Reactions of electrophilic substitution. Electrophilic substitution Mechanism of electrophilic substitution in arenes

Electrophilic substitution reactions(English) substitution electrophilic reaction ) - substitution reactions, in which the attack is carried out electrophile- a particle that is positively charged or has a deficit of electrons. When a new bond is formed, the outgoing particle - electrofuge split off without its electron pair. The most popular leaving group is the proton H+.

General view of electrophilic substitution reactions:

(cationic electrophile)

(neutral electrophile)

There are reactions of aromatic (widespread) and aliphatic (not common) electrophilic substitution. The specificity of electrophilic substitution reactions specifically for aromatic systems is explained by the high electron density of the aromatic ring, which is capable of attracting positively charged particles.

Aromatic electrophilic substitution reactions play an extremely important role in organic synthesis and are widely used both in laboratory practice and in industry.

Aromatic electrophilic substitution reactions

For aromatic systems, there is actually one mechanism of electrophilic substitution - S E Ar. Mechanism S E 1(by analogy with the mechanism S N 1) is extremely rare, and S E 2(corresponding by analogy S N 2) does not occur at all.

S E Ar reactions

reaction mechanism S E Ar or aromatic electrophilic substitution reactions(English) Electrophilic aromatic substitution ) is the most common and most important of the aromatic substitution reactions and consists of two steps. At the first stage, the electrophile is attached, at the second stage, the electrofuge is split off:

During the reaction, an intermediate positively charged intermediate is formed (in the figure - 2b). It bears the name Weland intermediate, aronium ion or σ-complex. This complex, as a rule, is very reactive and is easily stabilized by rapidly eliminating the cation.

The rate-limiting step in the vast majority of reactions S E Ar is the first stage.

Speed ​​reaction S E Ar is usually presented in the following form:

Relatively weak electrophiles usually act as an attacking particle, so in most cases the reaction S E Ar proceeds under the action of a Lewis acid catalyst. More often than others, AlCl 3, FeCl 3, FeBr 3, ZnCl 2 are used.

In this case, the reaction mechanism is as follows (using the example of benzene chlorination, FeCl 3 catalyst):

1. At the first stage, the catalyst interacts with the attacking particle to form an active electrophilic agent:

2. At the second stage, in fact, the mechanism is implemented S E Ar:

Typical aromatic electrophilic substitution reactions

In substituted benzenes, the so-called ipso-attack, that is, the replacement of an existing substitute with another:

Aliphatic electrophilic substitution reactions

Reactions S E 1

reaction mechanism S E 1 or monomolecular electrophilic substitution reactions(English) substitution electrophilic unimolecular ) is similar to the mechanism S N 1 includes the following stages:

1. Ionization of the substrate with the formation of a carbanion (slow stage):

2. Electrophilic attack of the carbanion (fast stage):

Most often an outgoing particle in extremely rare reactions S E 1 is a proton.

Reactions S E 2

reaction mechanism S E 2 or bimolecular electrophilic substitution reactions(English) electrophilic bimolecular substitution ) is similar to the mechanism S N 2, occurs in one stage, without intermediate formation of an intermediate:

The main difference from the nucleophilic mechanism is that the attack of the electrophile can be carried out both from the front and from the rear, which as a result can lead to a different stereochemical result: both racemization and inversion.

An example is the ketone-enol tautomerization reaction:

Ketone-enol tautomerization

Notes

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Arenes are characterized by three types of reactions:

1) electrophilic substitution S E Ar (destruction of the C-H bond);

2) addition reactions (destruction of the p-bond);

3) reactions with the destruction of the benzene ring.

Electrophilic substitution in arenes (S E Ar)

Electrophilic substitution reactions proceed according to the general scheme through the formation of π- and σ-complexes

As follows from the presented scheme, the aromatic substitution S E Ar proceeds by the addition-elimination mechanism. Behind the addition of an electrophilic agent X + an aromatic substrate with the formation of a σ-complex is followed by the elimination of a proton with the formation of a reaction product.

Electrophilic substitution reactions in arenes generally follow a second-order kinetic equation ( v = k2[X+]).

Let's consider the stepwise flow of the process.

Stage 1 Formation of π-complexes

.

π–Complexes – coordination compounds in which the electron donor is an aromatic compound having easily polarizable π-electrons. π-complexes – non-classical chemical compounds in which an electrophilic particle is associated with a covalent bond with some covalent atom of the reactant. Most π-complexes easily decompose when heated or when exposed to water.

The ability to form π-complexes in arenes increases in the series:

C 6 H 6< C 6 Н 5 СН 3 < п - СН 3 –С 6 Н 4 –СН 3 ~ п - СН 3 –О–С 6 Н 4 СН 3 <

<м - СН 3 –С 6 Н 4 -СН 3 < 1,3,5 (СН 3) 3 С 6 Н 3

The greater the π-electron density of a compound, the more easily it forms π-complexes.

Stage 2 Formation of σ-complexes

σ-Complexes are cations, in the formation of which the reagent X + forms a covalent bond with one of the carbon atoms due to 2 π-electrons of the benzene nucleus, while this C-atom passes from sp 2-states in sp 3-hybridization, in which all four of its valences are at an angle of ~109 0 . The symmetry of the benzene nucleus is broken. Group X and the hydrogen atom are in a plane perpendicular to the plane of the benzene nucleus.

The stability of σ-complexes increases with an increase in the basicity of the benzene ring

This step is the slowest step in the entire reaction and is called limiting.

Stage 3 Detachment of a proton from a σ-complex

In the last stage, the proton is split off from the σ-complex and the 6π-electron cloud (aromatic structure) is restored. This process proceeds with an energy gain of ~42 kJ/mol. In many reactions, the removal of a proton at the final stage is facilitated by the corresponding base present in the solution.

According to the considered mechanism, the following reactions proceed in arenes.

However, the proposed scheme should not be considered as absolutely proven and universal. In various processes, the course of the reaction is influenced by:

Ø substrate structure;

Ø chemical activity of the reagent;

Ø conditions for the process;

Ø the nature, activity of the catalyst and other factors, which may lead to deviation in particular cases from the proposed process scheme.

Consider some examples of electrophilic substitution in benzene.

Example 1 Bromination of benzene

Molecular bromine is too weak an electrophilic agent and, in the absence of a catalyst, does not react with benzene.

Most often, the benzene bromination reaction is carried out in the presence of iron (III) bromide, which plays the role of a Lewis acid, the latter is obtained in the reaction mass by direct interaction of bromine with iron

Stage 1 Formation of electrophilic reagent E + .

The bromine molecule is activated according to the scheme of an acid-base reaction with a Lewis acid.

Stage 2 Formation of π-complex 1.

Free bromonium ion or ion in the composition of an ion pair is an active electrophilic agent capable of reacting with benzene; in this case, the π-complex 1

The role of the electrophilic agent at this stage can also be performed by the donor-acceptor complex .

Stage 3 Rearrangement of π-complex 1 and formation of σ-complex, or arenonium ion.

This is the slowest step in the entire reaction.

Stage 4 Rearrangement of the σ-complex into the π-complex 2 of the substitution product. The proton is split off from the carbon atom, which is being replaced; in the cycle, an aromatic sextet of electrons is formed again - rearomatization is observed

Stage 5 Dissociation of the π-complex 2 with the formation of a substitution product

The mechanism of electrophilic bromination of benzene is illustrated by the energy diagram of the reaction shown in Fig.11.

Rice. 11. Energy diagram of the reaction

electrophilic bromination of benzene;

PS - transition state.

Stages 2 and 5, which include π-complexes of the starting arene and the substitution product, are often omitted in schemes of the mechanism of electrophilic aromatic substitution. With this approach, the proper electrophilic aromatic substitution includes only three stages.

Stage 1" - the formation of an electrophilic agent.

Stage 2" – formation of the σ-complex, bypassing the π-complex 1.

Stage 3" is the decay of the σ-complex with the formation of a substitution product, bypassing the π-Complex 2.

Example 2 Nitration of Arenes

Nitration consists in replacing the hydrogen atom of the benzene ring with the nitro group NO 2. Benzene reacts with concentrated nitric acid slowly even when heated. Therefore, nitration is most often carried out by the action of a more active nitrating agent - nitrating mixture- mixtures of concentrated nitric and sulfuric acids. Nitration of arenes with a nitrating mixture is the main method for obtaining aromatic nitro compounds.

Nitration of benzene with a nitrating mixture is carried out at 45–50 0 C. Since the nitration reaction is irreversible, nitric acid is used in a minimal excess (5–10%), achieving almost complete conversion of benzene.

Sulfuric acid in the composition of the nitrating mixture is necessary to increase the concentration of the electrophilic agent - nitronium ion NO 2 +.

Stage 1 Formation of an electrophilic agent.

The active electrophilic agent in nitration is the nitronium ion, which is potentially found in a whole genus of compounds.

For example: HO _ NO 2 , O 2 N _ O _ NO 2 , etc.

Their propensity to form a nitronium ion increases with an increase in the electronegativity of the substituent associated with the nitro group.

The hydroxyl group as such cannot be split off, therefore, the nitronium ion from nitric acid is formed only in an acidic environment.

In the simplest case, nitric acid can protonate itself ("self-protonization")

However, the equilibrium is shifted to the left, so nitric acid nitrates weakly.

When concentrated sulfuric acid is added, the concentration of - cation increases greatly

The nitrating effect of a mixture of nitric and sulfuric acid (nitrating mixture) is much stronger than that of nitric acid alone. A further increase in reactivity can be achieved by using fuming nitric acid and oleum.

Stage 2 Formation of the σ-complex

Stage 3 Ejection of a proton with the formation of a substitution product

In practice, it is necessary to coordinate the activity of the nitrating agent with the reactivity of the aromatic nucleus.

Thus, for example, phenols and ethers of phenols are nitrated with already dilute nitric acid, while the nitration of benzaldehyde, benzoic acid, nitrobenzene, etc. requires a mixture of fuming nitric acid with sulfuric acid.

m-Dinitrobenzene is hardly nitrated even with a mixture of fuming nitric and sulfuric acids (5 days, 110 0 C; 45% yield).

In nitration, the most common side reaction is oxidation. It is favored by an increase in the reaction temperature. The oxidation process is determined by the release of nitrogen oxides. Aldehydes, alkylaryl-ketones and, to a lesser extent, alkylbenzenes are also subject to oxidation during nitration.

Example 3 Alkylation of Arenes

R-HIg, ROH, R-CH=CH 2 can be used as alkylating agents in the presence of appropriate catalysts (eg AICI 3 , AIBr 3 , H 2 SO 4 ).

Catalysts generate (form) an electrophilic particle - carbocation

Alkylation reactions have three major limitations:

1) the reaction is difficult to stop at the stage of monoalkylation, i.e. it proceeds further, with the formation of polyalkylbenzenes; an excess of arene is usually used to suppress polyalkylation;

2) if there are only electroacceptor substituents in the arena (for example, -NO 2), then the alkylation reaction cannot be carried out;

3) the alkylation reaction is accompanied by a rearrangement of the alkyl radical.

The rearrangement of an alkyl radical into the most stable one is a characteristic property of carbocations

Orientation rules

Hydrogen substitution reactions in benzene proceed in the same way at any carbon atom, since the benzene molecule is symmetrical. However, if benzene already has a substituent, then the positions remaining free for electrophilic substitution reactions become unequal.

The patterns that determine the directions of substitution reactions in the benzene nucleus are called orientation rules.

–activating group- a substituent that makes the benzene ring more reactive in electrophilic substitution reactions compared to unsubstituted benzene.

–Deactivating group- a substituent that makes the benzene ring less reactive in electrophilic substitution reactions compared to unsubstituted benzene.

- o-, p-orientant- a substituent that directs the attack of the electrophile mainly to the o- or p-position of the benzene ring.

– m-orientator is a substituent that directs the attack of the electrophile mainly to the m-position of the benzene ring.

In general, electrophilic substitution in monosubstituted benzene can proceed in three directions

The reactivity of carbon atoms in this case is determined by three factors:

1) the nature of the existing substituent;

2) the nature of the acting agent;

3) reaction conditions.

According to their influence on the orientation in these reactions, all substituents are divided into two groups: substituents of the first kind (ortho-, para-orienting agents) and substituents of the second kind (meta-orienting agents).

Electrophilic substitution is undoubtedly the most important group of reactions for aromatic compounds. There is hardly any other class of reactions that has been studied in such detail, in depth and comprehensively, both from the point of view of the mechanism and from the point of view of application in organic synthesis. It was in the field of electrophilic aromatic substitution that the problem of the relationship between structure and reactivity was first posed, which is the main subject of study in physical organic chemistry. In general, this type of reactions of aromatic compounds can be represented as follows:

1. Literature review

1.1 Electrophilic substitution in the aromatic series

These reactions are characteristic not only for benzene itself, but also for the benzene ring in general, wherever it is located, as well as for other aromatic cycles - benzenoid and non-benzenoid. Electrophilic substitution reactions cover a wide range of reactions: nitration, halogenation, sulfonation and Friedel-Crafts reactions are characteristic of almost all aromatic compounds; reactions such as nitrosation and azo coupling are inherent only in systems with increased activity; reactions such as desulfurization, isotopic exchange, and numerous cyclization reactions, which at first glance seem quite different, but which also prove to be appropriate to refer to reactions of the same type.

Electrophilic agents E + , although the presence of a charge is not necessary, because an electrophile can also be an uncharged electron-deficient particle (for example, SO 3 , Hg(OCOCH 3) 2, etc.). Conventionally, they can be divided into three groups: strong, medium strength and weak.

NO 2 + (nitronium ion, nitroyl cation); complexes of Cl 2 or Br 2 with various Lewis acids (FeCl 3 , AlBr 3 , AlCl 3 , SbCl 5 etc.); H 2 OCl + , H 2 OBr + , RSO 2 + , HSO 3 + , H 2 S 2 O 7 . Strong electric saws interact with compounds of the benzene series containing both electron-donating and practically any electron-withdrawing substituents.

Medium strength electrophiles

Complexes of alkyl halides or acyl halides with Lewis acids (RCl . AlCl 3 , RBr . GaBr 3 , RCOCl . AlCl 3 etc.); complexes of alcohols with strong Lewis and Bronsted acids (ROH . BF 3 , ROH . H 3 PO 4 , ROH . HF). They react with benzene and its derivatives containing electron-donating (activating) substituents or halogen atoms (weak deactivating substituents), but usually do not react with benzene derivatives containing strong deactivating electron-withdrawing substituents (NO 2, SO 3 H, COR, CN, etc.) .

Weak electrophiles

Diazonium cations ArN +є N, iminium CH 2 \u003d N + H 2, nitrosonium NO + (nitrosoyl cation); carbon monoxide (IY) CO 2 (one of the weakest electrophiles). weak electrophiles interact only with benzene derivatives containing very strong electron-donating substituents (+M)-type (OH, OR, NH 2, NR 2 , O-, etc.).

1.1.2 Mechanism of electrophilic aromatic substitution

At present, aromatic electrophilic substitution is considered as a two-stage addition-elimination reaction with the intermediate formation of an arenonium ion, called the σ-complex

I-Arenium ion (

The first formula is the most commonly used. The σ-complex will be much better stabilized by donor substituents in the ortho and para positions than by donor substituents in the meta position.

π -Complexes

As is known, arenes are π-bases and can form donor-acceptor complexes with many electrophilic reagents. formation of molecular complexes of composition 1:1 (G.Brown, 1952).

These complexes are not colored; their solutions in aromatic hydrocarbons are nonconductive. Dissolution of gaseous DCl in benzene, toluene, xylenes, mesitylene, and pentamethylbenzene does not result in the exchange of H for D. Since the solutions of the complexes do not conduct electric current, they are not ionic particles; These are not arenonium ions.

Such donor-acceptor complexes are called π-complexes. For example, crystals of benzene complexes with bromine or chlorine with a composition of 1:1, according to X-ray diffraction data, consist of chains of alternating molecules of a π-donor of composition (C 6 H 6) and an acceptor (Cl 2 ,Br 2), in which the halogen molecule is located perpendicular to the plane of the ring along axis passing through its center of symmetry.

σ-complexes (arenonium ions)

When HCl and DCl are introduced into a solution in alkylbenzenes AlCl 3 or AlBr 3, the solution begins to conduct an electric current. Such solutions are colored and their color changes from yellow to orange-red when passing from para-xylene to pentamethylbenzene. In the ArH-DCl-AlCl 3 or ArH-DF-BF 3 systems, the hydrogen atoms of the aromatic ring are already exchanged for deuterium. The electrical conductivity of the solutions definitely indicates the formation of ions in the ternary system arene-hydrogen halide-aluminum halide. The structure of such ions was determined using 1 H and 13 C NMR spectroscopy in the ArH-HF (liquid)-BF 3 or ArH-HF-SbF 5 system in SO 2 ClF at low temperature.

1.1.3 Substituent classification

Monosubstituted C 6 H 5 X benzenes may be more or less reactive than benzene itself. If an equivalent mixture of C 6 H 5 X and C 6 H 6 is introduced into the reaction, then the substitution will occur selectively: in the first case, C 6 H 5 X will predominantly react, and in the second case, mainly benzene.

Currently, substituents are divided into three groups, taking into account their activating or deactivating effect, as well as the orientation of the substitution in the benzene ring.

1. Activating ortho-para-orienting groups. These include: NH 2 , NHR, NR 2 , NHAc, OH, OR, OAc, Alk, etc.

2. Deactivating ortho-para-orienting groups. These are the halogens F, Cl, Br and I.

3. Deactivating meta-orienting groups. This group consists of NO 2 , NO, SO 3 H, SO 2 R, SOR, C(O)R, COOH, COOR, CN, NR 3+ , CCl 3 and others. These are orientants of the second kind.

Naturally, there are also groupings of atoms of an intermediate nature, which determine the mixed orientation. These, for example, include: CH 2 NO, CH 2 COCH 3, CH 2 F, CHCl 2, CH 2 NO 2, CH 2 CH 2 NO 2, CH 2 CH 2 NR 3 +, CH 2 PR 3 +, CH 2 SR 2 + id.

1.2 Electrophilic substitution in π-excess heterocycles

Furan, pyrrole and thiophene are highly reactive with common electrophilic reagents. In this sense, they resemble the most reactive benzene derivatives, such as phenols and anilines. The increased sensitivity to electrophilic substitution is due to the asymmetric charge distribution in these heterocycles, resulting in a greater negative charge on the carbon atoms of the ring than in benzene. Furan is somewhat more reactive than pyrrole, while thiophene is the least reactive.

1.2.1 Electrophilic substitution of pyrrole

While pyrrole and its derivatives are not prone to nucleophilic addition and substitution reactions, they are very sensitive to electrophilic reagents, and the reactions of pyrroles with such reagents proceed almost exclusively as substitution reactions. Unsubstituted pyrrole, N- and C-monoalkylpyrroles, and, to a lesser extent, C,C-dialkyl derivatives polymerize in strongly acidic media, so most of the electrophilic reagents used in the case of benzene derivatives are not applicable to pyrrole and its alkyl derivatives.

However, in the presence of electron-withdrawing groups in the pyrrole ring that prevent polymerization, for example, such as ester groups, it becomes possible to use strongly acidic media, nitrating and sulfonating agents.

protonation

In solution, reversible addition of a proton is observed at all positions of the pyrrole ring. The nitrogen atom is protonated most rapidly, the addition of a proton at position 2 is twice as fast as at position 3. In the gas phase, when using acids of moderate strength, such as C 4 H 9 + and NH 4 + , pyrrole is protonated exclusively at carbon atoms , and the propensity to attach a proton at position 2 is higher than at position 3. The most thermodynamically stable cation, the 2H-pyrrolium ion, is formed upon addition of a proton at position 2, and the pKa value determined for pyrrole is associated precisely with this cation. The weak N-basicity of pyrrole is due to the absence of mesomeric delocalization of the positive charge in the 1H-pyrrolium cation.

By chemical properties, arenas differ from saturated and unsaturated hydrocarbons. This is due to the structural features of the benzene ring. The delocalization of six p-electrons in the cyclic system lowers the energy of the molecule, which leads to an increased stability (aromaticity) of benzene and its homologues. Therefore, arenes are not prone to undergo addition or oxidation reactions that lead to loss of aromaticity. For them, the most characteristic reactions proceed with the preservation of the aromatic system, namely, the substitution reactions of hydrogen atoms associated with the cycle. The presence of regions of increased p-electron density on both sides of the planar aromatic ring leads to the fact that the benzene ring is a nucleophile and, therefore, tends to be attacked by an electrophilic reagent. Thus, electrophilic substitution reactions are most typical for aromatic compounds.

Let us consider the mechanism of electrophilic substitution using the example of benzene nitration.

Benzene reacts with a nitrating mixture (a mixture of concentrated nitric and sulfuric acids):

nitrobenzene

Substitution reactions in the ring proceed only through the formation of positively charged intermediate particles.

p-complex s-complex

The particle to be replaced is the proton.

According to this mechanism, the reactions of alkylation, halogenation, sulfonation, nitration of aromatic compounds and others proceed, differing only in the way the active particle of the reaction is formed - the electrophile E +

a) sulfonation:

HO–SO 3 H + H–SO 4 H à HSO 3 + + HSO 4 –

b) halogenation

Cl 2 + AlCl 3 a Cl + + AlCl 4 –

c) alkylation:

CH 3 -CH 2 -Cl + AlCl 3 à CH 3 -CH 2 + + AlCl 4 -

d) acylation

CH 3 COCl + AlCl 3 à CH 3 C + \u003d O + AlCl 4 -

In the unsubstituted ring of benzene, all 6 positions are equivalent for the occurrence of a substituent group. The situation is more complicated if homologues or derivatives of benzene enter into the reaction. In this case, the newly entering group enters a certain place in the ring. This place depends on the substituent already present (or present) in the ring. For example, if there is an electron donor group in the ring like: alkyl-, -OH, -OCH 3, -NH 2, -NHR, NR 2, -NH-COR, -X (halogen)(substituents of the first kind), then the substituting group enters into ortho- or para-positions relative to the existing group:

If the ring already contains an electron-withdrawing group of the type: –NO 2 , –NO, –SO 3 H, –CX 3 , –COOH, –COH, –COR, –CN (substituents of the second kind), then the newly entering group becomes in a meta position to them:

table 2

Summary table of substituents and their electronic effects

If the ring has two deputies of different kind guiding substitution inconsistently, then the place of entry of the new group is determined by deputy of the first kind, for example.

Most characteristic of aromatic hydrocarbons reactions substitution. In this case, as a result of the reactions, the aromatic sextet of electrons is not destroyed. Numerous examples of reactions are also known. radical halogenation and oxidation side chains of alkylbenzenes. Processes in which a stable aromatic system is destroyed are not very typical.

IV.1 Electrophilic aromatic substitution (seAr)

BUT. MechanismS E Ar (Substitution Electrophilic in Arenes)

Electrophilic substitution in the aromatic ring is one of the most well studied and widely used organic reactions. Most often, the end result of electrophilic substitution is the replacement of a hydrogen atom in the aromatic nucleus with another atom or group of atoms:

Electrophilic substitution reactions in the aromatic nucleus (as well as electrophilic accession to C=C bonds) begin with the formation -complex - the electrophilic agent is coordinated with the benzene molecule due to the -electronic system of the latter:

In the benzene ring, the -system, being stable (stabilization energy; see Section II), is not disturbed as easily as in alkenes. Therefore, the corresponding -complex can not only be fixed using physicochemical methods, but also highlighted.(note 24)

As a rule, the stage of formation of the α-complex proceeds quickly and not limits speed the whole process.

Further, the aromatic system is broken, and a covalent bond of the electrophile with the carbon atom of the benzene ring occurs. In this case, the -complex turns into a carbocation (carbenium ion), in which the positive charge is delocalized in the diene system, and the carbon atom attacked by the electrophile passes from sp 2 - in sp 3 hybrid state. Such a cation is called -complex . Usually, stage of education-complex is the rate determining. The delocalization of the positive charge in the -complex is not carried out uniformly between five carbon atoms, but due to the 2,4,6-positions of the benzene ring (compare with the allyl cation, where the positive charge is distributed between the 1,3-positions):

In electrophilic addition to alkenes, an α-complex is also first formed, which then passes into an α-complex, however, the further fate of the α-complex in the case of electrophilic reactions of alkenes and arenes is different. -The complex formed from alkenes is stabilized by trance- addition of a nucleophile; -the complex formed from the aromatic system stabilizes with the regeneration of the aromatic sextet -electrons: (note 25)

Below is the energy profile of such a reaction (Note 27) (E a ​​is the activation energy of the corresponding step):

We emphasize once again that the reactions S Е Ar, which, according to the result, are substitution, in fact according to the mechanism, they are addition reactions followed by elimination.

B. Orientation of addition in monosubstituted benzenes

When considering electrophilic substitution reactions in monosubstituted benzenes, two problems arise: 1. A new substituent can enter into ortho-, meta- or pair-positions, as well as to replace an existing substitute (the latter, the so-called ipso substitution , less common - see section IV.1.E (nitration). 2. The rate of substitution may be greater or less than the rate of substitution in benzene.

The influence of the substituent present in the benzene ring can be explained on the basis of its electronic effects. On this basis, substituents can be divided into 3 main groups:

1. Substituents that speed up the reaction compared to unsubstituted benzene ( activating ) and substitution guides in ortho ,- pair - positions.

2. Substituents that slow down the reaction ( deactivating ) and substitution guides in ortho,-para- positions .

3. Substituents that slow down the reaction ( deactivating ) and substitution guides in meta - provisions.

Substituents noted in p.p. 1.2( ortho-, para-orientators ) are called substituents of the 1st kind ; noted in paragraph 3 ( meta orientants ) - substituents of the second kind . The assignment of commonly occurring substituents according to their electronic effects is given below.

Obviously, electrophilic substitution will occur the faster, the more electron-donating substituent in the nucleus, and the slower, the more electron-withdrawing substituent in the nucleus.

For explanation orientation substitution, consider the structure of -complexes under attack in ortho-, meta- and pair- positions of monosubstituted benzene (as already noted, the formation of - complexes is usually the rate-determining step of electrophilic substitution; therefore, the ease of their formation should determine the ease of substitution in a given position):

If the Z group is an electron donor (whether inductive or mesomeric), then at ortho- or pair-attack, it can be directly involved in the delocalization of the positive charge in the -complex (structures III, IV, VI, VII). If Z is an electron acceptor, then these structures will be energetically unfavorable (due to the presence of a partial positive charge on the carbon atom associated with the electron-withdrawing substituent), and in this case, a meta-attack is preferable, in which such structures do not arise.

The above explanation is given on the basis of the so-called dynamic effect , i.e. electron density distributions in the reacting molecule. The orientation of electrophilic substitution in monosubstituted benzenes can also be explained from the position static electronic effects - distribution of electron density in a non-reacting molecule. When considering the shift in the electron density over multiple bonds, it can be seen that in the presence of an electron-donating substituent, most of all increased electron density in ortho- and pair- positions, and in the presence of an electron-withdrawing substituent, these positions are most depleted electrons:

Halogens are a special case - being substituents in the benzene nucleus, they deactivate it in electrophilic substitution reactions, however, they are ortho-, pair-orientators. Deactivation (decrease in the rate of reaction with electrophiles) is due to the fact that, unlike other groups with unshared electron pairs (such as -OH, -NH 2, etc.), which have a positive mesomeric (+M) and negative inductive effect ( -I), halogens are characterized by the predominance of the inductive effect over the mesomeric one (+ M< -I).(прим.30)

At the same time, halogen atoms are ortho, couple-orientants, since they are able, due to the positive mesomeric effect, to participate in the delocalization of a positive charge in the -complex formed during ortho- or pair- attack (structures IV, VII in the above scheme), and thereby reduce the energy of its formation.

If the benzene nucleus has not one, but two substituents, then their orienting action may coincide ( agreed orientation ) or not match ( mismatched orientation ). In the first case, one can count on the predominant formation of some specific isomers, and in the second, complex mixtures will be obtained. (Note 31)

The following are some examples of the coordinated orientation of two substituents; the place of preferential entry of the third substituent is shown by an arrow.

Demand for benzene is determined by the development of industries that consume it. The main applications of benzene are the production of ethylbenzene, cumene and cyclohexane and aniline.