The main sources of production are oil and products of dry distillation (coking) of coal. The separation of aromatic hydrocarbons from coal tar is the oldest and until the 1950s the main method for their production. When heated above 1000 ºС without air access, coal decomposes with the formation of solid (coke), liquid (coal tar, ammonia water) and gaseous (coke oven gases) products of distillation.
Coke– mostly carbon; used in metallurgy.
coking gases- H 2 , CH 4 , CO, CO 2 , N 2 , ethylene hydrocarbons.
Coal tar- contains a large number of organic compounds of various nature. Resin yield about 3%. At the first stage, it is distilled into 4 fractions (Table 11).
Table 11
The remainder of the distillation (60%) is called pitch. It is a hard, dark-colored mass that softens when heated.
Individual organic compounds are isolated from the listed fractions by various methods.
In some types of oil, the content of aromatic hydrocarbons reaches 60%. Nevertheless, most of them are obtained from oil during chemical processing (oil aromatization) - pyrolysis and catalytic reforming, during which dehydrogenation (a) and dehydrocyclization (b) reactions occur:
(a) 
;
cyclohexane benzene
n-hexane benzene
A synthetic method for producing benzene is acetylene trimerization (see Section 5.2.5). Benzene homologues are obtained by alkylation according to the Friedel-Crafts method (Section 6.2.1) or the Wurtz-Fittig method:
bromobenzene butyl bromide butylbenzene
(R. Fittig in 1864 extended the reaction of S. Wurtz to aromatic hydrocarbons for the alkylation and acylation of benzene).
Arenas are extremely versatile.
Benzene, toluene, xylenes are widely used organic solvents and the basis of large-scale organic synthesis - dyes, explosives (TNT), plastics (polystyrene, lavsan), drugs, plant protection products, etc.
Bibliography
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4. Grandberg I.I. Organic Chemistry: Proc. allowance for agricultural universities. - 2nd ed., revised. and additional - M.: Higher School, 1980. - 463 p.
5. Karrer P. Course of organic chemistry. 2nd ed. - L .: Goshimizdat, 1962. - 1216 p.
6. Roberts J., Caserio M. Fundamentals of organic chemistry. - M.: Mir, 1968. - Part 1. - 592 p.; 1968. - Part 2. - 550 p.
7. Kahn R., Dermer O. Introduction to chemical nomenclature. - M.: Chemistry, 1983. - 224 p.
8. Volkov V.A. Vonsky E.V., Kuznetsova G.I. Outstanding Chemists of the World: A Biographical Guide. - M .: Higher School, 1991.
9. Brief chemical encyclopedia. – M.: Sov. Encyclopedia, 1961. - T. 1. - 1262 p.; 1963. - T. 2. - 1086 p.; 1964. - T. 3. - 1112 p.; 1965. - T. 4. - 1182 p.; 1967. - T. 5. - 1184 p.
10. Chmutov K.V. Chromatography. - M.: Chemistry, 1978. - 128 p.
11. Azimov A. The world of carbon. - M.: Chemistry, 1978. - 208 p.
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Benzene is obtained from coal tar formed during the coking of coal, oil, by synthetic methods.
1. Obtaining from aliphatic hydrocarbons. When straight-chain alkanes having at least six carbon atoms per molecule are passed over heated platinum or chromium oxide, dehydrocyclization- formation of an arene with the release of hydrogen: the method of B.A. Kazansky and A.F. Plate
2. Dehydrogenationcycloalkanes (N.D. Zelinsky) The reaction occurs by passing vapors of cyclohexane and its homologues over heated platinum at 3000 0 .
3. Obtaining benzene trimerization of acetylene over activated carbon at 600 0(N.D. Zelinsky )
| 3HC?CH | -- 600?C? |
4. Fusion of salts of aromatic acids with alkali or soda lime:
5. Chemical properties of arenes.
The benzene core has high strength. For arenes, the most typical reactions proceed according to the mechanism electrophilic substitution, denoted by the symbol S E (from the English substitution electrophilic).
1. Substitution reactions:
Halogenation . Benzene does not interact with chlorine or bromine under normal conditions. The reaction can proceed only in the presence of catalysts - anhydrous AlCl 3 , FeCl 3 , AlBr 3 . 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:
Nitration . Benzene reacts very slowly with concentrated nitric acid, even when heated strongly. However, with the so-called nitrating mixture (a mixture of concentrated nitric and sulfuric acids) The nitration reaction proceeds quite easily:
Sulfonation. The reaction easily takes place under the action of “fuming” sulfuric acid (oleum):
2. Friedel-Crafts Alkylation. 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 RCl on benzene in the presence of catalysts - aluminum halides. The role of the catalyst is reduced to the polarization of the RСl 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:
Alkylation with alkenes. These reactions are widely used in industry to produce ethylbenzene and isopropylbenzene (cumene). Alkylation is carried out in the presence of AlCl 3 catalyst. The reaction mechanism is similar to that of the previous reaction:
All the above reactions proceed according to the mechanism electrophilic substitution S E . 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.
3. Addition reactions proceeding with bond breaking:
Hydrogenation. The reaction of hydrogen addition to arenes proceeds under heating and high pressure in the presence of metal catalysts (Ni, Pt, Pd). Benzene turns to cyclohexane, a benzene homologues - into cyclohexane derivatives:
Radical halogenation. The interaction of benzene vapor with chlorine proceeds according to the radical mechanism only under the influence of hard ultraviolet radiation. In this case, benzene adds three molecules of chlorine and forms solid product - hexachlorocyclohexane (hexachlorane) C 6 H 6 Cl 6:
4. 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 V 2 O 5 catalyst, a mixture of maleic acid and its anhydride is obtained:
5. Benzene is on fire. (View experience) The flame of benzene is smoky due to the high carbon content in the molecule.
2 C 6 H 6 + 15 O 2 → 12CO 2 + 6H 2 O
6. The use of arenes.
Benzene and its homologues are used as chemical raw materials for the production of medicines, plastics, dyes, acetone, phenol, and formaldehyde plastics. pesticides and many other organic substances. Widely used as solvents. Benzene as an additive improves the quality of motor fuel. Ethylene is used to produce ethyl alcohol, polyethylene. It accelerates the ripening of fruits (tomatoes, citrus fruits) with the introduction of small amounts of it into the air of greenhouses. Propylene is used for the synthesis of glycerin, alcohol, for the extraction of polypropylene, which is used for the manufacture of ropes, ropes, and packaging material. Based on 1-butene, synthetic rubber is produced.
Acetylene is used for autogenous welding of metals. Polyethylene is used as a packaging material, for the manufacture of bags, toys, household utensils (bottles, buckets, bowls, etc.). Aromatic hydrocarbons are widely used in the production of dyes, plastics, chemical pharmaceuticals, explosives, synthetic fibers, motor fuels, and others. are the products of coal coking. From 1 T kam.-ug. resins can be isolated on average: 3.5 kg benzene, 1.5 kg toluene, 2 kg naphthalene. Of great importance is the production of A. at. from fatty hydrocarbons. For some A. at. purely synthetic methods are of practical importance. Thus, ethylbenzene is produced from benzene and ethylene, the dehydrogenation of which leads to styrene.
TASKS FOR SELF-CONTROL:
1. What compounds are called arenas?
2. What are the characteristic physical properties?
3. Task. From 7.8 g of benzene, 8.61 g of nitrobenzene was obtained. Determine the yield (in%) of the reaction product.
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 harsh conditions.
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.
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.
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 benzene homologues by the Friedel-Crafts reaction - see below.
5. Fusion of salts of aromatic acids with alkali:
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.
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:
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:
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 -electron 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 effect, 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.
1. Classification of aromatic hydrocarbons.
2. Homologous series of monocyclic arenes, nomenclature, preparation.
3. Isomerism, the structure of benzene and its homologues.
4. Properties of arenes.
Arenes are called carbon-rich cyclic hydrocarbons that contain a benzene ring in the molecule and have special physical and chemical properties. Arenes are divided into monocyclic (benzene and its homologues) and polycyclic (with condensed and isolated rings) compounds according to the number of benzene rings in the molecule and the way the rings are connected.
Arenes of the benzene series can be considered as products of substitution of hydrogen atoms in the benzene molecule for alkyl radicals. The general formula of such arenes is СnH 2 n- 6. In the name of monosubstituted arenes, the name of the radical and cycle (benzene) is indicated:
benzene methylbenzene (toluene) ethylbenzene.
In more substituted arenas, the position of the radicals is indicated by the smallest numbers; in disubstituted arenes, the position of the radicals is called: 1,2 - ortho ( o-)-, 1,3 - meta ( m-)- and 1,4 - pair ( P-)-:
1,3-dimethylbenzene 1,2-methylethylbenzene
m-dimethylbenzene ( m-xylene) O-methylethylbenzene ( O-xylene)
Trivial names are common for arenas (some names are in brackets).
Finding in nature.
Aromatic hydrocarbons are found in plant resins and balms. Phenantrene in a partially or fully hydrogenated form is found in the structures of many natural compounds, such as steroids, alkaloids.
Getting arenas:
1. dry distillation of coal;
2. dehydrogenation of cycloalkanes
3. dehydrocyclization of alkanes with 6 or more carbon atoms in the composition
4. alkylation
Isomerism. Structural isomerism is characteristic of benzene homologues: different structure of the carbon skeleton of the side radical and different composition and arrangement of radicals in the benzene ring. For example, isomers of aromatic hydrocarbons of the composition C 9 H 12 (propylbenzene, isopropylbenzene, o-methylethylbenzene and 1,2,4-trimethylbenzene):
Structure. Aromatic hydrocarbons have a number of features in the electronic structure of molecules.
The structural formula of benzene was first proposed by A. Kekule. It is a six-membered cycle with alternating double and single bonds, with the double bonds moving around in the structure:
In both formulas, carbon is tetravalent, all carbons are equivalent, and disubstituted benzenes exist as three isomers ( ortho-, meta-, pair-). However, such a structure of benzene contradicted its properties: benzene did not enter into addition reactions (for example, bromine) and oxidation (for example, with potassium permanganate) characteristic of unsaturated hydrocarbons; for it and its homologues, the main type of chemical transformation is substitution reactions.
The modern approach to describing the electronic structure of benzene resolves this contradiction in the following way. The carbon atoms in the benzene molecule are in sp 2 hybridization. Each of the carbon atoms forms three covalent σ-bonds - 2 bonds with neighboring carbon atoms (sp 2 -sp 2 - orbital overlap) and one with a hydrogen atom (sp 2 -s- orbital overlap). Unhybridized p-orbitals form a π-electron conjugated system (π,π-conjugation) containing six electrons due to lateral overlap. Benzene is a flat regular hexagon with a carbon-carbon bond length of 0.14 nm, a carbon-hydrogen bond of 0.11 nm, bond angles of 120 0:
The benzene molecule is more stable than cyclic compounds with isolated double bonds, therefore benzene and its homologues are prone to substitution reactions (the benzene ring is preserved), rather than addition and oxidation.
Other cyclic compounds also show similarity in structure and properties (aromaticity) with benzene. Aromaticity criteria (E. Hückel, 1931):
a) a flat cyclic structure, i.e. the atoms forming the cycle are in sp 2 hybridization; b) coupled electronic system; c) the number of electrons (N) in the ring is 4n+2, where n is any integer value - 0,1,2,3, etc.
The aromaticity criteria apply to both neutral and charged cyclic conjugates, so aromatics would be, for example:
furan cation of cyclopropenyl.
For benzene and other aromatic compounds, the most typical reactions are the substitution of hydrogen atoms at carbon atoms in the cycle, and the reactions of addition to the π-bond in the cycle are less typical.
physical properties.
Benzene and its homologues are colorless liquids and crystalline substances with a peculiar smell. They are lighter than water and do not dissolve well in it. Benzene is a non-polar compound (μ=0), alkylbenzenes -
polar compounds (μ≠0).
Chemical properties.
electrophilic substitution. The most characteristic transformation for arenes is electrophilic substitution - S E. The reaction proceeds in two stages with the formation of an intermediate σ-complex:
Reaction conditions: temperature 60-80 0 С, catalysts - Lewis acids or mineral acids.
Typical S E reactions:
a) halogenation(Cl 2, Br 2):
b) nitration:
v) sulfonation(H 2 SO 4, SO 3, oleum) :
d) Friedel-Crafts alkylation (1877)(RNal, ROH, alkenes) :
e) Friedel-Crafts alkylation(acid halides, anhydrides of carboxylic acids) :
In benzene homologues, as a result of the influence of the side radical (+I-effect, electron-donating group), the π-electron density of the benzene ring is unevenly distributed, increasing in the 2,4,6-positions. Therefore, S E -reactions proceed in a direction (in 2,4,6- or O- and P- provisions). The homologues of benzene are more reactive than benzene in reactions of this type.
toluene P-chlorotoluene O-chlorotoluene
Reactions of side radicals in alkylbenzenes (radical substitution -S R and oxidation).
Radical substitution reactions proceed, as in saturated hydrocarbons, by a chain mechanism and include the stages of chain initiation, growth, and chain termination. The chlorination reaction proceeds non-directionally, the bromination reaction is regioselective - the replacement of hydrogen occurs at the α-carbon atom.
In alkylbenzenes, the side chain is oxidized with potassium permanganate, potassium dichromate to form carboxylic acids. Regardless of the length of the side chain, the carbon atom associated with the benzene nucleus (α-carbon or benzyl carbon atom) is oxidized, the remaining carbon atoms are oxidized to CO 2 or carboxylic acids.
ethylbenzene benzoic acid
P-methylethylbenzene terephthalic acid
Reactions of benzene with violation of the aromatic system.
Aromatic hydrocarbons have a strong cycle, so reactions with violation of the aromatic system (oxidation, radical addition) proceed under harsh conditions (high temperatures, strong oxidizing agents).
a) radical addition:
1. hydrogenation
toluene cyclohexane
2. chlorination
benzene 1,2,3,4,5,6-hexachlorocyclohexane (hexachloran).
The product of this reaction is a mixture of spatial isomers.
Orientation of electrophilic substitution in aromatic compounds. Substituents in the benzene ring are divided into two types according to their orienting effect: ortho-, pair-orientants (substituents of the 1st kind) and meta-orientants (substituents of the 2nd kind).
Type 1 substituents are electron-donating groups that increase the electron density of the ring, increase the rate of electrophilic substitution reactions, and activate the benzene ring in these reactions:
D(+I-effect): -R, -CH 2 OH, -CH 2 NH 2, etc.
D(-I,+M-effects): -NH 2 , -OH, -OR, -NR 2 , -SH etc.
Substituents of the 2nd kind are electron-withdrawing groups that lower the electron density of the ring, reduce the rate of the electrophilic substitution reaction and deactivate the benzene ring in these reactions:
A (-I-effect): -SO 3 H, -CF 3, -CCl 3, etc.
A (-I, -M effect): -HC \u003d O, -COOH, -NO 2, etc.
Halogen atoms occupy an intermediate position - they lower the electron density of the ring, reduce the rate of electrophilic substitution reactions and deactivate the benzene ring in these reactions, however, this O-,P-orientators.
If there are two substituents in the benzene ring, then their orienting action may coincide ( agreed orientation) or not match ( mismatched orientation). In electrophilic substitution reactions, compounds with a consistent orientation form a smaller number of isomers; in the second case, a mixture of a larger number of isomers is formed. For instance:
P-
hydroxybenzoic acid m-
hydroxybenzoic acid
(consistent orientation) (inconsistent orientation)
Polycyclic condensed aromatic hydrocarbons (naphthalene, anthracene, phenanthrene, etc.) are basically similar in properties to benzene, but at the same time they have some differences.
Application:
1. aromatic hydrocarbons - raw materials for the synthesis of dyes, explosives, drugs, polymers, surfactants, carboxylic acids, amines;
2. liquid aromatic hydrocarbons are good solvents for organic compounds;
3. arenas - additives for the production of high-octane gasolines.
Do you know that-In 1649, German chemist Johann Glauber first obtained benzene.
In 1825, M. Faraday isolated a hydrocarbon from the lighting gas and established its composition - C 6 H 6 .
In 1830, Justus Liebig named the resulting compound benzene (from Arabic Ven-aroma + zoa-juice + Latin ol-oil).
In 1837, Auguste Laurent named the benzene radical C 6 H 5 - phenyl (from the Greek phenix - to illuminate).
In 1865, the German organic chemist Friedrich August Kekule proposed a formula for benzene with alternating double and single bonds in a six-membered ring.
In 1865-70s, V. Kerner suggested using prefixes to indicate the relative position of two deputies: 1,2 position - ortho-(orthos - straight); 1,3- meta(meta - after) and 1,4- pair(para - opposite).
Aromatic hydrocarbons are highly toxic substances that cause poisoning and damage to certain organs, such as the kidneys and liver.
Some aromatic hydrocarbons are carcinogens (substances that cause cancer), such as benzene (causes leukemia), one of the strongest is benzopyrene (found in tobacco smoke).
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 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 in the form of 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 bonds (0.154 nm) and longer than double ones (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.
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:
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.
