These are cyclic hydrocarbons with three double conjugated bonds in the cycle.
Benzene C 6 H 6- the ancestor of aromatic hydrocarbons. It was first isolated by Faraday in 1825 from lighting gas.
Each of the six carbon atoms in its molecule is in the state sp 2 hybridizations and is linked to two adjacent carbon atoms and a hydrogen atom by three σ-bonds. The bond angles between each pair of π-bonds are 120 0 .
Thus, the skeleton of σ bonds is a regular hexagon in which all carbon atoms and all C–C and C–H σ bonds lie in the same plane.
p-electrons of all carbon atoms form a single cyclic π-electron cloud, concentrated above and below the plane of the ring.
All C–C bonds in benzene are equivalent, their length is 0.140 nm, which corresponds to an intermediate value between single and double.
This means that in the benzene molecule there are no purely simple and double bonds between carbon atoms (as in the formula proposed in 1865 by the German chemist F. Kekule), and they are all aligned (delocalized).
General formula for the homologous series of benzene C n H 2n-6(n ≥ 6).
If there are two or more radicals, their position is indicated by the numbers of the carbon atoms in the ring to which they are attached. The ring is numbered so that the numbers of radicals are the smallest.
For disubstituted benzenes
R-C 6 H 4 -R"
Another way of constructing names is also used:
ortho- (O-)
substituents at adjacent carbon atoms of the ring, 1,2-;
meta- (m-)
substituents through one carbon atom (1,3-);
pair-(P-)
substituents on opposite sides of the (1,4-) ring.
Isomerism in arenes.
It is determined by the number of substituents, their location in the benzene ring and the possibility of isomerism of the carbon skeleton in substituents containing more than three carbon atoms.
For an aromatic hydrocarbon C 8 H 10 there are 4 isomers: ortho-, meta- and para-xylenes and ethylbenzene.
PRODUCTION OF AROMATIC HYDROCARBONS
Dehydrogenation of cycloalkanes
2. Dehydrocyclization (dehydrogenation and cyclization) of alkanes in the presence of a catalyst
3.Trimerization of acetylene over activated charcoal Zelinsky's reaction):
4.Alkylation of benzene with haloalkanes in the presence of anhydrous aluminum chloride, or alkenes:
PHYSICAL PROPERTIES.
Benzene and its closest homologues are colorless liquids with a characteristic odor, with a density of less than 1 g/ml. Flammable. Insoluble in water, but highly soluble in non-polar solvents. Benzene and toluene are poisonous (affect the kidneys, liver, bone marrow, blood).
The higher arenas are solids.
CHEMICAL PROPERTIES.
Due to the presence delocalized -system arenas are not characterized by addition or oxidation reactions that lead to a violation of aromaticity. They are most characteristic electrophilic substitution reactions hydrogen atoms associated with the cycle - S E.
1. REACTIONS OF ADDITION TO ARENES
In addition reactions leading to the destruction of the aromatic structure of the benzene ring, arenes can enter with great difficulty.
A. hydrogenation. The addition of hydrogen to benzene and its homologues occurs at elevated temperature and pressure in the presence of metal catalysts.
b. Radical chlorination. With the radical chlorination of benzene, hexachlorocyclohexane is obtained - "hexachloran" (a means of combating harmful insects).
2. REACTIONS OF RADICAL SUBSTITUTION OF HYDROGEN ATOMS IN SIDE CHAIN:
In the case of benzene homologues, under the action of chlorine in the light or on heating, the reaction occurs radical substitution in side chain:
3. Arene oxidation reactions
Benzene is not oxidized even under the influence of strong oxidizing agents (KMnO 4 , K 2 Cr 2 O 7 , etc.). Therefore, it is often used as an inert solvent in the oxidation reactions of other organic compounds.
Unlike benzene, its homologues are oxidized quite easily. Under the action of a solution of KMnO 4 in an acidic environment and heating in benzene homologues, only side chains undergo oxidation, while the carboxyl group remains from the side chain, and the rest goes into carbon dioxide:
5C 6 H 5 - CH 3+6KMnO 4 +9H 2 SO 4 à5C 6 H 5 - COOH+6MnSO4 +3K2SO4 +14H2O
5C 6 H 5 - CH 2-CH 3+12KMnO 4 +18H 2 SO 4 à5C 6 H 5 - COOH+5CO 2+12MnSO4 +
6K 2 SO 4 +28H 2 O
If oxidation occurs in a neutral solution when heated, then a salt of benzoic acid and potassium carbonate are formed:
C 6 H 5 - CH 2-CH 3+4KMnO 4 àC 6 H 5 – COO K+K 2 CO3+4MnO2 +KOH+2H2O
4. SUBSTITUTION REACTIONS IN THE BENZENE RING
Halogenation
The replacement of the hydrogen atom in the benzene ring by a halogen occurs in the presence of catalysts AlCl 3 , AlBr 3 , FeCl 3 , etc.:
Nitration
Benzene reacts with a nitrating mixture (a mixture of concentrated nitric and sulfuric acids):
Alkylation
Substitution of a hydrogen atom in the benzene ring with an alkyl group ( alkylation) occurs under the action alkyl halides in the presence of catalysts AlCl 3 , FeBr 3 or alkenes in the presence of phosphoric acid:
Lecture 16
POLYCYCLIC AROMATIC HYDROCARBONS
Lecture outline.
1. Polycyclic aromatic hydrocarbons with isolated rings
1.1 Biphenyl group
1.2. Polyphenylmethanes
2. Condensed benzenoid hydrocarbons
2.1 Naphthalene
2.2. Anthracene, phenanthrene
1. Polycyclic aromatic hydrocarbons with isolated rings
There are two groups of polycyclic aromatic hydrocarbons (arenes) with several benzene rings.
1. Hydrocarbons with isolated rings. These include biphenyl and di- and triphenylmethanes.
2. Hydrocarbons with condensed rings or benzoid hydrocarbons. These include naphthalene, anthracene, and phenanthrene.
1.1. Biphenyl group
Definition: Aromatic compounds in which two (or more) rings (rings) are connected to each other by a single bond are called polycyclic aromatic hydrocarbons with isolated rings.
The simplest aromatic hydrocarbon compound with isolated rings is biphenyl. The positions of the substituents in the biphenyl formula are indicated by numbers. In one ring, the numbers are not marked: 1, 2 ..... In the second ring, the numbers are marked with a stroke 1, 2, etc.:
Scheme 1.
Biphenyl is a crystalline substance with T pl. 70 0 C, T b.p. 254 0 C, has a wide application due to thermal and chemical resistance. It is used in industry as a high-temperature coolant. In industry, biphenyl is produced by pyrolysis of benzene:
Scheme 2.
The laboratory method of obtaining is the action of sodium or copper on iodobenzene
Scheme 3.
The reaction proceeds especially smoothly in the presence of electron-withdrawing substituents in the aryl halides, which increase the mobility of the halogen in the nucleus:
The most important derivative of biphenyl is the diamine benzidine. It is usually obtained by reducing nitrobenzene to hydrazobenzene and isomerizing the latter under the influence of acids:
Scheme 5.
Benzidine is the starting material for the production of many substantive (direct) dyes. The presence of two amino groups that can be diazotized makes it possible to obtain bis-azo dyes with a deep color. An example of a dye derived from benzidine is the Congo red indicator:
Scheme 6.
In the crystalline state, both benzene rings of biphenyl lie in the same plane. In solution and in the gaseous state, the angle between the planes of the rings is 45 0 . The exit of benzene rings from the plane is explained by the spatial interaction of hydrogen atoms in positions 2, 2 and 6, 6:
Scheme 7.
If there are large substituents in the ortho positions, then rotation about the C-C bond becomes difficult. If the substituents are not the same, then the corresponding derivatives can be separated into optical isomers. This form of spatial isomerism is called rotational optical isomerism or atropisomerism.
Scheme 8.
Biphenyl participates much more actively than benzene in electrophilic aromatic substitution reactions. Bromination of biphenyl with an equimolar amount of bromine leads to the formation of 4-bromobiphenyl. An excess of bromine leads to the formation of 4,4`-dibromobiphenyl:
Scheme 9.
Biphenyl nitration reactions, Friedel-Crafts acylation, and other electrophilic aromatic substitution reactions proceed similarly.
1.2. Polyphenylmethanes
Definition:
Aromatic compounds in which from two to four benzene rings are connected to one carbon atom in the state of sp 3 hybridization.
The founder of the homologous series of polyphenylmethane is toluene, the following compound is diphenylmethane:
Scheme 10.
Di- and triphenylmethane are produced using benzene by the Friedel-Crafts reaction by two methods:
1. From methylene chloride and chloroform:
Scheme 11.
2. From benzyl chloride and benzylidene chloride:
Scheme 12..
Diphenylmethane is a crystalline substance with T pl. 26-27 0 C, has the smell of orange.
When diphenylmethane is oxidized, benzophenone is formed:
Scheme 13.
Triphenylmethane is a crystalline substance with T pl. 92.5 0 C. With benzene gives a crystalline molecular compound T pl. 78 0 C. Triphenylmethane is easily oxidized to triphenylcarbinol. The hydrogen atom in its molecule is easily replaced by metals and halogens. In turn, triphenylcarbinol under the action of hydrogen chloride triphenylchloromethane. Triphenylchloromethane upon reduction forms triphenylmethane, and upon hydrolysis, triphenylcarbinol:
Scheme 14..
The structure of triphenylmethane forms the basis of the so-called dyes of the triphenylmethane series. Aminotriphenylmethanes are colorless substances, they are called leuco compounds (from the Greek leukos - white, colorless). When oxidized in an acidic medium, they form colored salts. In these salts, the color carrier (chromophore) is a conjugated ion with a positive charge distributed between the carbon and nitrogen atoms. The most prominent representative of this group is malachite green. It is obtained by the Friedel-Crafts reaction:
Scheme 15.
During the oxidation of the leuco compound, a system of conjugated bonds is formed through the benzene ring between the nitrogen atom and the carbon of the triphenylmethane system, which has passed into the state of sp 2 hybridization. Such a structure is called quinoid. The presence of a quinoid structure ensures the appearance of a deep intense color.
The widely used indicator phenolphthalein belongs to the group of triphenylmethane dyes. Obtained by the reaction of phenol and phthalic anhydride (phthalic anhydride) in the presence of sulfuric acid:
Scheme 16.
2. Condensed benzenoid hydrocarbons
Hydrocarbons containing two or more benzene rings sharing two carbon atoms are called fused benzenoid hydrocarbons.
2.1. Naphthalene
The simplest of the condensed benzoic hydrocarbons is naphthalene:
Scheme 17.
Positions 1,4,5 and 8 are designated "α", positions 2, 3,6,7 are designated "β". Accordingly, for naphthalene, the existence of two monosubstituted ones, which are called 1 (α)- and 2 (β)-derivatives, and ten disubstituted isomers is possible, for example:
Scheme 18.
Ways to get.
The bulk of naphthalene is obtained from coal tar.
In laboratory conditions, naphthalene can be obtained by passing benzene and acetylene vapors over charcoal:
Scheme 19.
Dehydrocyclization over platinum of benzene homologues with a side chain of four or more carbon atoms:
Scheme 20.
By the reaction of the diene synthesis of 1,3-butadiene with P-benzoquinone:
Scheme 21.
A convenient laboratory method for obtaining naphthalene and its derivatives is a method based on the Friedel-Crafts reaction:
Scheme 22.
Naphthalene is a crystalline substance with T pl. 80 0 C, characterized by high volatility.
Naphthalene enters into electrophilic substitution reactions more easily than benzene. In this case, the first substituent almost always becomes in the α-position, since in this case an energetically more favorable σ-complex is formed than with substitution in the β-position. In the first case, the σ-complex is stabilized by the redistribution of electron density without disturbing the aromaticity of the second ring; in the second case, such stabilization is not possible:
Scheme 23.
A number of electrophilic substitution reactions in naphthalene:
Scheme 24.
Entry of an electrophilic agent into the β-position is less common. As a rule, this occurs in specific conditions. In particular, the sulfonation of naphthalene at 60 0 C proceeds as a kinetically controlled process, with the predominant formation of 1-naphthalenesulfonic acid. Sulfonation of naphthalene at 160 0 C proceeds as a thermodynamically controlled process and leads to the formation of 2-naphthalenesulfonic acid:
Scheme 25.
The place of entry of the second substituent into the naphthalene system is determined by:
1. orientational influence of an existing substituent;
2. Differences in the reactivity of the α and β positions.
In this case, the following conditions are met:
1. If one of the naphthalene rings has a substituent of the first kind, then the new substituent enters the same ring. A substituent of the first kind in the 1(α)-position sends the second substituent, mainly to 4( pair)-position. Isomer with a second substituent in 2( ortho)-position is formed in small quantities, for example:
Scheme 26.
The electron-withdrawing substituents located in the naphthalene molecule direct the attack to another ring in the 5th and 8th positions:
Scheme 27.
Scheme 28.
Oxidation of naphthalene with atmospheric oxygen using vanadium pentoxide as a catalyst leads to the formation of phthalic anhydride:
Scheme 29.
Naphthalene can be reduced by the action of various reducing agents with the addition of 1, 2 or 5 moles of hydrogen:
Scheme 30.
2.2. Anthracene, phenanthrene
By building up another ring from naphthalene, two isomeric hydrocarbons can be obtained - anthracene and phenanthrene:
Scheme 31..
Positions 1, 4, 5 and 8 are designated "α", positions 2, 3, 6 and 7 are designated "β", positions 9 and 10 are designated "γ" or "meso" - the middle position.
Ways to get.
The bulk of anthracene is obtained from coal tar.
Under laboratory conditions, anthracene is obtained by the Friedel-Crafts reaction from benzene or with tetrabromoethane:
Scheme 32.
or by reaction with phthalic anhydride:
Scheme 33.
As a result of the first stage of the reaction, anthraquinone is obtained, which is easily reduced to anthracene, for example, with sodium borohydride.The Fittig reaction is also used, according to which the anthracene molecule is obtained from two molecules ortho- bromobenzyl bromide:
Scheme 34.
Properties:
Anthracene is a crystalline substance with T pl. 213 0 C. All three benzene rings of anthracene lie in the same plane.
Anthracene easily adds hydrogen, bromine and maleic anhydride to positions 9 and 10:
Scheme 35.
The bromine addition product easily loses hydrogen bromide to form 9-bromoanthracene.
Under the action of oxidizing agents, anthracene is easily oxidized to anthraquinone:
Scheme 36.
Phenantrene, as well as anthracene, is a constituent of coal tar.
Just like anthracene, phenanthrene adds hydrogen and bromine to the 9 and 10 positions:
Scheme 37.
Under the action of oxidizing agents, phenanthrene is easily oxidized to phenanthrenquinone, which is further oxidized to 2,2`-bifenic acid:
Scheme 36.
Demonstration material for the lecture
Scheme 1. Structural formula of biphenyl and the order of designation of the position of substituents in the biphenyl molecule.
Scheme 2.
Scheme for the synthesis of biphenyl by pyrolysis of benzene.
Scheme 3. Scheme for the synthesis of biphenyl from iodobenzene.
Scheme 4.
Scheme for the synthesis of biphenyl according to the Ullmann reaction.
Scheme 5. Scheme for the synthesis of benzidine.
Scheme 6. Congo indicator is red.
Scheme 7. Scheme of steric interactions of hydrogen atoms in ortho- and ortho-provisions.
Scheme 8. Rotational optical isomers.
Scheme 9. Scheme of the electrophilic substitution reaction.
The following compound is diphenylmethane: 
Scheme 10. Polyphenylmethanes.
Scheme 11. Scheme for the synthesis of di- and triphenylmethane, methylene chloride and chloroform.
Scheme 12. Scheme for the synthesis of di- and triphenylmethane from benzyl chloride and benzylidene chloride.
Scheme 13. Scheme of diphenylmethane oxidation.
Scheme 14. Reactions involving derivatives of triphenylmethane.
Scheme 15. Scheme for the synthesis of malachite green dye.
Scheme 16. Scheme for the synthesis of the indicator phenolphthalein.
Scheme 18. Naphthalene derivatives.
Ways to get.
POLYCYCLIC AROMATIC HYDROCARBONS WITH ISOLATED CYCLES
Aromatic hydrocarbons with multiple benzene rings are divided into:
1. Hydrocarbons with non-condensed cycles. These include biphenyl and di- and triphenylmethanes.
2. Hydrocarbons with condensed cycles. These include naphthalene, anthracene and phenanthrene.
Biphenyl group
Definition: Aromatic compounds in which two (or more) rings (rings) are connected to each other by a single bond are called polycyclic aromatic hydrocarbons with isolated rings.
Biphenyl is considered as an example:
In industry, biphenyl is produced by pyrolysis of benzene:
The laboratory method of preparation is the action of sodium or copper on iodobenzene, or in the presence of electron-withdrawing substituents in the aryl halides, which increase the mobility of the halogen in the nucleus:
Biphenyl is a crystalline substance with T pl. 70 0 C, T b.p. 254 0 C. Thermodynamically stable. It is used in industry as a high-temperature coolant.
Biphenyl participates much more actively than benzene in electrophilic aromatic substitution reactions. Bromination of biphenyl with an equimolar amount of bromine leads to the formation of 4-bromobiphenyl. An excess of bromine leads to the formation of 4,4`-dibromobiphenyl:
Biphenyl nitration reactions, Friedel-Crafts acelation, and other electrophilic aromatic substitution reactions proceed similarly.
Polyphenylmethanes
Definition: Aromatic compounds in which from two to four benzene rings are connected to one carbon atom in the state of sp 3 hybridization.
The founder of the homologous series of polyphenylmethane is toluene, the following compound is diphenylmethane:
Di- and triphenylmethane are produced using benzene by the Friedel-Crafts reaction by two methods:
1. From methylene chloride and chloroform:
2. From benzyl chloride and benzylidene chloride:
Diphenylmethane is a crystalline substance with T pl. 26-27 0 C, has the smell of orange.
When diphenylmethane is oxidized, benzophenone is formed:
The structure of triphenylmethane forms the basis of the so-called dyes of the triphenylmethane series:
1. Malachite green (brilliant green) is obtained by the Friedel-Crafts reaction:
2. Phenolphthalein.
Obtained by the reaction of phenol and phthalic anhydride (phthalic anhydride) in the presence of sulfuric acid:
CONDENSED BENZOID HYDROCARBONS
Hydrocarbons containing two or more benzene rings sharing two carbon atoms are called fused benzenoid hydrocarbons.
Naphthalene
The simplest of the condensed benzoic hydrocarbons is naphthalene:
Positions 1,4,5 and 8 are designated "α", positions 2, 3,6,7 are designated "β".
Ways to get.
The bulk of naphthalene is obtained from coal tar.
In laboratory conditions, naphthalene can be obtained by passing benzene and acetylene vapors over charcoal:
Dehydrocyclization over platinum of benzene homologues with a side chain of four or more carbon atoms:
By the reaction of the diene synthesis of 1,3-butadiene with P-benzoquinone:
Naphthalene is a crystalline substance with T pl. 80 0 C, characterized by high volatility.
Naphthalene enters into electrophilic substitution reactions more easily than benzene. In this case, the first substituent almost always becomes in the α-position:
Entry of an electrophilic agent into the β-position is less common. As a rule, this occurs in specific conditions. In particular, the sulfonation of naphthalene at 60 0 C proceeds as a kinetically controlled process with the predominant formation of 1-naphthalenesulfonic acid. Sulfonation of naphthalene at 160 0 C proceeds as a thermodynamically controlled process and leads to the formation of 2-naphthalenesulfonic acid:
When a second substituent is introduced into the naphthalene molecule, the orientation is determined by the nature of the substituent already present in it. Electron donor substituents located in the naphthalene molecule direct the attack to the same ring in the 2nd and 4th positions.
S.Yu. Eliseev
The concept of aromatic hydrocarbons, their application, physico-chemical and fire-explosive properties.
Modern understanding of the structure of the benzene molecule. Homologous series of benzene, nomenclature, isomerism. Arene toxicity.
Basic chemical reactions:
substitutions (halogenation, nitration, sulfonation, alkylation)
additions (hydrogen and halogens);
oxidation (incomplete oxidation, features of the combustion process, tendency to spontaneous combustion upon contact with strong oxidizing agents);
Rules of substitution in the benzene ring. Deputy first and second row.
Industrial methods for obtaining aromatic hydrocarbons.
Brief description of the main aromatic hydrocarbons: toluene, benzene, xylene, ethylbenzene, isopropylbenzene, styrene, etc.
Aromatic nitro compounds, physicochemical and fire hazardous properties of nitrobenzene, toluene. Reactions to receive them.
Aromatic amines: nomenclature, isomerism, production methods, individual representatives (aniline, diphenylamine, dimethylaniline).
Aromatic hydrocarbons (arenes)
Aromatic compounds are usually called carbocyclic compounds, in the molecules of which there is a special cyclic group of six carbon atoms - the benzene ring. The simplest substance containing such a group is the hydrocarbon benzene; all other aromatic compounds of this type are considered to be derivatives of benzene.
Due to the presence of a benzene ring in aromatic compounds, they differ significantly in some properties from saturated and unsaturated alicyclic compounds, as well as from compounds with an open chain. The distinctive properties of aromatic substances, due to the presence of a benzene nucleus in them, are usually called aromatic properties, and the benzene nucleus, respectively, the aromatic nucleus.
It should be noted that the very name "aromatic compounds" no longer has its original direct meaning. This was the name of the first studied benzene derivatives, because they had an aroma or were isolated from natural aromatic substances. At present, aromatic compounds include many substances that have both unpleasant odors or no smell at all if their molecule contains a flat ring with (4n + 2) generalized electrons, where n can take on the values 0, 1, 2, 3, etc. .d., is Hückel's rule.
Aromatic hydrocarbons of the benzene series.
The first representative of aromatic hydrocarbons - benzene - has the composition C6H6. This substance was discovered by M. Faraday in 1825 in a liquid formed during compression or cooling of the so-called. lighting gas, which is obtained during the dry distillation of coal. Subsequently, benzene was discovered (A. Hoffman, 1845) in another product of the dry distillation of coal - in coal tar. It turned out to be a very valuable substance and found wide application. Then it was found that very many organic compounds are derivatives of benzene.
The structure of benzene.
For a long time the question of the chemical nature and structure of benzene remained unclear. It would seem that it is a strongly unsaturated compound. After all, its composition C6H6 according to the ratio of carbon and hydrogen atoms corresponds to the formula CnH2n-6, while the saturated hydrocarbon corresponding to the number of carbon atoms hexane has the composition C6H14 and corresponds to the formula CnH2n+2. However, benzene does not give reactions characteristic of unsaturated compounds; for example, it does not provide bromine water and KMnO4 solution; under normal conditions, it is not prone to addition reactions, it does not oxidize. On the contrary, benzene in the presence of catalysts enters into substitution reactions characteristic of saturated hydrocarbons, for example, with halogens:
C6H6 + Cl2 ® C6H5Cl + HCl
It turned out, however, that under certain conditions, benzene can also enter into addition reactions. There, in the presence of catalysts, it is hydrogenated, adding 6 hydrogen atoms:
C6H6 + 3H2 ® C6H12
Under the action of light, benzene slowly adds 6 halogen atoms:
C6H6 + 3Cl2 ® C6H6Cl6
Some other addition reactions are also possible, but they all proceed with difficulty, many times less actively than addition to double bonds in substances with an open goal or in alicyclic compounds.
Further, it was found that monosubstituted derivatives of benzene C6H5X do not have isomers. This showed that all hydrogen and all carbon atoms in its molecule are equivalent in their position, which also did not find an explanation for a long time.
For the first time, the formula for the structure of benzene was proposed in 1865. German chemist August Kekule. He suggested that the 6 carbon atoms in benzene form a cycle, connecting to each other by alternating single and double bonds, and, in addition, each of them is connected to one hydrogen atom: CH CH CH CH CH Kekule suggested that the double bonds in benzene not motionless; according to him, they continuously move (oscillate) in the ring, which can be represented by the scheme: CH (I) CH (II) Formulas I and II, according to Kekule, CH CH CH CH are completely equivalent and only ½½<=>½½ express 2 mutually passing CH CH CH CH CH phases of the compound of the benzene molecule. CH CHKekule came to this conclusion on the basis that if the position of double bonds in benzene was fixed, then its disubstituted derivatives C6H4X2 with substituents at neighboring carbons should exist in the form of isomers at the position of single and double bonds:
½ (III) ½ (IV)
C CNS S-X NS S-X
½½½<=>½½½
The Kekule formula has become widespread. It is consistent with the concept of tetravalent carbon, explains the equivalence of hydrogen atoms in benzene. The presence of a six-membered cycle in the latter has been proved; in particular, it is confirmed by the fact that during hydrogenation benzene forms cyclohexane, in turn, cyclohexane turns into benzene by dehydrogenation.
However, the Kekule formula has significant drawbacks. Assuming that there are three double bonds in benzene, she cannot explain why benzene in this case hardly enters into addition reactions, is resistant to the action of oxidizing agents, i.e. does not show the properties of unsaturated compounds.
The study of benzene using the latest methods indicates that in its molecule between carbon atoms there are neither ordinary simple nor ordinary double bonds. For example, the study of aromatic compounds using X-rays showed that 6 carbon atoms in benzene, forming a cycle, lie in the same plane at the vertices of a regular hexagon and their centers are at equal distances from each other, constituting 1.40 A. These distances are smaller, than the distances between the centers of carbon atoms connected by a single bond (1.54 A), and more than m. connected by a double bond (1.34 A). Thus, in benzene, carbon atoms are connected using special, equivalent bonds, which were called aromatic bonds. By their nature, they differ from double and single bonds; their presence determines the characteristic properties of benzene. From the point of view of modern electronic concepts, the nature of aromatic bonds is explained as follows.
