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), not participating 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 is due to characteristic properties benzene core. 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 is methylbenzene or toluene, C 6 H 4 (CH 3) 2 is dimethylbenzene or xylene, C 6 H 5 -C 2 H 5 is 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 matter. 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 homologs 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.

Not happening chemical reaction(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).

Physical Properties

Benzene and its closest homologues are colorless liquids with a specific odor. Aromatic hydrocarbons are lighter than water and do not dissolve in it, but they easily dissolve in organic solvents - alcohol, ether, acetone.

Benzene and its homologues are themselves good solvents for many organic substances. All arenas burn with a smoky flame due to the high carbon content of their molecules.

The physical properties of some arenes are presented in the table.

Table. Physical properties of some arenas

Name	Formula	t°.pl., °C	t°.bp., °C
Benzene	C 6 H 6	5,5	80,1
Toluene (methylbenzene)	C 6 H 5 CH 3	95,0	110,6
Ethylbenzene	C 6 H 5 C 2 H 5	95,0	136,2
Xylene (dimethylbenzene)	C 6 H 4 (CH 3) 2
ortho-		25,18	144,41
meta-		47,87	139,10
pair-		13,26	138,35
Propylbenzene	C 6 H 5 (CH 2) 2 CH 3	99,0	159,20
Cumene (isopropylbenzene)	C 6 H 5 CH(CH 3) 2	96,0	152,39
Styrene (vinylbenzene)	C 6 H 5 CH \u003d CH 2	30,6	145,2

Benzene - low-boiling ( tkip= 80.1°C), colorless liquid, insoluble in water

Attention! Benzene - poison, acts on the kidneys, changes the blood formula (with prolonged exposure), can disrupt the structure of chromosomes.

Most aromatic hydrocarbons are life threatening and toxic.

Obtaining arenes (benzene and its homologues)

In the laboratory

1. Fusion of salts of benzoic acid with solid alkalis

C 6 H 5 -COONa + NaOH t → C 6 H 6 + Na 2 CO 3

sodium benzoate

2. Wurtz-Fitting reaction: (here G is halogen)

From 6H 5 -G+2Na + R-G →C 6 H 5 - R + 2 NaG

WITH 6 H 5 -Cl + 2Na + CH 3 -Cl → C 6 H 5 -CH 3 + 2NaCl

In industry

• isolated from oil and coal by fractional distillation, reforming;
• from coal tar and coke oven gas

1. Dehydrocyclization of alkanes with more than 6 carbon atoms:

C 6 H 14 t , kat→C 6 H 6 + 4H 2

2. Trimerization of acetylene(only for benzene) – R. Zelinsky:

3C 2 H2 600°C, Act. coal→C 6 H 6

3. Dehydrogenation cyclohexane and its homologues:

Soviet Academician Nikolai Dmitrievich Zelinsky established that benzene is formed from cyclohexane (dehydrogenation of cycloalkanes

C 6 H 12 t, cat→C 6 H 6 + 3H 2

C 6 H 11 -CH 3 t , kat→C 6 H 5 -CH 3 + 3H 2

methylcyclohexanetoluene

4. Alkylation of benzene(obtaining homologues of benzene) – r Friedel-Crafts.

C 6 H 6 + C 2 H 5 -Cl t, AlCl3→C 6 H 5 -C 2 H 5 + HCl

chloroethane ethylbenzene

Chemical properties of arenes

I. OXIDATION REACTIONS

1. Combustion (smoky flame):

2C 6 H 6 + 15O 2 t→12CO 2 + 6H 2 O + Q

2. Benzene at normal conditions does not decolorize bromine water and water solution potassium permanganate

3. Benzene homologues are oxidized by potassium permanganate (discolor potassium permanganate):

A) in an acidic environment to benzoic acid

Under the action of potassium permanganate and other strong oxidizing agents on benzene homologues 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:

Homologues containing two side chains give dibasic acids:

5C 6 H 5 -C 2 H 5 + 12KMnO 4 + 18H 2 SO 4 → 5C 6 H 5 COOH + 5CO 2 + 6K 2 SO 4 + 12MnSO 4 + 28H 2 O

5C 6 H 5 -CH 3 + 6KMnO 4 + 9H 2 SO 4 → 5C 6 H 5 COOH + 3K 2 SO 4 + 6MnSO 4 + 14H 2 O

Simplified :

C 6 H 5 -CH 3 + 3O KMnO4→C 6 H 5 COOH + H 2 O

B) in neutral and slightly alkaline to salts of benzoic acid

C 6 H 5 -CH 3 + 2KMnO 4 → C 6 H 5 COO K + K OH + 2MnO 2 + H 2 O

II. ADDITION REACTIONS (harder than alkenes)

1. Halogenation

C 6 H 6 + 3Cl 2 h ν → C 6 H 6 Cl 6 (hexachlorocyclohexane - hexachloran)

2. Hydrogenation

C 6 H 6 + 3H 2 t , PtorNi→C 6 H 12 (cyclohexane)

3. Polymerization

III. SUBSTITUTION REACTIONS – ionic mechanism (lighter than alkanes)

b) benzene homologues upon irradiation or heating

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, heating or UV irradiation leads to a radical substitution reaction in the side chain. The influence of the benzene ring on alkyl substituents leads to the fact that the hydrogen atom is always replaced at the carbon atom directly bonded to the benzene ring (a-carbon atom).

1) C 6 H 5 -CH 3 + Cl 2 h ν → C 6 H 5 -CH 2 -Cl + HCl

c) benzene homologues in the presence of a catalyst

C 6 H 5 -CH 3 + Cl 2 AlCl 3 → (mixture of orta, pair of derivatives) +HCl

2. Nitration (with nitric acid Oh)

C 6 H 6 + HO-NO 2 t, H2SO4→C 6 H 5 -NO 2 + H 2 O

nitrobenzene - smell almond!

C 6 H 5 -CH 3 + 3HO-NO 2 t, H2SO4→ WITH H 3 -C 6 H 2 (NO 2) 3 + 3H 2 O

2,4,6-trinitrotoluene (tol, trotyl)

The use of benzene and its homologues

Benzene C 6 H 6 is a good solvent. Benzene as an additive improves the quality of motor fuel. It serves as a raw material for the production of many aromatic organic compounds - nitrobenzene C 6 H 5 NO 2 (solvent, aniline is obtained from it), chlorobenzene C 6 H 5 Cl, phenol C 6 H 5 OH, styrene, etc.

Toluene C 6 H 5 -CH 3 - a solvent used in the manufacture of dyes, drugs and explosives (trotyl (tol), or 2,4,6-trinitrotoluene TNT).

Xylene C 6 H 4 (CH 3) 2 . Technical xylene is a mixture of three isomers ( ortho-, meta- and pair-xylenes) - is used as a solvent and starting product for the synthesis of many organic compounds.

Isopropylbenzene C 6 H 5 -CH (CH 3) 2 serves to obtain phenol and acetone.

Chlorine derivatives of benzene used for plant protection. Thus, the product of substitution of H atoms in benzene with chlorine atoms is hexachlorobenzene C 6 Cl 6 - a fungicide; it is used for dry seed dressing of wheat and rye against hard smut. The product of the addition of chlorine to benzene is hexachlorocyclohexane (hexachloran) C 6 H 6 Cl 6 - an insecticide; it is used to control harmful insects. These substances refer to pesticides - chemical means of combating microorganisms, plants and animals.

Styrene C 6 H 5 - CH \u003d CH 2 polymerizes very easily, forming polystyrene, and copolymerizing with butadiene - styrene-butadiene rubbers.

VIDEO EXPERIENCES

aromatic hydrocarbons- compounds of carbon and hydrogen, in the molecule of which there is a benzene ring. The most important representatives of aromatic hydrocarbons are benzene and its homologues - the products of substitution of one or more hydrogen atoms in the benzene molecule for hydrocarbon residues.

The structure of the benzene molecule

The first aromatic compound, benzene, was discovered in 1825 by M. Faraday. Its molecular formula was established - C 6 H 6. If we compare its composition with the composition of the saturated hydrocarbon containing the same number of carbon atoms, hexane (C 6 H 14), we can see that benzene contains eight hydrogen atoms less. As is known, the appearance of multiple bonds and cycles leads to a decrease in the number of hydrogen atoms in a hydrocarbon molecule. In 1865, F. Kekule proposed its structural formula as cyclohexantriene - 1, 3, 5.

So the molecule corresponding to Kekule formula, contains double bonds, therefore, benzene must have an unsaturated character, i.e., it is easy to enter into addition reactions: hydrogenation, bromination, hydration, etc.

However, the data of numerous experiments have shown that benzene enters into addition reactions only under harsh conditions (at high temperatures and light), and is resistant to oxidation. The most characteristic of it are substitution reactions, therefore, benzene is closer in character to the marginal hydrocarbons.

Trying to explain these inconsistencies, many scientists have proposed various options for the structure of benzene. The structure of the benzene molecule was finally confirmed by the reaction of its formation from acetylene. In fact, the carbon-carbon bonds in benzene are equivalent, and their properties are not similar to those of either single or double bonds.

Currently, benzene is denoted either by the Kekule formula, or by a hexagon in which a circle is depicted.

So what is the peculiarity of the structure of benzene? Based on the researchers' data and calculations, it was concluded that all six carbon atoms are in the state sp 2 hybridization and lie in the same plane. unhybridized p-orbitals of carbon atoms that make up double bonds (Kekule formula) are perpendicular to the plane of the ring and parallel to each other.

They overlap with each other, forming a single π-system. Thus, the system of alternating double bonds depicted in the Kekule formula is a cyclic system of conjugated, overlapping α-bonds. This system consists of two toroidal (donut-like) regions of electron density lying on both sides of the benzene ring. Thus, it is more logical to depict benzene as a regular hexagon with a circle in the center (π-system) than as cyclohexatriene-1,3,5.

The American scientist L. Pauling proposed to represent benzene as two boundary structures that differ in the distribution of electron density and constantly transform into each other, that is, to consider it an intermediate compound, an "averaging" of two structures.

The measured bond lengths confirm these assumptions. It was found that all C-C bonds in benzene have the same length (0.139 nm). They are somewhat shorter than single C-C ties(0.154 nm) and longer doubles (0.132 nm).

There are also compounds whose molecules contain several cyclic structures.

Isomerism and nomenclature

The benzene homologues are characterized by position isomerism of several substituents. The simplest benzene homologue, toluene (methylbenzene), does not have such isomers; the following homologue is presented as four isomers:

The basis of the name of an aromatic hydrocarbon with small substituents is the word benzene. Atoms in an aromatic ring are numbered from the highest substituent to the youngest:

According to the old nomenclature, positions 2 and 6 are called ortho positions, 4 - pair-, and 3 and 5 - metapositions.

Physical Properties
Benzene and its simplest homologues under normal conditions are very toxic liquids with a characteristic unpleasant odor. They are poorly soluble in water, but well - in organic solvents.

Chemical properties of benzene

Substitution reactions. Aromatic hydrocarbons enter into substitution reactions.
1. Bromination. When reacting with bromine in the presence of a catalyst, iron bromide (ΙΙΙ), one of the hydrogen atoms in the benzene ring can be replaced by a bromine atom:

2. Nitration of benzene and its homologues. When an aromatic hydrocarbon interacts with nitric acid in the presence of sulfuric acid (a mixture of sulfuric and nitric acids is called a nitrating mixture), a hydrogen atom is replaced by a nitro group -NO 2:

By reducing the nitrobenzene formed in this reaction, aniline is obtained - a substance that is used to obtain aniline dyes:

This reaction is named after the Russian chemist Zinin.
Addition reactions. Aromatic compounds can also enter into addition reactions to the benzene ring. In this case, cyclohexane or its derivatives are formed.
1. hydrogenation. The catalytic hydrogenation of benzene proceeds at a higher temperature than the hydrogenation of alkenes:

2. Chlorination. The reaction proceeds under illumination with ultraviolet light and is a free radical:

Benzene homologues

The composition of their molecules corresponds to the formula C n H 2 n-6. The closest homologues of benzene are:

All benzene homologues following toluene have isomers. Isomerism can be associated both with the number and structure of the substituent (1, 2), and with the position of the substituent in the benzene ring (2, 3, 4). Compounds of the general formula C 8 H 10:

According to the old nomenclature used to indicate the relative position of two identical or different substituents in the benzene ring, prefixes are used ortho- (abbreviated o-) - substituents are located at neighboring carbon atoms, meta-(m-) - through one carbon atom and pair— (P-) - substitutes against each other.
The first members of the homologous series of benzene are liquids with a specific odor. They are lighter than water. They are good solvents.

Benzene homologues react substitution ( bromination, nitration). Toluene is oxidized by permanganate when heated:

Benzene homologues are used as solvents, for the production of dyes, plant protection products, plastics, and medicines.

DEFINITION

Aromatic hydrocarbons (arenes)- substances whose molecules contain one or more benzene rings. General formula homologous series of benzene C n H 2 n -6

The simplest representatives of aromatic hydrocarbons are benzene - C 6 H 6 and toluene - C 6 H 5 -CH 3. Hydrocarbon radicals derived from arenes are called: C 6 H 5 - - phenyl (Ph-) and C 6 H 5 -CH 2 - - benzyl.

All six carbon atoms in the benzene molecule are in the sp 2 hybrid state. Each carbon atom forms 3σ bonds with two other carbon atoms and one hydrogen atom lying in the same plane. Six carbon atoms form a regular hexagon (σ-skeleton of the benzene molecule).

Each carbon atom has one unhybridized p-orbital, which contains one electron. Six p-electrons form a single π-electron cloud (aromatic system), which is depicted as a circle inside a six-membered cycle.

Chemical properties of arenes

Benzene and its homologues are characterized by substitution reactions proceeding according to the electrophilic mechanism:

- halogenation (benzene interacts with chlorine and bromine in the presence of catalysts - anhydrous AlCl 3, FeCl 3, AlBr 3)

C 6 H 6 + Cl 2 \u003d C 6 H 5 -Cl + HCl

- nitration (benzene easily reacts with a nitrating mixture - a mixture of concentrated nitric and sulfuric acids)

- alkylation with alkenes

C 6 H 6 + CH 2 \u003d CH-CH 3 → C 6 H 5 -CH (CH 3) 2

Addition reactions to benzene lead to the destruction of the aromatic system and proceed only under harsh conditions:

- hydrogenation (the reaction proceeds when heated, the catalyst is Pt)

- addition of chlorine (occurs under the action of UV radiation with the formation of a solid product - hexachlorocyclohexane (hexachlorane) - C 6 H 6 Cl 6)

Physical properties of arenes

The first members of the benzene homologous series are colorless liquids with a specific odor. They are lighter than water and practically insoluble in it. They dissolve well in organic solvents and are themselves good solvents.

Getting arenas

The main methods for obtaining benzene and its homologues:

- dehydrocyclization of alkanes (catalysts - Pt, Cr 3 O 2)

– dehydrogenation of cycloalkanes (the reaction proceeds when heated, the catalyst is Pt)

– trimerization of acetylene (the reaction proceeds when heated to 600C, the catalyst is activated carbon)

3HC≡CH → C 6 H 6

- alkylation of benzenes (Friedel-Crafts reaction) (catalyst - aluminum chloride or phosphoric acid)

Examples of problem solving

EXAMPLE 1

Exercise

The vapor density of the substance is 3.482 g/l. Its pyrolysis yielded 6 g of soot and 5.6 liters of hydrogen. Determine the formula for this substance.

Solution

Find the amount of soot (carbon) substance: v(C) = m(C)/M(C) v(C) = 6/12 = 0.5 mol Find the amount of hydrogen substance: v (H 2) \u003d V (H 2) / V m v (H 2) \u003d 5.6 / 22.4 \u003d 0.25 mol Therefore, the amount of substance of one hydrogen atom will be equal to: v(H) \u003d 2 × 0.25 \u003d 0.5 mol Let's denote the number of carbon atoms in the hydrocarbon molecule as x, and the number of hydrogen atoms as y, then the ratio of these atoms in the molecule: x:y = 0.5: 0.5 = 1:1 The simplest formula of the hydrocarbon CH The molecular weight of a hydrocarbon is: M(C x H y) \u003d ρ × V m \u003d 3.482 × 22.4 \u003d 78 g / mol The molecular weight of a molecule of CH composition is: M(CH) = 13 g/mol n \u003d M (C x H y) / M (CH) \u003d 78/13 \u003d 6, therefore, the coefficients x and y must be multiplied by 6, then the desired hydrocarbon has the composition C 6 H 6 - this is benzene

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.

VAll 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- (O-) 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, O-, 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 π -electronic system 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.