positions (5->55-):

A similar effect is exerted by a-electron donors - alkyl groups (R), atoms of alkali and alkaline earth metals. As a result, the rates of electrophilic substitution upon the introduction of these groups into the nucleus greatly increase, i.e., the benzene nucleus is activated to electrophilic substitution. Especially strong activators of the benzene core are ionized phenolic hydroxyl -O", hydroxyl itself -OH, alkylated hydroxyl -OR and all amino groups. Azasubstitution in benzene by one, two and three aza-atoms strongly deactivates the benzene core.

Pyrazine n(^)n, pyrimidine * pyridazine

0 /F

Symmetrical triazine HI )/ have a very low capacity for electrophilic substitution reactions. Five-membered aromatic rings - pyrrole (^^NH , furan (^^O, iron(II) cyclopentadienyl

go-Fe O nap P 0TIV have an extremely high capacity for electrophilic substitution reactions. So, if benzene does not enter into the reaction of C 6 H 6 + 1 2 iodination even with catalysts, then pyrrole, furan and iron cyclopentadienyl (ferrocene) enter into a substitution reaction with 1 2 without catalysts.

The second most important element of the interaction of the functional group with the benzene core, as a consequence of the mutual influence of atoms in molecules, is the guiding action of the functional group in ortho-, in pairs lorto-, para- (ortho-, nuclei-orientants) or in meta-(.ieta-orientants) positions.

So, all electron acceptors that deactivate the benzene nucleus, except for halogens, orient the electrophilic reagent to the positions:

Wherein ortho- and ldrd isomers are formed in very small amounts.

Alkyl groups and mi-electron donors, which increase the electron density more strongly in ortho- and lord positions than in meta-, are ortho-, ldrd-orientators. To ortho-, pair-orientants also include all halogens.

Bromination of phenol is threefold and occurs at a tremendous speed (almost instantly):

Benzene itself, as well as its alkyl, amino and hydroxy derivatives, do not enter into the nucleophilic substitution reaction. Halogenbenzenes replace their halogen with a nucleophile only under very harsh conditions. So, for substitution by -OH, a catalyst and a high temperature are required:

The reaction takes place under extremely harsh conditions.

If the nucleophile is very strong (NHj-amide ion), then substitution proceeds easily, but according to a special mechanism called the elimination-attachment mechanism:

The nucleophilic substitution of the functional group in polysubstituted arenes easily passes when there are several strong electron acceptors in the nucleus, for example, the substitution of chlorine in chloropicrin:

Sulfonic acids of arenes will undergo nucleophilic substitution - S0 2 0H to -OH when fused with NaOH or other alkalis:

The mechanism of nucleophilic substitution reactions has not been studied in detail. It can be assumed that the reaction proceeds through the a-complex (conjugated carbanion):

The reaction takes place in two stages. At the first bimolecular stage, which is limiting, a conjugated carbanion is formed. At the second stage, the (-)-charge with the leaving group X is removed. This stage has higher rates, therefore, the overall rate of the nucleophilic substitution reaction in arenes and other aromatic systems depends little on the nature of the halogen, i.e., on the strength of the C- bond G.

F" is usually noticeably easier to remove, since it is more easily solvated by proton-acceptor molecules.

Aromatic molecules have the ability to equally lose or gain electrons and electron pairs due to the special properties of frontier orbitals and the delocalization of n-electrons. The removal of one or two electrons from the bonding orbital does not lead to a catastrophic decrease in the order of chemical bonds and their rupture, since there are six C - C bonds in the benzene nucleus. Therefore, when one n-electron is removed, the bond order decreases from 1.5 to 1.42 , when removing two from 1.5 to 1.33. The situation is similar when one or a pair of electrons is added to the loosening H 7 * orbital.

Thus, aromatic systems, especially macrocyclic ones (porphyrins, phhalocyanines), have a margin of stability in redox states.

For this reason, electrophilic and nucleophilic substitution reactions in arenes can proceed through conjugated carbocationic and carbanionic forms.

Mechanisms of substitution reactions in the carboxyl group. Carboxylic acids R-COOH, their derivatives in the carboxyl group - acid chlorides RCOC1, anhydrides RCO-O-COR, esters RCO-OR", amides RCO-NH 2 are capable of nucleophilic substitution of the group (X-) in RCO-X , where X \u003d OH, F, Cl, Br, OR, OCOR, NH 2.

Substitution reactions take longer than with a saturated carbon atom. Therefore, the -C=0 group favors substitution at the oxidized carbon atom.

In the first stage of the reaction (6.33)

the n-bond opens due to C=0 ionization. Electrons electron-

of the reagent pair Y occupy a vacant orbital at the carbon atom.

The chemical bond knot - C is flat and easy to attack

reagent Y of the carbon atom above and below its plane.

The mechanism of the reaction of substitution of the X- group by the Y- anion can be represented as follows. The nucleophilic reagent (V) with its electron pair attacks the loosening 4 / * - orbital of the C \u003d 0 bond, as a result of which the order of the n-bond approaches zero and it experiences heterolytic + -

sky decay to -C-O.

The electron pair of the reactant occupies R: ~carbocation orbital with simultaneous 5p 3 hybridization of all a-bonds of the carbon atom. In the transition state, the C-X bond weakens and the electronegative group X (-OH, -G, -OR, etc.) gets the opportunity to leave the carbon atom in the form of an anion X ":

According to the mechanism (6.34), acid chlorides, anhydrides, amides and esters are hydrolyzed in a neutral and alkaline medium, as well as anhydrides, acid chlorides and esters are amidated:

The promotion of a substitution reaction by the hydroxyl group, leading to an increase in the rate, is called alkaline catalysis.

Otherwise, the reaction of the formation of esters and their hydrolysis in an acidic medium takes place:

All stages of ester formation and its hydrolysis (reverse reaction) in an acidic medium are reversible. These reactions are acid catalyzed. They do not pass in a neutral environment.

Rearrangement reactions of substituents in hydrocarbons. A large and practically important group of reactions are intramolecular rearrangements in polysubstituted alkanes, alkenes, alkynes, and arenes. These rearrangements are usually associated with the delocalization of a chemical bond, with the internal ionization of molecules. As a result, the migration of n-bonds along the carbon chain, the migration of the proton, alkyl and aryl groups, and halogens from one atom to another is possible.

The n-electron rearrangements are simpler. So, allenes having a cumulated n-bond are converted into alkynes under the action of alkalis (A. E. Favorsky, 1888):

Reaction (6.35) is reversible. Reactions of the type (6.35) should be classified as Favorsky rearrangements.

So, cyclooctadeca-1,3,7,9,13,15-hexaine (I) under the influence of alcoholate tert-butanol (the reaction is similar to the Favorsky reaction) turns into conjugated cyclooctadeca-1,3,7,9,13,15-hexaen-5,11,17-triyne (I):

When hydrogenated on a Pb/BaCO 3 catalyst, hexaentriin is converted to -annulene. This method is used for industrial synthesis - annulene.

One of the most probable mechanisms for the transformation of a triple bond into a double bond with the movement of the n-bond along the chain of carbon atoms (Favorsky reactions) is associated with the addition - elimination of an alkaline reagent, and in the transition state, the formation of a carbene (-C -) is possible:

Apparently, ORT plays the role of a bridge for the transfer of a proton from one terminal carbon atom to another. In the chemistry of coordination compounds, OH” very often plays the role of a bridging electron-donor group and, at the same time, a proton transmitter.

Apparently, a similar role of the bridging group is played by one of the hydroxyls in the process ttacolin rearrangement:

Numerous studies of various substituted glycols and 1,2-amino alcohols have shown that the migrating group (H, alkyl, aryl, halogen) acts as a nucleophilic reagent, attacking the (+) charged carbon atom that appears after the elimination of one of the protonated hydroxyls. The formation of the carbocation and the transition state can be imagined as follows:

It was found that the jumping substituent (in our example, alkyl R) always approaches the carbon atom from the side opposite to the position of the lost hydroxyl. This means that there is no rotation of the C–C bond in the carbocation, probably due to the strong donor-acceptor interaction of the remaining -OH with the (+)-charge carrier. The migrating group (H, CH 3 , C 6 H 5 , etc.) in the transition state is bound to both carbon atoms by a three-center (single-electron) bond. The second hydroxide ion, having played its active role as a carbocation stabilizer, again returns to its (C|) carbon atom, but already in the form of a protonated C=0 group.

The rearrangement requires energy costs to break one st-bond (C|-R) and redistribute the electron density between chemical bonds. Unfortunately, the activation entropies of these processes have not been studied, and the role of solvation in the rearrangement is unknown. It was found that the rearrangement result for unsymmetrically substituted glycols strongly depends on the conformations of the substituents and the entire molecule as a whole.

Practically and theoretically important is beisidine rearrangement(A. Hoffman, 1863), in the course of which N,N-diphenylhydrazine (hydrazobenzene) can be converted into u,n-diamino diphenyl (benzidine):

This rearrangement is typical of all structural analogs of hydrazobenzene containing in the a/d-iodizations of phenyl groups or hydrogen atoms or functional groups capable of leaving the molecule under the reaction conditions. Ease of splitting groups when regrouping:

Groups R, NH 2 , NHCOCH3 are not split off.

Obviously, in the course of rearrangement (6.36), which almost always occurs in a strongly proton-donor medium (H 2 S0 4 , HC1, CH3COOH), the covalent N-N connections and the formation of a C-C connection. Numerous studies of reactions of the type (6.36), carried out over the past century, made it possible to establish its main structural regularities and the stoichiometric mechanism. It has been shown that, along with benzidine, o-benzidine, diphenyline, etc. can occur in various amounts:

The reaction, depending on the nature of the diarylhydrazine and the solvent, may have a third or second kinetic order. For hydrazobenzene (GB), it is always first, while for proton it can sometimes be first, and most often second.

Therefore, the reaction (6.36) may have not one, but two or more routes that lead to benzidine. The general kinetic equation can be written:

where with p- current concentration of hydrazobenzene; and to 2 are the rate constants of routes with the active participation of one and two protons. If the rates of both flows (routes) of the reaction are very different from each other, then one of the terms of equation (6.37) will disappear and the reaction will have either the second or third order, which was most often observed experimentally. If the speeds are comparable, then the order in [Н + ] will be fractional (from 1 to 2).

Based on these and other data, it can be concluded that the most likely mechanism would be the following:

In the course of the reaction, rapid single protonation of hydrazobenzene occurs, which does not require significant energy costs for activation. This is the first stage. At the second limiting stage, the protonation of the second nitrogen atom without strong activation of the reacting system is impossible, since a strong stretching of the N–N bond and deformation are necessary.

tetrahedral angles -C in the direction of their strong increase, since

it is necessary to get closer P- and P"-reaction centers of phenyl nuclei. Therefore, the transition state (#) is formed only under the influence of the second proton, which brings the aromatic n-system of benzene nuclei to a non-aromatic, conjugated, characteristic of the st-complex in simple substitution reactions in arenes (see mechanisms of electrophilic and nucleophilic substitution in arenes).

The rate-limiting stage is of the first order in [b1]. However, the first fast stage is reversible. Application of the law of mass action leads to the second order of the reaction (6.36) with respect to the proton. If the system is already strongly activated by the first attached proton, then the second proton will not enter the kinetic equation (or will enter as a component of the solvent) and the kinetic order of the reaction (6.36) will be the first in H*.

The protonless benzidine rearrangement discovered in 1949 by Krolik and Lukashevich remained without a theoretical interpretation. It took place at 80–130°C in ethanol, acetone, or benzene in the absence of acids. Unfortunately, the difficulties in determining the energies and entropies of activation in reactions associated with intramolecular rearrangements do not allow one to seriously study and substantiate the activation processes and the structural and energy aspects of transition states. Apparently, these gaps will soon be eliminated.

Mechanisms of polymerization reactions. The polymeric state of matter is the highest form of complication of molecules. It is characterized by such features as an extremely large, predominant role of conformational transformations of molecular chains and spatial screening of reaction centers, a gradual transition from physicochemical properties to functions, as a result of which such polymer molecules can be capable of metabolism, fixing changes associated with the action of surrounding atoms and molecules, physical fields, etc.

Only polymer molecules, due to the variety of spatial structures, can be the basis of the most complex associates of molecules of various chemical nature and create biological apparatuses of living organisms, such as photosynthetic, respiratory, hemoglobin, enzymatic oxidative based on cytochromes, etc. For this reason, the mechanisms of reactions of formation of polymeric molecules and the mechanisms of their degradation (decay) are of primary interest. Polymer molecules can be formed in two different ways - with the help of polymerization reactions and condensation reactions.

polymerization reaction- this is the interaction of two or more molecules of unsaturated compounds containing simple or conjugated 71-bonds, or easily opened rings, called monomers, which leads to the formation of dimers, trimers and, ultimately, high polymers due to α, n-electron conversion with subsequent localization of electrons on newly emerging σ-bonds.

Polymerization is never accompanied by the elimination of any atoms or atomic groups. On the contrary, it is associated with the additional attachment of the so-called fragments of molecules at the ends of the high polymer. polymerization initiators.

Free radicals, anions the weakest acids and cations with extremely high electron-withdrawing properties.

In this regard, free radical, anionic and cationic polymerization are distinguished.

free radical polymerization. The polymerization initiator is a molecule that is a source of a free radical. So, H 2 0 2 (or any of its derivatives) can cause the polymerization of ethylene. The mechanism is described in four stages.

The first stage is the decay of the initiator:

The second stage is the initiation of polymerization by a free radical:

on which the initiator opens the n-bond, joins one of the atoms of the n-bond, and the second n-electron becomes free, forming a more complex free radical.

The third stage can be extremely long, repetitive (P- 1) times, where w is the degree of polymerization. This stage of polymer chain growth:

In this case, a free (unpaired electron) is constantly transferred from the terminal atom of the growing polymer to a new C 2 H 4 molecule.

The fourth stage is a chain break. It ends the polymerization process, since a random attack by any particle containing an unpaired electron (OR, H, etc.) produces a terminal a-bond and stops chain growth, which cannot be infinite:

The chain breaking stage is a random process that can take place at any value P- from units to tens, hundreds and many thousands. Therefore, the resulting polymer is not uniform along the length of the molecule, but is polydisperse. The polymer is characterized by an average degree of polymerization M cf.

anionic polymerization. The strongest bases - alkali metal amides, alcoholates, organomagnesium compounds, organolithium compounds, etc. act as initiators of the polymerization of alkenes, conjugated dienes.

Further, at the stage of chain growth, the anionic charge polarizes the n-bond of the styrene molecule in such a way that the positive end of the dipole attaches the initiated monomer. In this case, a carbanion appears at the end of the newly attached molecule:

The polymer chain growth step eventually leads to chain termination (this can be done by NH 3, water) and high polymer polystyrene arises:

The kinetic equation of the polymerization reaction has a fractional order:

The second order in styrene means that the chain growth step is bimolecular. This can be seen from the above chemical reaction. The chain is terminated by the solvent - liquid NH 3. Anionic polymerization is widely used in the polymerization of acrylic acid derivatives (acrylonitrile CH 2 =CH-C=N, esters - acrylates, methyl acrylates, styrene, caprolactam).

cationic polymerization. Strong acids and superacids (complex acids HBF 4 , HO-BFj" , HSbF 6 , etc.) can be used as polymerization initiators.

In this way, polyisobutylene is obtained in technology:

The reaction is carried out with BF 3 as initiator. The addition of H 2 0 or alcohols is necessary. The stage of initiation (nucleation of the chain) is associated with the addition of a proton:

Chain growth (second stage) occurs due to the polarization of the n-bond of the new isobutylene molecule, i.e., the carbocation acts as an electrophilic reagent. An electrophilic addition of a carbocation to the n-bond occurs:

up to CH 3 -C (CH 3) 2 - (CH 2 C) No. -CH 2 C (CH 3),

The chain termination step most often occurs when it encounters some proton acceptor (F" or another anion):

Activation mechanisms and transition states of polymerization reactions have been little studied.

coordination polymerization. In the early 1950s, K. V. Ziegler and D. Natta proposed new type polymerization initiators, which they called coordination polymerization. It is understood that the polymerization is initiated by a complex compound, which is attached to the monomer as a whole. One of the catalysts whose authors were awarded Nobel Prize, consists of a mixture of triethylaluminum and TiCl 4 . As a result of the interaction of these components, the cationic form TiClj is formed

which causes the process of cationic polymerization:

Polycondensation reactions, like polymerizations, lead to high polymers. Polycondensation- this is the interaction of two unsaturated or saturated molecules, accompanied by an intermolecular displacement of a hydrogen atom or the elimination of stable molecules (H 2 0, NH 3, CH3OH, etc.) and a successive increase in the length of the polymer molecule. An example of a condensation reaction that does not lead to high polymers is the reaction of aldol and crotonic condensation of aldehydes in an alkaline medium.

The mechanism of aldol condensation repeats the mechanism of nucleophilic addition at the carbonyl group. It is assumed that the ionized form of vinyl ether, the vinylate anion, which is formed as a result of enolization of the aldehyde and its subsequent acid dissociation, acts as a nucleophile:

Negative charge in the vinylate anion as a result of n,n conjugation

Crotonic condensation has the same mechanism, but is accompanied by the elimination of water when an alkaline solution of aldehyde is heated.

Similarly, aldol-crotonic condensation occurs in ketones, in mixtures of aldehydes and ketones.

Mechanisms of oxidation-reduction reactions of organic compounds. This type of reaction remains one of the least studied among other types of reactions, including in inorganic compounds. The situation is most often associated with a very high complexity of redox reactions. When they occur, not only the intra- and intermolecular transfer of a single electron often takes place, but also the transfer of a proton and other atomic-molecular particles. From the reduction reaction of the permanganate ion in an acidic medium

It can be seen that this is a multi-stage reaction. The reduction of Mn0 4 passes through several intermediate oxidation states (+6, +5, +4, +3), which correspond to strictly defined particles Mn0 4, Mn0 4 ~, Mn0 2+, Mn 4+, Mn 3+, two of which Mn0 4 ~ and MlO 2 * have not yet been studied. Usually, anions (SG, VG, Cr 2 0 4 ", etc.) are oxidized by permanganate, which act as ligands with respect to the cationic forms of manganese, cations (Fe 2 +, Sn 2 Mn 2 Cr 2+, etc.) and organic molecules of the most diverse nature (alcohols, carbohydrates, thio derivatives, amines, etc.). Depending on the nature of the reducing agent, various coordination forms (complex compounds) arise between the permanganate ion and the reducing agent-ligand, which undergoes oxidation, in which chemical bonds-bridges are formed between the carrier of excess electrons (reducing agent) and the oxidizing agent, through which the transition of the electron to oxidant coordination center. Thus, in a strongly acidic medium at a high concentration of SG, which undergoes oxidation with the help of KMn0 4, acid chloride forms of the type C1-Mn0 3, C1 2 Mn0 2, MnC1 4, etc., arise, many of which are extremely unstable and therefore not studied . Some of them easily decay, for example, into C1 2 and Mn0 2 , C1 2 and MnC1 2 .

In the case of the oxidation of organic compounds, the problem is fundamentally more complicated. The concept of oxidation-reduction of organic compounds, more precisely the carbon atom, is not so obvious when compared with the oxidation of inorganic compounds.

In the so-called oxidation of the carbon atom in organic molecules, the complete transfer of electrons, as required by the definition of the redox process and equation (6.38), does not occur. In those cases where it is believed that the carbon atom is oxidized, there is only a slight change in the polarity of the C-X chemical bond.

There is also no quantitative change in the coordination environment of the carbon atom. This is evident from the reactions

The difference in the oxidizing ability of hydrated and dehydrated forms of carbon compounds with oxygen is not known for certain.

The introduction of an oxygen atom into the C-H bond changes its polarity from almost zero to 3.66 * 10 -30 C m for alcohols, 8.32-10 -30 C m for aldehydes and ketones.

A similar phenomenon occurs when C-H is replaced by C-F, C-C1, C-Br and C-1. In all these cases it is assumed that the saturated carbon atom is oxidized. Conversely, the removal of halogen or oxygen atoms from the coordination environment of a carbon atom is called reduction. The conditionality of such representations is quite obvious, since there is no explicit oxidation-reduction process, which is measured by the loss or acquisition of an integer number of electrons.

There are great difficulties in applying the concept of "oxidation state" to carbon. It applies only to ionic or highly polar compounds that can be mentally converted into ionic ones:

Such a procedure with a sulfuric acid molecule allows us to establish that the oxidation state of the S atom in it is +6. There is also a convention in this case, since the oxidation state of +1 is always assigned to the hydrogen atom, and -2 to the oxygen atom. The reduction of the S atom in H 2 S0 4 can occur electrochemically at the cathode, resulting in the formation of H 2 S0 3 , H 2 S0 2 and many other particles up to H 2 S, in which the oxidation state of the sulfur atom is -2. In this case, the reduction changes the coordination surrounding the sulfur atom from 4 to 2. The entire chain of redox transformations of the S atom can be carried out on the anode in the reverse order, starting with H 2 S. This is due to the reversibility of the oxidation-reduction reactions of many inorganic compounds. In contrast, the so-called redox transformations of carbon compounds on C-X bonds are irreversible.

As is known, in order to equalize complex oxidation-reduction reactions, the reaction is divided into two electrode processes - oxidative at the anode and reduction at the cathode, even if it is impossible to implement them in a galvanic cell. So, the oxidation reaction of Fe 2+ with permanganate in an acidic medium is as follows:

The number of electrons passing from the reducing agent to the oxidizing agent is determined by the particle charges on the left and right sides.

With the complete oxidation of methanol with manganese dioxide in an acidic medium, we have:

From this example, it can be seen that for the oxidation of CH 3 OH to CO 2, “removal” is required 6e~ y and not 7, as would be required based on the zero oxidation state of the carbon atom in CH 3 OH.

For the oxidation reaction of H 2 C \u003d 0 with manganese dioxide, we can write:

Whence it follows that "removal" is required for the oxidation of formaldehyde 4e~. It can be seen that two electrons are donated by hydrogen H 2 CO, and two by the carbon of the carbonyl group. Consequently, in CH 3 OH, carbon is oxidized once, and in H 2 CO, i.e., in aldehydes and ketones, twice.

In carboxylic acids R-COOH, the carbon of the carboxyl group is oxidized three times. The oxidation state of the carbon atom rises from +1 in alcohols to +4 in CO 2 .

It is of interest to determine the formal oxidation state of carbon in alkenes, alkynes, and arenes. The use of their complete oxidation to CO 2 and H 2 0 using Mn0 2 shows that, regardless of the hybrid state of the carbon atom and the type of chemical bond (a, P) the degree of oxidation of the C atom in hydrocarbons is zero (oxidation of the C atom requires the removal of 40" of the H atom is also zero.

Application of the above redox approach to the study of haloalkyls shows that in CH 3 G, CH 2 G 2 , CIS 3 and SG 4 the carbon atom is also oxidized and the oxidation state varies from +1 to +4. At the same time, carrying out electrode reactions for the oxidation of nitrogen-, sulfur- and phosphorus-containing derivatives of hydrocarbons does not make it possible to establish the degree of oxidation of these atoms. In thioalcohols and thioethers, for both C and S atoms, it is equal to zero.

At present, it can be stated that oxidation in a series of organic compounds is a hidden process of moving electron density from a carbon atom to an atom of oxygen or halogens. Recovery in this series means the process of transformation C-O connections, С=0 and С-Г in С-Н-bond with reverse movement of electron density to the carbon atom. Consider the mechanism of the main oxidation reactions.

Of the typical oxidation reactions in a number of organic compounds, the most interesting are:

oxidative dihydroxylation of alkenes

oxidative cleavage of n-bonds in alkenes and arenes by ozone

oxidation of alkyl groups in arenes to carboxyl

and different kinds catalytic oxidation.

Oxidative dihydroxylation of alkenes. This method is used to synthesize glycols from alkenes. As oxidizing agents, an alkaline solution of KMn0 4 or performic acid HCOOOH is used (a mixture of anhydrous HCOOH and 30% H 2 0 2 is taken instead).

With permanganate, the reaction results in i/ms addition of both hydroxyl groups if a cyclic alkene is taken. It is assumed that

MlO *, recovering to MnO * ", forms an unstable intermediate as an intermediate compound - an ester of glycol and permanganous acid:

Nothing is known about the electronic mechanism of this reaction.

The transfer of an oxygen atom to the n-bond is more clearly observed in the oxidation of alkenes with performic acid. It is assumed that epoxy compounds (alkene oxides) are initially formed, which then react with H 2 0 to cleave the so-called oxirane-

th cycle *G.O:

Oxidative breakdown by ozone. The reaction with ozone proceeds with the splitting of both components (l and a) of the C=C double bond. Ozone, which has a very high redox potential (E\u003d +2.07 V) and low stability, first joins at the n-bond, and then also breaks the st-component of the double bond, forming a five-membered heterocycle containing three oxygen atoms in the cycle, including the peroxide chain:

Ozonides are oily liquids that explode easily. They are not isolated, but subjected to the hydrolytic action of water. The ozonolysis reaction is used to locate the double bond in the carbon chain. This is easy to do once the ketones and aldehydes formed during ozonolysis have been identified and quantified. This method of oxidative cleavage of alkenes is good because, unlike other oxidizing agents, it does not produce by-products.

Oxidation of alkyl groups in arenes. The oxidation of alkylbenzenes is the main method for the synthesis of aromatic carboxylic acids:

As oxidizing agents, a hot alkaline solution of KMn0 4, a chromium mixture (K 2 SIO 4 + H 2 S0 4) diluted with HN0 3 are used. As is known, alkanes and benzene are inert with respect to these oxidizing agents. However, due to the activation of the C-H bond in the a-position to the benzene ring, the bond becomes active in the oxidation reaction.

The increased polarity of this bond allows the formation of a hydrogen bridge with the oxidizing agent, which is necessary as a conductor, along which an electron is removed from the bonding h? orbital and to the central atom of the oxidizing agent:

After the removal of an electron, a proton leaves the alkylbenzene molecule. Further, we can assume the usual route of the reaction: the free radical of alkylbenzene interacts with atomic oxygen, which is released during the decomposition of MnO2 in an alkaline solution:

Oxidative processes involving organic compounds are a source of energy for plants and animals: their detailed study is vital.

Oxidation of organic compounds at the cellular level occurs only with the participation of enzymes. Due to the high complexity of enzymatic processes, their mechanism is not well understood. It cannot be studied until the details of the mechanism, including stoichiometric and activation, are known for simple organic oxidation reactions, including those mentioned above.

Reactions of hydrogenation and dehydrogenation. Despite the fact that these

reactions are redox reactions, not all of them are. Thus, the hydrogenation of unsaturated compounds with any type of n-bonds is not an oxidation-reduction, since the degree of oxidation of carbon and hydrogen atoms in all hydrocarbons is zero, i.e., the same. The degree of oxidation of hydrogen atoms H 2 or active atomic hydrogen H also does not change during hydrogenation.

The reactions listed above can be attributed to the reactions of saturation of the chemical affinity of the carbon atom in unsaturated hydrocarbons.

Redox reactions, as follows from the previous section of this chapter, include the reactions of hydrogenation of alcohols, aldehydes (ketones), carboxylic acids, and dehydrogenation of alcohols.

Despite the difference between these two types of hydrogenation reactions, their mechanisms have much in common. Both types of hydrogenation reactions are chemical affinity saturation reactions; the reactant in them is active hydrogen H, which either arises in the chemical reaction of decomposition of amalgams, hydrides, or the reduction of H* in acid solutions, or is transferred from the coordination sphere of complexes (platinum metal hydrides) and active centers of heterogeneous catalysts (Raney nickel, platinum black , palladium, etc.).

In K. Ingold's unique monograph "Theoretical Foundations of Organic Chemistry", the mechanisms of this type of reactions, as well as oxidation reactions, are not considered, i.e., by the end of the 60s, these reactions had not yet been studied.

Hydrogenation of a single TC bond in alkenes is an exothermic reaction. During hydrogenation, an average of about 125 kJ/mol is released. The reactions of alkynes are of considerable interest for revealing the mechanism of n-bond hydrogenation. These latter can be hydrogenated stepwise to alkenes. In this case, various sources of hydrogen give either cis- or trinsapken:

Reactions leading to the formation of only one of the possible isomers are called stereoselective. The formation of r/1/s-butene-2 ​​during hydrogenation over catalysts (specially prepared Pd or over nickel boride) means that the hydrogenation reaction proceeds trimolecularly, when a linear alkyne molecule “finds” a pair of active hydrogen atoms on the catalyst surface.

Hydrogenation of benzene, alkylbenzenes and more complex arenes occurs on finely divided nickel (Raney nickel) or on Pd-, Pt-catalysts. The process of hydrogenation of benzene (arenes) and its mechanism differs from the hydrogenation of alkynes and alkenes in greater complexity. According to the structural theory of catalysis by A. A. Balandin, who developed the concept of multiplets as active centers with high chemical affinity unsaturation and grouped into certain structural ensembles (multiplets), the benzene molecule “searches” on the catalyst surface for a multiplet corresponding to its structure ( Fig. 6.8). Activation of the benzene molecule, which is hydrogenated stepwise

consists, in all likelihood, in the destruction of the electronic structure of aromatic benzene with equivalent C-“C-bonds of one and a half multiplicity and the transformation of the molecule into a state close to 1,3,5-cyclohexatriene. This requires 150 kJ/mol. Activation of this kind can occur on a multiplet consisting of three strongly n-acceptor centers, which are capable of polarizing the symmetric n-cloud of the initial molecule so that the n-electron density is concentrated on 1,3,5 C-C bonds.

The H 2 molecule also requires activation associated with a strong stretching of the H-H bond. A complete break of this bond absorbs 435 kJ/mol. it

Rice. 6.8. One of the multiplets on the surface of the catalyst in the hydrogenation of benzene: (x) - active sites of the catalyst

very high energy consumption. Therefore, stretching connections H-H may not be accompanied by its complete rupture. The most suitable for the hydrogenation reaction of C 6 H 6 will be the four-center transition state shown in Fig. 6.8. It will probably be the lowest energetically. The hydrogenation of benzene to cyclohexadiene (first stage) is energetically unfavorable and requires an expenditure of 20 kJ/mol, i.e., the reaction is endothermic. Hydrogenation of the second and third n-bonds of cyclohexadiene is energetically favorable and is accompanied by the release of (108 + 114) kJ/mol.

The dehydrogenation reactions of cyclohexane and its derivatives occur on the same catalysts that are used in their synthesis from arenes during hydrogenation. The transition states of the reactions of formation of the first, second, and third n-bonds repeat those that are realized during hydrogenation. This is due to the reversibility of catalytic hydrogenation reactions:

Balandin's theory of multiplets is also applied here.

Of great interest are the reactions of dehydrogenation of alkanes to alkenes and conjugated denenes, which occur on oxide catalysts (A1 2 0 3 , Cr 2 0 3 mixed with other oxides). The action of an oxide catalyst is reduced to the abstraction of a hydrogen atom by the active site of the catalyst:

Thus, butadiene-1,3 is formed from butylene and //-butane, and isoprene (2-methylbutadiene-1,3) is formed from isopentane:

Conjugated dienes are widely used in the production of rubbers and other valuable substances.

CH 3 -CH 3 + Cl 2 - (hv) ---- CH 3 -CH 2 Cl + HCl

C 6 H 5 CH 3 + Cl 2 --- 500 C --- C 6 H 5 CH 2 Cl + HCl

Addition reactions

Such reactions are characteristic of organic compounds containing multiple (double or triple) bonds. Reactions of this type include addition reactions of halogens, hydrogen halides and water to alkenes and alkynes

CH 3 -CH \u003d CH 2 + HCl ---- CH 3 -CH (Cl) -CH 3

Cleavage (elimination) reactions

These are reactions that lead to the formation of multiple bonds. When splitting off hydrogen halides and water, a certain selectivity of the reaction is observed, described by the Zaitsev rule, according to which a hydrogen atom is split off from the carbon atom at which there are fewer hydrogen atoms. Reaction Example

CH3-CH(Cl)-CH 2 -CH 3 + KOH →CH 3 -CH=CH-CH 3 + HCl

Polymerization and polycondensation

n(CH 2 \u003d CHCl)  (-CH 2 -CHCl) n

redox

The most intense of the oxidative reactions is combustion, a reaction characteristic of all classes of organic compounds. In this case, depending on the combustion conditions, carbon is oxidized to C (soot), CO or CO 2, and hydrogen is converted into water. However, of great interest to organic chemists are oxidation reactions carried out under much milder conditions than combustion. Used oxidizing agents: solutions of Br2 in water or Cl2 in CCl 4 ; KMnO 4 in water or dilute acid; copper oxide; freshly precipitated hydroxides of silver (I) or copper (II).

3C 2 H 2 + 8KMnO 4 + 4H 2 O→3HOOC-COOH + 8MnO 2 + 8KOH

Esterification (and its reverse hydrolysis reaction)

R 1 COOH + HOR 2 H+  R 1 COOR 2 + H 2 O

Cycloaddition

YR Y-R

‖ + ‖ → ǀ ǀ

R Y R Y

‖ + →

11. Classification of organic reactions by mechanism. Examples.

The reaction mechanism involves a detailed step-by-step description of chemical reactions. At the same time, it is established which covalent bonds are broken, in what order and in what way. Equally carefully describe the formation of new bonds in the course of the reaction. Considering the reaction mechanism, first of all, attention is paid to the method of breaking the covalent bond in the reacting molecule. There are two such ways - homolytic and heterolytic.

Radical reactions proceed by homolytic (radical) breaking of the covalent bond:

Non-polar or low-polar covalent bonds (C–C, N–N, C–H) undergo radical rupture at high temperature or under the action of light. The carbon in the CH 3 radical has 7 outer electrons (instead of the stable octet shell in CH 4). Radicals are unstable, they tend to capture the missing electron (up to a pair or up to an octet). One of the ways to form stable products is dimerization (combination of two radicals):

CH 3 + CH 3 CH 3 : CH 3,

H + H H : N.

Radical reactions - these are, for example, the reactions of chlorination, bromination and nitration of alkanes:

Ionic reactions occur with heterolytic bond cleavage. In this case, short-lived organic ions are intermediately formed - carbocations and carbanions - with a charge on the carbon atom. In ionic reactions, the binding electron pair does not separate, but passes entirely to one of the atoms, turning it into an anion:

Strongly polar (H–O, C–O) and easily polarizable (C–Br, C–I) bonds are prone to heterolytic cleavage.

Distinguish nucleophilic reactions (nucleophile- looking for the nucleus, a place with a lack of electrons) and electrophilic reactions (electrophile looking for electrons). The statement that this or that reaction is nucleophilic or electrophilic, conditionally always refers to the reagent. Reagent- a substance participating in the reaction with a simpler structure. substrate is the starting material with a more complex structure. Leaving group is a displaceable ion that has been bonded to carbon. reaction product- new carbon-containing substance (written on the right side of the reaction equation).

To nucleophilic reagents(nucleophiles) include negatively charged ions, compounds with lone pairs of electrons, compounds with double carbon-carbon bonds. To electrophilic reagents(electrophiles) include positively charged ions, compounds with unfilled electron shells (AlCl 3, BF 3, FeCl 3), compounds with carbonyl groups, halogens. An electrophile is any atom, molecule, or ion that can accept a pair of electrons in the process of forming a new bond. The driving force of ionic reactions is the interaction of oppositely charged ions or fragments of different molecules with a partial charge (+ and -).

Examples of ionic reactions of various types.

Nucleophilic substitution :

Electrophilic substitution :

Nucleophilic addition (first CN - joins, then H +):

electrophilic addition (first H + joins, then X -):

Elimination under the action of nucleophiles (bases) :

Elimination on action electrophiles (acids) :

Reactions organic matter can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization).

Obviously, the whole variety of reactions of organic compounds cannot be reduced to the proposed classification (for example, combustion reactions). However, such a classification will help to establish analogies with the reactions already familiar to you that occur between inorganic substances.

As a rule, the main organic compound involved in the reaction is called substrate, and the other component of the reaction is conditionally considered as reagent.

Substitution reactions

Substitution reactions- these are reactions that result in the replacement of one atom or group of atoms in the original molecule (substrate) with other atoms or groups of atoms.

Substitution reactions involve saturated and aromatic compounds such as alkanes, cycloalkanes or arenes. Let us give examples of such reactions.

Under the action of light, hydrogen atoms in a methane molecule can be replaced by halogen atoms, for example, by chlorine atoms:

Another example of replacing hydrogen with halogen is the conversion of benzene to bromobenzene:

The equation for this reaction can be written differently:

With this form of writing reagents, catalyst, reaction conditions write above the arrow, and inorganic reaction products- under it.

Addition reactions

Addition reactions are reactions in which two or more molecules of reactants combine into one.

Unsaturated compounds, such as alkenes or alkynes, enter into addition reactions. Depending on which molecule acts as a reagent, hydrogenation (or reduction), halogenation, hydrohalogenation, hydration, and other addition reactions are distinguished. Each of them requires certain conditions.

1. hydrogenation- the reaction of adding a hydrogen molecule to a multiple bond:

2. Hydrohalogenation- hydrogen halide addition reaction (hydrochlorination):

3. Halogenation- halogen addition reaction:

4. Polymerization- a special type of addition reactions, during which molecules of a substance with a small molecular weight are combined with each other to form molecules of a substance with a very high molecular weight - macromolecules.

polymerization reactions- these are the processes of combining many molecules of a low molecular weight substance (monomer) into large molecules (macromolecules) of a polymer.

An example of a polymerization reaction is the production of polyethylene from ethylene (ethene) under the action of ultraviolet radiation and a radical polymerization initiator R .

The covalent bond most characteristic of organic compounds is formed when atomic orbitals overlap and the formation of common electron pairs. As a result of this, an orbital common to two atoms is formed, on which a common electron pair is located. When the bond is broken, the fate of these common electrons can be different.

Types of reactive particles in organic chemistry

An orbital with an unpaired electron belonging to one atom can overlap with an orbital of another atom that also contains an unpaired electron. This is where education takes place covalent bond by exchange mechanism:

The exchange mechanism for the formation of a covalent bond is realized if a common electron pair is formed from unpaired electrons belonging to different atoms.

The process opposite to the formation of a covalent bond by the exchange mechanism is disconnection at which one electron goes to each atom. As a result, two uncharged particles with unpaired electrons are formed:

Such particles are called free radicals.

free radicals- atoms or groups of atoms having unpaired electrons.

Free radical reactions are reactions that occur under the action and with the participation of free radicals.

In the course of inorganic chemistry, these are reactions of interaction of hydrogen with oxygen, halogens, combustion reactions. Reactions of this type are characterized by high speed, release of a large amount of heat.

A covalent bond can also form donor-acceptor mechanism. One of the orbitals of an atom (or anion), which contains an unshared electron pair, overlaps with an unfilled orbital of another atom (or cation), which has an unfilled orbital, while forming covalent bond, for example:

Breaking a covalent bond leads to the formation of positively and negatively charged particles; since in this case both electrons from a common electron pair remain with one of the atoms, the other atom has an unfilled orbital:

Consider electrolytic dissociation acids:

One can easily guess that a particle having lone electron pair R: -, i.e., a negatively charged ion, will be attracted to positively charged atoms or to atoms on which there is at least a partial or effective positive charge. Particles with lone electron pairs are called nucleophilic agents(nucleus - "nucleus", the positively charged part of the atom), that is, the "friends" of the nucleus, a positive charge.

Nucleophiles(Nu) - anions or molecules that have a lone pair of electrons that interact with parts of the molecules on which an effective positive charge is concentrated.

Examples of nucleophiles: Cl - (chloride ion), OH - (hydroxide anion), CH 3 O - (methoxide anion), CH 3 COO - (acetate anion).

Particles that have unfilled orbital, on the contrary, will tend to fill it and, therefore, will be attracted to the regions of the molecules where there is an increased electron density, a negative charge, an unshared electron pair. They are electrophiles, "friends" of the electron, negative charge or particles with increased electron density.

electrophiles- cations or molecules that have an unfilled electron orbital, tending to fill it with electrons, as this leads to a more favorable electronic configuration of the atom.

Not every particle is an electrophile with an empty orbital. So, for example, alkali metal cations have the configuration of inert gases and do not tend to acquire electrons, since they have a low electron affinity. From this we can conclude that despite the presence of an unfilled orbital, such particles will not be electrophiles.

Main reaction mechanisms

There are three main types of reacting particles - free radicals, electrophiles, nucleophiles- and three corresponding types of reaction mechanism:

Free radical;

Electrophilic;

Nuleophilic.

In addition to classifying reactions according to the type of reacting particles, in organic chemistry there are four kinds of reactions according to the principle of changing the composition of molecules: accession, substitution, splitting off, or elimination (from the English to eliminate - remove, split off) and rearrangements. Since addition and substitution can occur under the action of all three types of reactive species, several main reaction mechanisms can be distinguished.

1. Free radical substitution:

2. Free radical addition:

3. Electrophilic substitution:

4. Electrophilic addition:

5. Nucleophilic addition:

In addition, consider the cleavage or elimination reactions that take place under the influence of nucleophilic particles - bases.

6. Elimination:

Rule of V. V. Markovnikov

A distinctive feature of alkenes (unsaturated hydrocarbons) is the ability to enter into addition reactions. Most of these reactions proceed by the mechanism of electrophilic addition.

Hydrohalogenation (addition of hydrogen halide):

This reaction obeys the rule of V. V. Markovnikov.

When a hydrogen halide is added to an alkene, hydrogen is attached to a more hydrogenated carbon atom, i.e., an atom at which there are more hydrogen atoms, and a halogen to a less hydrogenated one.

Reference material for passing the test:

periodic table

Solubility table