ARENA (aromatic hydrocarbons)

Arenes or aromatic hydrocarbons - these are compounds whose molecules contain stable cyclic groups of atoms (benzene nuclei) with a closed system of conjugated bonds.

Why "Aromatic"? Because some of the substances have a pleasant smell. However, at present, a completely different meaning is put into the concept of "aromaticity".

Aromaticity of a molecule means its increased stability due to the delocalization of π-electrons in a cyclic system.

Arenes aromaticity criteria:

1. carbon atoms in sp 2 -hybridized state form a cycle.
2. The carbon atoms are arranged in one plane(the cycle has a flat structure).

A closed system of conjugated bonds contains

4n+2π electrons ( n is an integer).

The benzene molecule fully complies with these criteria. C 6 H 6.

The concept “ benzene ring” requires decryption. To do this, it is necessary to consider the structure of the benzene molecule.

ATAll bonds between carbon atoms in benzene are the same (there are no double or single bonds as such) and have a length of 0.139 nm. This value is intermediate between the single bond length in alkanes (0.154 nm) and the double bond length in alkenes (0.133 nm).

The equivalence of links is usually depicted as a circle inside the cycle

Circular conjugation gives an energy gain of 150 kJ/mol. This value is conjugation energy - the amount of energy that must be expended to break the aromatic system of benzene.

General formula: C n H 2n-6(n ≥ 6)

Homologous series:

Benzene homologues are compounds formed by replacing one or more hydrogen atoms in a benzene molecule with hydrocarbon radicals (R):

ortho- (about-) substituents at adjacent carbon atoms of the ring, i.e. 1,2-;
meta- (m-) substituents through one carbon atom (1,3-);
pair- (P-) substituents on opposite sides of the (1,4-) ring.

aryl

C 6H5- (phenyl) and C6H Aromatic monovalent radicals have the common name " aryl". Of these, two are most common in the nomenclature of organic compounds:

C 6H5- (phenyl) and C 6 H 5 CH 2- (benzyl). 5 CH 2- (benzyl).

Isomerism:

structural:

1) positions of deputies for di-, three- and tetra-substituted benzenes (for example, about-, m- and P-xylenes);

2) carbon skeleton in the side chain containing at least 3 carbon atoms:

3) isomerism of substituents R, starting from R = C 2 H 5 .

Chemical properties:

Arenes are more characteristic of reactions going with preservation of the aromatic system, namely, substitution reactions hydrogen atoms associated with the cycle.

2. Nitration

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

3. Alkylation

Substitution of a hydrogen atom in the benzene ring with an alkyl group ( alkylation) occurs under the action alkyl halides or alkenes in the presence of catalysts AlCl 3 , AlBr 3 , FeCl 3 .

Substitution in alkylbenzenes:

Benzene homologues (alkylbenzenes) are more active in substitution reactions than benzene.

For example, when nitrating toluene C 6 H 5 CH 3 substitution of not one, but three hydrogen atoms can occur with the formation of 2,4,6-trinitrotoluene:

and facilitates substitution in these positions.

On the other hand, under the influence of the benzene ring, the methyl group CH 3 in toluene becomes more active in oxidation and radical substitution reactions compared to methane CH 4.

Toluene, unlike methane, oxidizes under mild conditions (discolors the acidified solution of KMnO 4 when heated):

Easier than in alkanes, radical substitution reactions proceed in side chain alkylbenzenes:

This is explained by the fact that stable intermediate radicals are easily (at a low activation energy) formed at the limiting stage. For example, in the case toluene a radical is formed benzyl Ċ H 2 -C 6 H 5 . It is more stable than alkyl free radicals ( Ċ H 3 Ċ H 2 R), because its unpaired electron is delocalized due to interaction with the π-electron system of the benzene ring:

Orientation rules

1. The substituents present in the benzene ring direct the newly entering group to certain positions, i.e. have an orienting effect.

2. According to their guiding action, all substituents are divided into two groups:orientators of the first kind and orientators of the second kind.

   Orientants of the 1st kind(ortho pair-orientants) direct the subsequent substitution mainly inortho- and pair-provisions.

   These include electron donor groups (electronic effects of groups are indicated in brackets):

R( +I); - Oh(+M,-I); - OR(+M,-I); - NH2(+M,-I); - NR 2(+M,-I) +M-effect in these groups is stronger than -I-effect.

Orientants of the 1st kind increase the electron density in the benzene ring, especially on carbon atoms inortho- and pair-positions, which favors the interaction of these atoms with electrophilic reagents.

Orientants of the 1st kind, by increasing the electron density in the benzene ring, increase its activity in electrophilic substitution reactions compared to unsubstituted benzene.

A special place among the orientants of the 1st kind is occupied by halogens, which exhibitelectron-withdrawing properties:

-F (+M<–I ), -Cl (+M<–I ), -Br (+M<–I ).

Being ortho pair-orientants, they slow down electrophilic substitution. Reason is strong –I-the effect of electronegative halogen atoms, which lowers the electron density in the ring.

Orientators of the 2nd kind ( meta-orientants) direct subsequent substitution predominantly to meta-position.
These include electron-withdrawing groups:

-NO 2 (-M, -I); -COOH (-M, -I); -CH=O (-M, -I); -SO 3 H (–I); -NH3+ (–I); -CCl 3 (–I).

Orientants of the 2nd kind reduce the electron density in the benzene ring, especially in ortho- and pair-provisions. Therefore, the electrophile attacks carbon atoms not in these positions, but in meta-position, where the electron density is somewhat higher.
Example:

All orientants of the 2nd kind, reducing the overall electron density in the benzene ring, reduce its activity in electrophilic substitution reactions.

Thus, the ease of electrophilic substitution for compounds (given as examples) decreases in the series:

toluene C 6 H 5 CH Unlike benzene, its homologues are oxidized quite easily.

The concept of "benzene ring" immediately requires deciphering. To do this, it is necessary to at least briefly consider the structure of the benzene molecule. The first structure of benzene was proposed in 1865 by the German scientist A. Kekule:

The most important aromatic hydrocarbons include benzene C 6 H 6 and its homologues: toluene C 6 H 5 CH 3, xylene C 6 H 4 (CH 3) 2, etc.; naphthalene C 10 H 8 , anthracene C 14 H 10 and their derivatives.

The carbon atoms in the benzene molecule form a regular flat hexagon, although it is usually drawn elongated.

The structure of the benzene molecule was finally confirmed by the reaction of its formation from acetylene. The structural formula shows three single and three double alternating carbon-carbon bonds. But such an image does not convey the true structure of the molecule. In fact, the carbon-carbon bonds in benzene are equivalent, and they have properties that are not similar to those of either single or double bonds. These features are explained by the electronic structure of the benzene molecule.

Electronic structure of benzene

Each carbon atom in the benzene molecule is in a state of sp 2 hybridization. It is linked to two adjacent carbon atoms and a hydrogen atom by three σ-bonds. As a result, a flat hexagon is formed: all six carbon atoms and all C-C and C-H σ-bonds lie in the same plane. The electron cloud of the fourth electron (p-electron), which is not involved in hybridization, has the shape of a dumbbell and is oriented perpendicular to the plane of the benzene ring. Such p-electron clouds of neighboring carbon atoms overlap above and below the plane of the ring.

As a result, six p-electrons form a common electron cloud and a single chemical bond for all carbon atoms. Two regions of the large electronic plane are located on both sides of the plane of σ-bonds.

The p-electron cloud causes a reduction in the distance between carbon atoms. In the benzene molecule, they are the same and equal to 0.14 nm. In the case of a single and double bond, these distances would be 0.154 and 0.134 nm, respectively. This means that there are no single and double bonds in the benzene molecule. The benzene molecule is a stable six-membered cycle of identical CH-groups lying in the same plane. All bonds between carbon atoms in benzene are equivalent, which determines the characteristic properties of the benzene nucleus. This is most accurately reflected by the structural formula of benzene in the form of a regular hexagon with a circle inside (I). (The circle symbolizes the equivalence of bonds between carbon atoms.) However, the Kekule formula is often used, indicating double bonds (II):

The benzene nucleus has a certain set of properties, which is commonly called aromaticity.

Homologous series, isomerism, nomenclature

Conventionally, the arenas can be divided into two rows. The first includes benzene derivatives (for example, toluene or diphenyl), the second - condensed (polynuclear) arenes (the simplest of them is naphthalene):

The homologous series of benzene has the general formula C n H 2 n -6 . Homologues can be considered as derivatives of benzene, in which one or more hydrogen atoms are replaced by various hydrocarbon radicals. For example, C 6 H 5 -CH 3 - methylbenzene or toluene, C 6 H 4 (CH 3) 2 - dimethylbenzene or xylene, C 6 H 5 -C 2 H 5 - ethylbenzene, etc.

Since all carbon atoms in benzene are equivalent, its first homologue, toluene, has no isomers. The second homologue, dimethylbenzene, has three isomers that differ in the mutual arrangement of methyl groups (substituents). This is an ortho- (abbreviated as o-), or 1,2-isomer, in which substituents are located at neighboring carbon atoms. If the substituents are separated by one carbon atom, then it is the meta (abbreviated m-) or 1,3-isomer, and if they are separated by two carbon atoms, then it is the para- (abbreviated p-) or 1,4-isomer. In the names, substituents are indicated by letters (o-, m-, p-) or numbers.

Physical properties

The first members of the homologous series of benzene are colorless liquids with a specific odor. Their density is less than 1 (lighter than water). Insoluble in water. Benzene and its homologues are themselves good solvents for many organic substances. Arenas burn with a smoky flame due to the high carbon content in their molecules.

Chemical properties

Aromaticity determines the chemical properties of benzene and its homologues. The six-electron π-system is more stable than conventional two-electron π-bonds. Therefore, addition reactions are less typical for aromatic hydrocarbons than for unsaturated hydrocarbons. The most typical for arenes are substitution reactions. Thus, aromatic hydrocarbons in their chemical properties occupy an intermediate position between saturated and unsaturated hydrocarbons.

I. Substitution reactions

1. Halogenation (with Cl 2, Br 2)

2. Nitration

3. Sulfonation

4. Alkylation (benzene homologues are formed) - Friedel-Crafts reactions

Alkylation of benzene also occurs when it interacts with alkenes:

Dehydrogenation of ethylbenzene produces styrene (vinylbenzene):

II. Addition reactions

1. Hydrogenation

2. Chlorination

III. Oxidation reactions

1. Combustion

2C 6 H 6 + 15O 2 → 12CO 2 + 6H 2 O

2. Oxidation under the action of KMnO 4, K 2 Cr 2 O 7, HNO 3, etc.

No chemical reaction occurs (similar to alkanes).

Properties of benzene homologues

In benzene homologues, a core and a side chain (alkyl radicals) are distinguished. In terms of chemical properties, alkyl radicals are similar to alkanes; the effect of the benzene nucleus on them is manifested in the fact that hydrogen atoms at the carbon atom directly bonded to the benzene nucleus always participate in substitution reactions, as well as in the easier oxidizability of C-H bonds.

The effect of an electron-donating alkyl radical (for example, -CH 3) on the benzene core is manifested in an increase in the effective negative charges on carbon atoms in the ortho and para positions; as a result, the substitution of their associated hydrogen atoms is facilitated. Therefore, benzene homologues can form trisubstituted products (and benzene usually forms monosubstituted derivatives).

General review.

Aromatic hydrocarbons (arenes) are substances whose molecules contain one or more benzene rings - cyclic groups of carbon atoms with a special nature of bonds.

The concept of "benzene ring" immediately requires deciphering. To do this, it is necessary to at least briefly consider the structure of the benzene molecule. The first structure of benzene was proposed in 1865 by the German scientist A. Kekule:

This formula correctly reflects the equivalence of six carbon atoms, but does not explain a number of special properties of benzene. For example, despite the unsaturation, benzene does not show a tendency to addition reactions: it does not decolorize bromine water and potassium permanganate solution, i.e. does not give qualitative reactions typical for unsaturated compounds.

The features of the structure and properties of benzene were fully explained only after the development of the modern quantum mechanical theory of chemical bonds. According to modern concepts, all six carbon atoms in the benzene molecule are in the -hybrid state. Each carbon atom forms -bonds with two other carbon atoms and one hydrogen atom lying in the same plane. The bond angles between the three -bonds are 120°. Thus, all six carbon atoms lie in the same plane, forming a regular hexagon (-skeleton of the benzene molecule).

Each carbon atom has one unhybridized p orbital.

Six such orbitals are located perpendicular to the flat -skeleton and parallel to each other (Fig. 21.1, a). All six p-electrons interact with each other, forming -bonds, not localized in pairs, as in the formation of ordinary double bonds, but combined into a single -electron cloud. Thus, circular conjugation occurs in the benzene molecule (see § 19). The highest -electron density in this conjugated system is located above and below the -skeleton plane (Fig. 21.1, b).

Rice. 21.1. The structure of the benzene molecule

As a result, all bonds between carbon atoms in benzene are aligned and have a length of 0.139 nm. This value is intermediate between the single bond length in alkanes (0.154 nm) and the double bond length in alkenes (0.133 nm). The equivalence of connections is usually depicted as a circle inside the cycle (Fig. 21.1, c). Circular conjugation gives an energy gain of 150 kJ/mol. This value is the conjugation energy - the amount of energy that needs to be expended to break the aromatic system of benzene (compare - the conjugation energy in butadiene is only 12 kJ / mol).

This electronic structure explains all the features of benzene. In particular, it is clear why benzene is difficult to enter into addition reactions - this would lead to a violation of conjugation. Such reactions are possible only under very severe conditions.

Nomenclature and isomerism.

Conventionally, the arenas can be divided into two rows. The first includes benzene derivatives (for example, toluene or diphenyl), the second - condensed (polynuclear) arenes (the simplest of them is naphthalene):

We will consider only the homologous series of benzene with the general formula .

Structural isomerism in the homologous series of benzene is due to the mutual arrangement of substituents in the nucleus. Monosubstituted benzene derivatives do not have position isomers, since all atoms in the benzene nucleus are equivalent. Disubstituted derivatives exist in the form of three isomers that differ in the mutual arrangement of substituents. The position of the substituents is indicated by numbers or prefixes:

Aromatic hydrocarbon radicals are called aryl radicals. The radical is called phenyl.

physical properties.

The first members of the homologous series of benzene (for example, toluene, ethylbenzene, etc.) are colorless liquids with a specific odor. They are lighter than water and insoluble in water. They dissolve well in organic solvents. Benzene and its homologues are themselves good solvents for many organic substances. All arenas burn with a smoky flame due to the high content of carbon in their molecules.

Ways to get.

1. Obtaining from aliphatic hydrocarbons. When straight-chain alkanes having at least 6 carbon atoms in a molecule are passed over heated platinum or chromium oxide, dehydrocyclization occurs - the formation of an arene with the release of hydrogen:

2. Dehydrogenation of cycloalkanes. The reaction occurs when passing vapors of cyclohexane and its homologues over heated platinum:

3. Preparation of benzene by trimerization of acetylene - see § 20.

4. Obtaining homologues of benzene by the Friedel-Crafts reaction - see below.

5. Fusion of salts of aromatic acids with alkali:

Chemical properties.

General review. Possessing a mobile six -electrons, the aromatic nucleus is a convenient object for attack by electrophilic reagents. This is also facilitated by the spatial arrangement of the -electron cloud on both sides of the flat -skeleton of the molecule (Fig. 21.1, b)

For arenes, reactions proceeding according to the mechanism of electrophilic substitution, denoted by the symbol (from the English substitution electrophilic), are most characteristic.

The mechanism of electrophilic substitution can be represented as follows. The electrophilic reagent XY (X is an electrophile) attacks the electron cloud, and an unstable -complex is formed due to the weak electrostatic interaction. The aromatic system is not yet disturbed. This stage is fast. At the second, slower stage, a covalent bond is formed between the electrophile X and one of the carbon atoms of the ring due to two α-electrons of the ring. This carbon atom changes from to the -hybrid state. The aromaticity of the system is thus disturbed. The four remaining -electrons are distributed among five other carbon atoms, and the benzene molecule forms a carbocation, or -complex.

Violation of aromaticity is energetically unfavorable, therefore the structure of the -complex is less stable than the aromatic structure. To restore aromaticity, a proton is split off from the carbon atom associated with the electrophile (third stage). In this case, two electrons return to the -system, and thereby aromaticity is restored:

Electrophilic substitution reactions are widely used for the synthesis of many benzene derivatives.

Chemical properties of benzene.

1. Halogenation. Benzene does not interact with chlorine or bromine under normal conditions. The reaction can proceed only in the presence of anhydrous catalysts. As a result of the reaction, halogen-substituted arenes are formed:

The role of the catalyst is to polarize the neutral halogen molecule with the formation of an electrophilic particle from it:

2. Nitration. Benzene reacts very slowly with concentrated nitric acid, even when heated strongly. However, under the action of the so-called nitrating mixture (a mixture of concentrated nitric and sulfuric acids), the nitration reaction proceeds quite easily:

3. Sulfonation. The reaction easily takes place under the action of "fuming" sulfuric acid (oleum):

4. Alkylation according to Friedel-Crafts. As a result of the reaction, an alkyl group is introduced into the benzene core to obtain benzene homologues. The reaction proceeds under the action of haloalkanes on benzene in the presence of catalysts - aluminum halides. The role of the catalyst is reduced to the polarization of the molecule with the formation of an electrophilic particle:

Depending on the structure of the radical in the haloalkane, different homologues of benzene can be obtained:

5. Alkylation with alkenes. These reactions are widely used in industry to produce ethylbenzene and isopropylbenzene (cumene). Alkylation is carried out in the presence of a catalyst. The reaction mechanism is similar to that of the previous reaction:

All the reactions discussed above proceed by the mechanism of electrophilic substitution.

Addition reactions to arenes lead to the destruction of the aromatic system and require large amounts of energy, so they proceed only under harsh conditions.

6. Hydrogenation. The reaction of hydrogen addition to arenes proceeds under heating and high pressure in the presence of metal catalysts (Ni, Pt, Pd). Benzene is converted to cyclohexane, and benzene homologues are converted to cyclohexane derivatives:

7. Radical halogenation. The interaction of benzene vapor with chlorine proceeds by a radical mechanism only under the influence of hard ultraviolet radiation. In this case, benzene adds three molecules of chlorine and forms a solid product - hexachlorocyclohexane:

8. Oxidation by atmospheric oxygen. In terms of resistance to oxidizing agents, benzene resembles alkanes. Only with strong heating (400 ° C) of benzene vapor with atmospheric oxygen in the presence of a catalyst, a mixture of maleic acid and its anhydride is obtained:

Chemical properties of benzene homologues.

Benzene homologues have a number of special chemical properties associated with the mutual influence of the alkyl radical on the benzene ring, and vice versa.

Reactions in the side chain. In terms of chemical properties, alkyl radicals are similar to alkanes. Hydrogen atoms in them are replaced by halogens by a free radical mechanism. Therefore, in the absence of a catalyst during heating or UV irradiation, a radical substitution reaction occurs in the side chain. The effect of the benzene ring on alkyl substituents always results in the replacement of the hydrogen atom at the carbon atom directly bonded to the benzene ring (a-carbon atom).

Substitution in the benzene ring is possible only by the mechanism in the presence of a catalyst:

Below you will find out which of the three isomers of chlorotoluene are formed in this reaction.

Under the action of potassium permanganate and other strong oxidants on the homologues of benzene, the side chains are oxidized. No matter how complex the substituent chain is, it is destroyed, with the exception of the -carbon atom, which is oxidized into a carboxyl group.

Homologues of benzene with one side chain give benzoic acid:

Orientation (substitution) rules in the benzene ring.

The most important factor determining the chemical properties of a molecule is the distribution of electron density in it. The nature of the distribution depends on the mutual influence of the atoms.

In molecules that have only -bonds, the mutual influence of atoms is carried out through the inductive effect (see § 17). In molecules that are conjugated systems, the action of the mesomeric effect is manifested.

The influence of substituents, transmitted through a conjugated system of -bonds, is called the mesomeric (M) effect.

In a benzene molecule, the -electron cloud is distributed evenly over all carbon atoms due to conjugation.

If, however, some substituent is introduced into the benzene ring, this uniform distribution is disturbed, and the electron density in the ring is redistributed. The place of entry of the second substituent into the benzene ring is determined by the nature of the already existing substituent.

Substituents are divided into two groups depending on the effect they exhibit (mesomeric or inductive): electron support and electron acceptor.

Electron-donor substituents exhibit an effect and increase the electron density in the conjugated system. These include the hydroxyl group -OH and the amino group. The lone pair of electrons in these groups enters into general conjugation with the -electronic system of the benzene ring and increases the length of the conjugated system. As a result, the electron density is concentrated in the ortho and para positions:

Alkyl groups cannot participate in general conjugation, but they exhibit an effect under which a similar redistribution of -electron density occurs.

Electron-withdrawing substituents exhibit the -M effect and reduce the electron density in the conjugated system. These include the nitro group, the sulfo group, the aldehyde group -CHO and the carboxyl group -COOH groups. These substituents form a common conjugated system with the benzene ring, but the overall electron cloud shifts towards these groups. Thus, the total electron density in the ring decreases, and it decreases least of all in the meta positions:

For example, toluene containing a substituent of the first kind is nitrated and brominated in the para and ortho positions:

Nitrobenzene containing a substituent of the second kind is nitrated and brominated in the meta position:

In addition to the orienting action, substituents also affect the reactivity of the benzene ring: orientants of the 1st kind (except for halogens) facilitate the introduction of the second substituent; orientants of the second kind (and halogens) make it difficult.

Ways to get. one. Obtaining from aliphatic hydrocarbons. To obtain benzene and its homologues in industry, they use aromatization saturated hydrocarbons that are part of the oil. When alkanes with a straight chain consisting of at least six carbon atoms are passed over heated platinum or chromium oxide, dehydrogenation occurs with simultaneous ring closure ( dehydrocyclization). In this case, benzene is obtained from hexane, and toluene is obtained from heptane.

2. Dehydrogenation of cycloalkanes also leads to aromatic hydrocarbons; for this, a pair of cyclohexane and its homologues is passed over heated platinum.

3. Benzene can be obtained from acetylene trimerization, why acetylene is passed over activated carbon at 600 °C.

4. Benzene homologues are obtained from benzene by its interaction with alkyl halides in the presence of aluminum halides (alkylation reaction, or Friedel-Crafts reaction).

5. When fusion salts of aromatic acids with alkali, arenes are released in gaseous form.

Chemical properties. The aromatic nucleus, which has a mobile system of n-electrons, is a convenient object for attack by electrophilic reagents. This is also facilitated by the spatial arrangement of the n-electron cloud on both sides of the flat a-skeleton of the molecule (see Fig. 23.1, b).

For arenes, the most typical reactions proceed according to the mechanism electrophilic substitution, denoted by the symbol S E(from English, substitution, electrophilic).

Mechanism S E can be represented as follows:

At the first stage, the electrophilic particle X is attracted to the n-electron cloud and forms an n-complex with it. Then two of the six n-electrons of the ring form an a-bond between X and one of the carbon atoms. In this case, the aromaticity of the system is violated, since only four n-electrons remain in the ring, distributed among five carbon atoms (a-complex). To preserve aromaticity, the a-complex ejects a proton, and two C-H bond electrons pass into the n-electron system.

The following reactions of aromatic hydrocarbons proceed according to the mechanism of electrophilic substitution.

1. Halogenation. Benzene and its homologues react with chlorine or bromine in the presence of anhydrous A1C1 3 , FeCl 3 , A1Br 3 catalysts.

This reaction produces a mixture from toluene. ortho- and para-isomers (see below). The role of the catalyst is to polarize the neutral halogen molecule with the formation of an electrophilic particle from it.

2. Nitration. Benzene reacts very slowly with concentrated nitric acid, even when heated strongly. However, when acting nitrating mixture(mixtures of concentrated nitric and sulfuric acids), the nitration reaction proceeds quite easily.

3. Sulfonation. The reaction easily passes with "fuming" sulfuric acid (oleum).

• 4. Friedel-Crafts Alkylation- see above methods for obtaining benzene homologues.
• 5. Alkylation with alkenes. These reactions are widely used in industry to produce ethylbenzene and isopropylbenzene (cumene). Alkylation is carried out in the presence of a catalyst A1C1 3 . The reaction mechanism is similar to that of the previous reaction.

All the above reactions proceed according to the mechanism electrophilic substitution S E .

Along with substitution reactions, aromatic hydrocarbons can enter into addition reactions, however, these reactions lead to the destruction of the aromatic system and therefore require large amounts of energy and proceed only under harsh conditions.

6. hydrogenation benzene goes under heating and high pressure in the presence of metal catalysts (Ni, Pt, Pd). Benzene is converted to cyclohexane.

Hydrogenation of benzene homologues gives cyclohexane derivatives.

7. Radical halogenation benzene occurs when its vapor interacts with chlorine only under the influence of hard ultraviolet radiation. At the same time, benzene joins three chlorine molecules and forms solid product hexachlorocyclohexane (hexachloran) C 6 H 6 C1 6 (hydrogen atoms are not indicated in the structural formulas).

8. Oxidation by atmospheric oxygen. In terms of resistance to the action of oxidizing agents, benzene resembles alkanes - the reaction requires harsh conditions. For example, the oxidation of benzene with atmospheric oxygen occurs only when its vapor is strongly heated (400 °C) in air in the presence of a V 2 0 5 catalyst; the products are a mixture of maleic acid and its anhydride.

Benzene homologues. The chemical properties of benzene homologues are different from those of benzene, which is due to the mutual influence of the alkyl radical and the benzene ring.

Reactions in the side chain. In terms of chemical properties, alkyl substituents in the benzene ring are similar to alkanes. Hydrogen atoms in them are replaced by halogens by the radical mechanism (S R). That's why in the absence of a catalyst upon heating or UV irradiation, a radical substitution reaction occurs in the side chain. However, the influence of the benzene ring on alkyl substituents leads to the fact that, first of all, the hydrogen at the carbon atom directly bonded to the benzene ring is replaced (and -atom carbon).

Substitution on the benzene ring by mechanism S E Maybe only in the presence of a catalyst(A1C1 3 or FeCl 3). Substitution in the ring occurs in ortho- and para positions to the alkyl radical.

Under the action of potassium permanganate and other strong oxidizing agents on benzene homologues, the side chains are oxidized. No matter how complex the chain of the substituent is, it is destroyed, with the exception of the a-carbon atom, which is oxidized into a carboxyl group.

Homologues of benzene with one side chain give benzoic acid.

ARENA (aromatic hydrocarbons)

Arenes or aromatic hydrocarbons - these are compounds whose molecules contain stable cyclic groups of atoms (benzene nuclei) with a closed system of conjugated bonds.

Why "Aromatic"? Because some of the substances have a pleasant smell. However, at present, a completely different meaning is put into the concept of "aromaticity".

Aromaticity of a molecule means its increased stability due to the delocalization of π-electrons in a cyclic system.

Arenes aromaticity criteria:

1. carbon atoms in sp 2 -hybridized state form a cycle.
2. The carbon atoms are arranged in one plane(the cycle has a flat structure).

A closed system of conjugated bonds contains

4n+2π electrons ( n is an integer).

The benzene molecule fully complies with these criteria. C 6 H 6.

The concept “ benzene ring” requires decryption. To do this, it is necessary to consider the structure of the benzene molecule.

ATAll bonds between carbon atoms in benzene are the same (there are no double or single bonds as such) and have a length of 0.139 nm. This value is intermediate between the single bond length in alkanes (0.154 nm) and the double bond length in alkenes (0.133 nm).

The equivalence of links is usually depicted as a circle inside the cycle

Circular conjugation gives an energy gain of 150 kJ/mol. This value is conjugation energy - the amount of energy that must be expended to break the aromatic system of benzene.

General formula: C n H 2n-6(n ≥ 6)

Homologous series:

Benzene homologues are compounds formed by replacing one or more hydrogen atoms in a benzene molecule with hydrocarbon radicals (R):

ortho- (about-) substituents at adjacent carbon atoms of the ring, i.e. 1,2-;
meta- (m-) substituents through one carbon atom (1,3-);
pair- (P-) substituents on opposite sides of the (1,4-) ring.

aryl

C 6H5- (phenyl) and C6H Aromatic monovalent radicals have the common name " aryl". Of these, two are most common in the nomenclature of organic compounds:

C 6H5- (phenyl) and C 6 H 5 CH 2- (benzyl). 5 CH 2- (benzyl).

Isomerism:

structural:

1) positions of deputies for di-, three- and tetra-substituted benzenes (for example, about-, m- and P-xylenes);

2) carbon skeleton in the side chain containing at least 3 carbon atoms:

3) isomerism of substituents R, starting from R = C 2 H 5 .

Chemical properties:

Arenes are more characteristic of reactions going with preservation of the aromatic system, namely, substitution reactions hydrogen atoms associated with the cycle.

2. Nitration

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

3. Alkylation

Substitution of a hydrogen atom in the benzene ring with an alkyl group ( alkylation) occurs under the action alkyl halides or alkenes in the presence of catalysts AlCl 3 , AlBr 3 , FeCl 3 .

Substitution in alkylbenzenes:

Benzene homologues (alkylbenzenes) are more active in substitution reactions than benzene.

For example, when nitrating toluene C 6 H 5 CH 3 substitution of not one, but three hydrogen atoms can occur with the formation of 2,4,6-trinitrotoluene:

and facilitates substitution in these positions.

On the other hand, under the influence of the benzene ring, the methyl group CH 3 in toluene becomes more active in oxidation and radical substitution reactions compared to methane CH 4.

Toluene, unlike methane, oxidizes under mild conditions (discolors the acidified solution of KMnO 4 when heated):

Easier than in alkanes, radical substitution reactions proceed in side chain alkylbenzenes:

This is explained by the fact that stable intermediate radicals are easily (at a low activation energy) formed at the limiting stage. For example, in the case toluene a radical is formed benzyl Ċ H 2 -C 6 H 5 . It is more stable than alkyl free radicals ( Ċ H 3 Ċ H 2 R), because its unpaired electron is delocalized due to interaction with the π-electron system of the benzene ring:

Orientation rules

1. The substituents present in the benzene ring direct the newly entering group to certain positions, i.e. have an orienting effect.

2. According to their guiding action, all substituents are divided into two groups:orientators of the first kind and orientators of the second kind.

   Orientants of the 1st kind(ortho pair-orientants) direct the subsequent substitution mainly inortho- and pair-provisions.

   These include electron donor groups (electronic effects of groups are indicated in brackets):

R( +I); - Oh(+M,-I); - OR(+M,-I); - NH2(+M,-I); - NR 2(+M,-I) +M-effect in these groups is stronger than -I-effect.

Orientants of the 1st kind increase the electron density in the benzene ring, especially on carbon atoms inortho- and pair-positions, which favors the interaction of these atoms with electrophilic reagents.

Orientants of the 1st kind, by increasing the electron density in the benzene ring, increase its activity in electrophilic substitution reactions compared to unsubstituted benzene.

A special place among the orientants of the 1st kind is occupied by halogens, which exhibitelectron-withdrawing properties:

-F (+M<–I ), -Cl (+M<–I ), -Br (+M<–I ).

Being ortho pair-orientants, they slow down electrophilic substitution. Reason is strong –I-the effect of electronegative halogen atoms, which lowers the electron density in the ring.

Orientators of the 2nd kind ( meta-orientants) direct subsequent substitution predominantly to meta-position.
These include electron-withdrawing groups:

-NO 2 (-M, -I); -COOH (-M, -I); -CH=O (-M, -I); -SO 3 H (–I); -NH3+ (–I); -CCl 3 (–I).

Orientants of the 2nd kind reduce the electron density in the benzene ring, especially in ortho- and pair-provisions. Therefore, the electrophile attacks carbon atoms not in these positions, but in meta-position, where the electron density is somewhat higher.
Example:

All orientants of the 2nd kind, reducing the overall electron density in the benzene ring, reduce its activity in electrophilic substitution reactions.

Thus, the ease of electrophilic substitution for compounds (given as examples) decreases in the series:

toluene C 6 H 5 CH Unlike benzene, its homologues are oxidized quite easily.