Benzene
The simplest representative of arenes is benzene. Let's take a closer look at its properties.
Benzene is a transparent, colorless, volatile liquid with a characteristic odor (it is because of the strong odor that aromatic compounds got their name). Melting point 5.5°C, boiling point - 80°C. Not miscible with water, but miscible with most organic solvents. It is a solvent for non-polar organic substances. Burns with a smoky flame (incomplete combustion) to form, except for carbon dioxide and water, a significant amount of soot. It is poisonous both as a liquid and in the form of vapors when inhaled.
Getting benzene
1. In industry, benzene is obtained by reforming oil, which is essentially the dehydrogenation of oil alkanes with the formation of a cyclic skeleton. In its "pure" form, the main reforming reaction is the dehydrogenation of hexane:
In addition, benzene is one of the volatile coking products. Coking is the heating of coal to 1000°C without air. This also produces many other valuable reagents for organic synthesis and coke used in metallurgy. Benzene can also be obtained by trimerizing acetylene over activated carbon at 100°C.
2. Of course, benzene is not obtained in the laboratory, but theoretically there are methods for its synthesis (they are used to obtain its derivatives). Both industrial and laboratory methods are shown in the chart below.
Scheme methods for obtaining benzene
industrial methods.
Chemical properties benzene
The chemical properties of benzene are determined, of course, by its p-system. Just as in the case of alkenes, it can be attacked by an electrophilic particle. However, in the case of aromatic compounds, the result of such an attack will be completely different. The high stability of the p-system leads to the fact that at the end of the reaction it is usually reduced and the result of the reaction is not addition (which would destroy
p-system), and electrophilic substitution. Let's take a closer look at its mechanism.
At the first stage, the attack of the AB molecule containing the electrophilic center A leads to the formation of an extremely unstable p-complex (stage 1). At the same time, the aromatic system was not disturbed. Further formed covalent bond one of the atoms of the ring with particle A (stage 2). At the same time, firstly, it breaks A-B connection, and secondly, the p-system is destroyed. The resulting unstable positively charged molecule is called an s-complex. As already mentioned, the restoration of the p-system is energetically very favorable, and this leads to a rupture of either the C-A bond (and then the molecule returns to its original state), or S-N connections(stage 3). In the latter case, the reaction ends, and the product of substitution of hydrogen for A is obtained.
Most Reactions aromatic compounds have just such a mechanism (electrophilic substitution, abbreviated S E). Let's consider some of them.
1. Halogenation. Occurs only in the presence of catalysts - Lewis acids (see "Lewis theory"). The task of the catalyst is to polarize the halogen molecule to form a good electrophilic center:
| AlCl 3 +Cl 2 "Cl + [AlCl 4] - The resulting particle has an electrophilic chlorine atom, and
reaction takes place:
To Nitration. It is carried out with a mixture of nitric and sulfuric acids (nitrating mixture). The following reaction takes place in the nitrating mixture:
HNO 3 + H 2 SO 4 "NO + 2 + H 2 O
In the resulting nitronium hydrosulfate there is a powerful electrophilic center - the nitronium ion NO + 2. Accordingly, the reaction general equation which:
3. Sulfonation. In concentrated sulfuric acid, there is an equilibrium:
2H 2 SO 4 "SO 3 H + - + H 2 O
In the molecule on the right side of the equilibrium there is a strong electrophile SO 3 H + , which reacts with benzene. Resulting reaction:
Alkylation according to Friedel-Crafts. When benzene reacts with alkyl chlorides or alkenes in the presence of Lewis acids (usually aluminum halides), alkyl-substituted benzenes are obtained. In the case of alkyl halides, the first stage of the process:
RCl + AlCl 3 "R + [AlCl 4] - In the second stage, the electrophilic particle R + attacks the p-system:
In the case of alkenes, the Lewis acid polarizes the double bond of the alkene, and again an electrophilic center is formed on carbon:
Non-electrophilic reactions include:
1. hydrogenation of benzene. This reaction proceeds with the destruction of the p-system and requires severe conditions ( high pressure, temperature, catalyst - platinum metals):
2. radical chlorination. In the absence of Lewis acids and under severe ultraviolet irradiation, benzene can react with chlorine by a radical mechanism. In this case, the p-system is destroyed and the product of chlorine addition is formed - solid hexachlorane, which was formerly used as an insecticide:
Benzene homologues
Nomenclature and isomerism of arenes
All arenas can be conditionally divided into two rows. The first row - benzene derivatives (toluene, diphenyl): the second row - condensed (polynuclear) arenes (naphthalene, anthracene).
Consider the homologous series of benzene, the compounds of this series have the general formula C n H 2 n. 6. 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,
I Group C 6 H 5 is called phenyl. Phenyl and substituted phenyl groups are called aryl. Some benzene derivatives are shown below:
Reaction scheme for benzene
Isomers with two substituents in positions 1,2; 1,3 and 1,4 are called ortho-, meta- and para-isomers:
Nomenclature of aromatic compounds
Below are the names of some aromatic compounds:
C 6 H 5 NH 3 + Cl - Phenylammonium chloride (anilinium chloride)
C b H 5 CO 2 H Benzenecarboxylic acid (benzoic acid)
C 6 H 5 CO 2 C 2 H 5 Benzenecarboxylic acid ethyl ester (ethyl benzoate)
C 6 H 5 COCl Benzenecarbonyl chloride (benzoyl chloride)
C 6 H 5 CONH 2 Benzenecarboxamide (benzamide)
C 6 H 5 CN Benzenecarbonitrile (benzonitrile)
C 6 H 5 CHO Benzenecarbaldehyde (benzaldehyde)
C 6 H 5 COCH 3 Acetophenone
C 6 H 5 OH Phenol
C 6 H 5 NH 2 Phenylamine (aniline)
C 6 H 5 OCH 3 Methoxybenzene (anisole)
These names correspond to the IUPAC nomenclature. In parentheses Indicated traditional names, which are still widely accepted and quite acceptable.
Arena nomenclature
The name of a benzene derivative with two or more substituents on the benzene ring is constructed in this way. The carbon atom of the benzene ring, to which the substituent closest to the beginning of the above list is attached, receives the number 1. Further, the carbon atoms of the benzene ring are numbered so that the locant - the number of the second substituent - is the smallest.
3-hydroxybenzenecarboxylic acid (3-hydroxybenzoic acid)
The carboxyl group is treated as the main group and is assigned the locant "1". The numbering of the ring is constructed so that the hydroxyl group receives a smaller ("3", not "5") locant.
2-aminobenzenecarbaldehyde (2-aminobenzaldehyde)
The -CHO group is treated as the main group. She gets a locant of "1". The NH 2 group is in the "2" position, not the "6" position. In addition, the name o-aminobenzaldehyde is acceptable.
1-bromo-2-nitro-4-chlorobenzene These groups are listed in alphabetical order.
Getting arenas
Obtaining from aliphatic hydrocarbons. When straight-chain alkanes with at least 6 carbon atoms per molecule are passed over heated platinum or chromium (III) oxide, dehydrocyclization occurs - the formation of an arene with the release of hydrogen. For example:
2. Dehydrogenation of cycloalkanes. The reaction occurs when passing vapors of cyclohexane and its homologues over heated platinum:
|. Preparation of benzene by trimerization of acetylene. According to the method of N. D. Zelinsky and B. A. Kazansky, benzene can be obtained by passing acetylene through a tube with activated carbon heated to 100 ° C. The whole process can be represented by a diagram:
4. Preparation of benzene homologues by the Friedel-Crafts reaction(see Chemical properties of benzene).
5. Fusion of salts of aromatic acids with alkali: C 6 H 6 -COONa + NaOH ® C 6 H 6 + Na 2 CO 3
Application of arenes
Arenas are used as chemical raw materials for the production of medicines, plastics, dyes, pesticides and many other organic substances. Arenes are widely used as solvents.
Dehydrogenation reactions make it possible to use oil hydrocarbons to produce hydrocarbons of the benzene series. They indicate the relationship between different groups of hydrocarbons and their mutual transformation into each other.
Arenas or aromatic(benzenoid) hydrocarbons are compounds whose molecules contain stable cyclic groups of atoms (benzene rings) with a closed system of conjugated bonds.
The aromatic character of arenes is explained electronic structure benzene ring.
Aromaticity criteria. Based on the study of cyclic conjugated systems, it has been established that a compound is aromatic if it obeys Hückel's rule: Increased thermodynamic stability (aromaticity) is possessed only by such monocyclic conjugated systems (polyenes) that have a planar structure and contain 4n + 2 -electrons in a closed conjugation chain (where n is an integer: 0, 1, 2, 3, etc.). e). Rings containing 4n -electrons are antiaromatic (destabilized).
Aromaticity conditions the benzene molecule C 6 H 6 fully corresponds, in the conjugation system of which 6 -electrons participate - an aromatic sextet (according to the formula 4n + 2 at n = 1). Aromatic rings are much more stable than conjugated acyclic analogs with the same number of π-electrons, i.e. benzene is more stable than CH 2 =CH–CH=CH–CH=CH 2 (hexatriene-1,3,5).
Nomenclature. Trivial names are widely used (toluene, xylene, cumene, etc.). The systematic names of benzene homologues are built from the name of the hydrocarbon radical (prefix) and the word benzene (root): C 6 H 5 -CH 3 - methylbenzene (toluene), C 6 H 5 -C 2 H 5 - ethylbenzene, C 6 H 5 -CH (CH 3) 2 - isopropylbenzene (cumene). If there are two or more radicals, their position is indicated by the numbers of carbon atoms in the ring to which they are associated. The numbering of the ring is carried out so that the numbers of radicals are the smallest.
For disubstituted benzenes RC 6 H 4 R, another method of constructing names is also used, in which the position of substituents is indicated before the trivial name of the compound by prefixes: ortho- (o-) substituents at neighboring carbon atoms of the ring, i.e. in position 1,2-; meta- (m-) substituents through one carbon atom (1,3-); para-(p-) substituents on opposite sides (1,4-).
isomerism. In the series of benzene homologues, structural isomerism: 1) positions of substituents for di-, tri- and tetra-substituted benzenes (for example, o-, m- and p-xylenes); 2) a carbon skeleton in a side chain containing at least 3 carbon atoms (C 6 H 5 -CH 2 CH 2 CH 3 - n-propylbenzene and C 6 H 5 -CH (CH 3) 2 - isopropylbenzene or cumene); 3) isomerism of substituents R, starting from R = C 2 H 5, (for example, 4 isomers correspond to the molecular formula C 8 H 10: o-, m-, p-xylenes CH 3 -C 6 H 4 -CH 3 and ethylbenzene C 6 H 5 -C 2 H 5); 4) interclass isomerism with unsaturated compounds (for example, the formula C 6 H 6 except benzene
have compounds CH 2 = CH–C≡C–CH=CH 2, CH≡C–CH=CH–CH=CH 2, etc., as well as unsaturated cycles). Spatial isomerism relative to the benzene ring in substituted arenes is absent.
Hückel's rule:
Aromatic are molecules that obey the Hückel rule: an aromatic is a planar monocyclic conjugated system containing (4n + 2)π-electrons (where n = 0,1,2…).
Mechanism of electrophilic substitution:
1-I stage: formation of p-complex. In this case, a weak bond is formed between the p-electron cloud of the benzene ring and an electrophilic reagent with an electron density deficit, while maintaining the aromatic sextet. The electrophilic reagent is usually located perpendicular to the plane of the ring along its axis of symmetry. This step is fast and does not affect the rate of the reaction. The existence of the p-complex is proved by UV spectroscopy...
2nd stage: formation of the b-complex. This stage is slow and practically irreversible. A covalent b-bond is formed between the electrophile and the carbon atom of the benzene ring, while the carbon atom passes from the spI to the spi-valence state with the violation of the aromatic sextet and the formation of a cyclohexadienyl cation (benzolenium ion). The benzene cation together with the counterion form an ionic compound that conducts well electricity. In the benzene ion, all carbon atoms are located in the same plane, and the substituents of the уspі-hybridized carbon atom are perpendicular to it.
3rd and 4th stages: formation of the second p-complex and aromatization. the b-complex can be converted into a new, slightly stable p-complex, which is deprotonated under the influence of a base, usually a counterion.
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:
- carbon atoms in sp 2 -hybridized state form a cycle.
- 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.
AT
All bonds between carbon atoms in benzene are the same (there are no double or single bonds as such) and have a length of 0.139 nm. This value is intermediate between the single bond length in alkanes (0.154 nm) and the double bond length in alkenes (0.133 nm).The equivalence of links is usually depicted as a circle inside the cycle
Circular conjugation gives an energy gain of 150 kJ/mol. This value is conjugation energy - the amount of energy that must be expended to break the aromatic system of benzene.
General formula: C n H 2n-6(n ≥ 6)
Homologous series:
Benzene homologues are compounds formed by replacing one or more hydrogen atoms in a benzene molecule with hydrocarbon radicals (R):
ortho- (about-)
substituents at adjacent carbon atoms of the ring, i.e. 1,2-;
meta- (m-)
substituents through one carbon atom (1,3-);
pair- (P-)
substituents on opposite sides of the (1,4-) ring.
aryl
C 6H5- (phenyl) and C6H
Aromatic monovalent radicals have the common name " aryl". Of these, two are most common in the nomenclature of organic compounds:C 6H5- (phenyl) and C 6 H 5 CH 2- (benzyl). 5 CH 2- (benzyl).
Isomerism:
structural:
1) positions of deputies for di-, three- and tetra-substituted benzenes (for example, about-, m- and P-xylenes);
2) carbon skeleton in the side chain containing at least 3 carbon atoms:
3) isomerism of substituents R, starting from R = C 2 H 5 .
Chemical properties:
Arenes are more characteristic of reactions going with preservation of the aromatic system, namely, substitution reactions hydrogen atoms associated with the cycle.
2. Nitration
Benzene reacts with a nitrating mixture (a mixture of concentrated nitric and sulfuric acids):
3. Alkylation
Substitution of a hydrogen atom in the benzene ring with an alkyl group ( alkylation) occurs under the action alkyl halides or alkenes in the presence of catalysts AlCl 3 , AlBr 3 , FeCl 3 .
Substitution in alkylbenzenes:
Benzene homologues (alkylbenzenes) are more active in substitution reactions than benzene.
For example, when nitrating toluene C 6 H 5 CH 3 substitution of not one, but three hydrogen atoms can occur with the formation of 2,4,6-trinitrotoluene:
and facilitates substitution in these positions.On the other hand, under the influence of the benzene ring, the methyl group CH 3 in toluene becomes more active in oxidation and radical substitution reactions compared to methane CH 4.
Toluene, unlike methane, oxidizes under mild conditions (discolors the acidified solution of KMnO 4 when heated):
Easier than in alkanes, radical substitution reactions proceed in side chain alkylbenzenes:
This is explained by the fact that stable intermediate radicals are easily (at a low activation energy) formed at the limiting step. For example, in the case toluene a radical is formed benzyl Ċ H 2 -C 6 H 5 . It is more stable than alkyl free radicals ( Ċ H 3 Ċ H 2 R), because its unpaired electron is delocalized due to interaction with the π-electron system of the benzene ring:
Orientation rules
- The substituents present in the benzene ring direct the newly entering group to certain positions, i.e. have an orienting effect.
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:
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.Ways to get. one. Obtaining from aliphatic hydrocarbons. To obtain benzene and its homologues in industry, they use aromatization saturated hydrocarbons that are part of the oil. When alkanes with a straight chain consisting of at least six carbon atoms are passed over heated platinum or chromium oxide, dehydrogenation occurs with simultaneous ring closure ( dehydrocyclization). In this case, benzene is obtained from hexane, and toluene is obtained from heptane.
2. Dehydrogenation of cycloalkanes also leads to aromatic hydrocarbons; for this, a pair of cyclohexane and its homologues is passed over heated platinum.
3. Benzene can be obtained from acetylene trimerization, why acetylene is passed over activated carbon at 600 °C.
4. Benzene homologues are obtained from benzene by its interaction with alkyl halides in the presence of aluminum halides (alkylation reaction, or Friedel-Crafts reaction).
5. When fusion salts of aromatic acids with alkali, arenes are released in gaseous form.
Chemical properties. The aromatic nucleus, which has a mobile system of n-electrons, is a convenient object for attack by electrophilic reagents. This is also facilitated by the spatial arrangement of the n-electron cloud on both sides of the flat a-skeleton of the molecule (see Fig. 23.1, b).
For arenes, the most typical reactions proceed according to the mechanism electrophilic substitution, denoted by the symbol S E(from English, substitution, electrophilic).
Mechanism S E can be represented as follows:
At the first stage, the electrophilic particle X is attracted to the n-electron cloud and forms an n-complex with it. Then two of the six n-electrons of the ring form an a-bond between X and one of the carbon atoms. In this case, the aromaticity of the system is violated, since only four n-electrons remain in the ring, distributed among five carbon atoms (a-complex). To preserve aromaticity, the a-complex ejects a proton, and two C-H bond electrons pass into the n-electron system.
The following reactions of aromatic hydrocarbons proceed according to the mechanism of electrophilic substitution.
1. Halogenation. Benzene and its homologues react with chlorine or bromine in the presence of anhydrous A1C1 3 , FeCl 3 , A1Br 3 catalysts.
This reaction produces a mixture from toluene. ortho- and para-isomers (see below). The role of the catalyst is to polarize the neutral halogen molecule with the formation of an electrophilic particle from it.
2. Nitration. Benzene reacts very slowly with concentrated nitric acid, even when heated strongly. However, when acting nitrating mixture(mixtures of concentrated nitric and sulfuric acids), the nitration reaction proceeds quite easily.
3. Sulfonation. The reaction easily passes with "fuming" sulfuric acid (oleum).
- 4. Friedel-Crafts Alkylation- see above methods for obtaining benzene homologues.
- 5. Alkylation with alkenes. These reactions are widely used in industry to produce ethylbenzene and isopropylbenzene (cumene). Alkylation is carried out in the presence of a catalyst A1C1 3 . The reaction mechanism is similar to that of the previous reaction.
All the above reactions proceed according to the mechanism electrophilic substitution S E .
Along with substitution reactions, aromatic hydrocarbons can enter into addition reactions, however, these reactions lead to the destruction of the aromatic system and therefore require large amounts of energy and proceed only under harsh conditions.
6. hydrogenation benzene goes under heating and high pressure in the presence of metal catalysts (Ni, Pt, Pd). Benzene is converted to cyclohexane.
Hydrogenation of benzene homologues gives cyclohexane derivatives.
7. Radical halogenation benzene occurs when its vapor interacts with chlorine only under the influence of hard ultraviolet radiation. At the same time, benzene joins three chlorine molecules and forms solid product hexachlorocyclohexane (hexachloran) C 6 H 6 C1 6 (hydrogen atoms are not indicated in the structural formulas).
8. Oxidation by atmospheric oxygen. In terms of resistance to the action of oxidizing agents, benzene resembles alkanes - the reaction requires harsh conditions. For example, the oxidation of benzene with atmospheric oxygen occurs only when its vapor is strongly heated (400 °C) in air in the presence of a V 2 0 5 catalyst; the products are a mixture of maleic acid and its anhydride.
Benzene homologues. The chemical properties of benzene homologues are different from those of benzene, which is due to the mutual influence of the alkyl radical and the benzene ring.
Reactions in the side chain. The chemical properties of alkyl substituents in the benzene ring are similar to alkanes. Hydrogen atoms in them are replaced by halogens by a radical mechanism (S R). So in the absence of a catalyst, when heated or UV irradiated, a radical substitution reaction occurs in the side chain. However, the influence of the benzene ring on alkyl substituents leads to the fact that, first of all, the hydrogen at the carbon atom directly bonded to the benzene ring is replaced (and -atom carbon).
Substitution on the benzene ring by mechanism S E possibly only in the presence of a catalyst(A1C1 3 or FeCl 3). Substitution in the ring occurs in ortho- and para positions to the alkyl radical.
Under the action of potassium permanganate and other strong oxidizing agents on benzene homologues, the side chains are oxidized. No matter how complex the substituent chain 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.
General review.
Aromatic hydrocarbons (arenes) are substances whose molecules contain one or more benzene rings - cyclic groups of carbon atoms with a special nature of bonds.
The concept of "benzene ring" immediately requires deciphering. To do this, it is necessary to at least briefly consider the structure of the benzene molecule. The first structure of benzene was proposed in 1865 by the German scientist A. Kekule:
This formula correctly reflects the equivalence of six carbon atoms, but does not explain a number of special properties of benzene. For example, despite the unsaturation, benzene does not show a tendency to addition reactions: it does not decolorize bromine water and potassium permanganate solution, i.e. does not give qualitative reactions typical of 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.
Nomenclature and isomerism.
Conventionally, the arenas can be divided into two rows. The first includes benzene derivatives (for example, toluene or diphenyl), the second - condensed (polynuclear) arenes (the simplest of them is naphthalene):
We will consider only the homologous series of benzene with the general formula .
Structural isomerism in homologous series benzene is due mutual arrangement substituents in the nucleus. Monosubstituted benzene derivatives do not have position isomers, since all atoms in the benzene nucleus are equivalent. Disubstituted derivatives exist in the form of three isomers that differ in the mutual arrangement of substituents. The position of the substituents is indicated by numbers or prefixes:
Aromatic hydrocarbon radicals are called aryl radicals. The radical is called phenyl.
physical properties.
The first members of the homologous series of benzene (for example, toluene, ethylbenzene, etc.) are colorless liquids with a specific odor. They are lighter than water and insoluble in water. They dissolve well in organic solvents. Benzene and its homologues are themselves good solvents for many organic substances. All arenas burn with a smoky flame due to the high content of carbon in their molecules.
Ways to get.
1. Obtaining from aliphatic hydrocarbons. When straight-chain alkanes having at least 6 carbon atoms in a molecule are passed over heated platinum or chromium oxide, dehydrocyclization occurs - the formation of an arene with the release of hydrogen:
2. Dehydrogenation of cycloalkanes. The reaction occurs when passing vapors of cyclohexane and its homologues over heated platinum:
3. Preparation of benzene by trimerization of acetylene - see § 20.
4. Obtaining benzene homologues by the Friedel-Crafts reaction - see below.
5. Fusion of salts of aromatic acids with alkali:
Chemical properties.
General review. Possessing a mobile six -electrons, the aromatic nucleus is a convenient object for attack by electrophilic reagents. This is also facilitated by the spatial arrangement of the -electron cloud on both sides of the flat -skeleton of the molecule (Fig. 21.1, b)
For arenes, reactions proceeding according to the mechanism of electrophilic substitution, denoted by the symbol (from the English substitution electrophilic), are most characteristic.
The mechanism of electrophilic substitution can be represented as follows. The electrophilic reagent XY (X is an electrophile) attacks the electron cloud, and an unstable -complex is formed due to the weak electrostatic interaction. The aromatic system is not yet disturbed. This stage is fast. At the second, slower stage, a covalent bond is formed between the electrophile X and one of the carbon atoms of the ring due to two α-electrons of the ring. This carbon atom changes from to the -hybrid state. The aromaticity of the system is thus disturbed. The four remaining -electrons are distributed among five other carbon atoms, and the benzene molecule forms a carbocation, or -complex.
Violation of aromaticity is energetically unfavorable, therefore the structure of the -complex is less stable than the aromatic structure. To restore aromaticity, a proton is split off from the carbon atom associated with the electrophile (third stage). In this case, two electrons return to the -system, and thereby aromaticity is restored:
Electrophilic substitution reactions are widely used for the synthesis of many benzene derivatives.
Chemical properties of benzene.
1. Halogenation. Benzene does not interact with chlorine or bromine under normal conditions. The reaction can proceed only in the presence of anhydrous catalysts. As a result of the reaction, halogen-substituted arenes are formed:
The role of the catalyst is to polarize the neutral halogen molecule with the formation of an electrophilic particle from it:
2. Nitration. Benzene reacts very slowly with concentrated nitric acid, even when heated strongly. However, under the action of the so-called nitrating mixture (a mixture of concentrated nitric and sulfuric acids), the nitration reaction proceeds quite easily:
3. Sulfonation. The reaction easily takes place under the action of "fuming" sulfuric acid (oleum):
4. Alkylation according to Friedel-Crafts. As a result of the reaction, an alkyl group is introduced into the benzene core to obtain benzene homologues. The reaction proceeds under the action of haloalkanes on benzene in the presence of catalysts - aluminum halides. The role of the catalyst is reduced to the polarization of the molecule with the formation of an electrophilic particle:
Depending on the structure of the radical in the haloalkane, different homologues of benzene can be obtained:
5. Alkylation with alkenes. These reactions are widely used in industry to produce ethylbenzene and isopropylbenzene (cumene). Alkylation is carried out in the presence of a catalyst. The reaction mechanism is similar to that of the previous reaction:
All the reactions discussed above proceed by the mechanism of electrophilic substitution.
Addition reactions to arenes lead to the destruction of the aromatic system and require large amounts of energy, so they proceed only under harsh conditions.
6. Hydrogenation. The reaction of hydrogen addition to arenes proceeds under heating and high pressure in the presence of metal catalysts (Ni, Pt, Pd). Benzene is converted to cyclohexane, and benzene homologues are converted to cyclohexane derivatives:
7. Radical halogenation. The interaction of benzene vapor with chlorine proceeds by a radical mechanism only under the influence of hard ultraviolet radiation. In this case, benzene adds three molecules of chlorine and forms a solid product - hexachlorocyclohexane:
8. Oxidation by atmospheric oxygen. In terms of resistance to oxidizing agents, benzene resembles alkanes. Only with strong heating (400 ° C) of benzene vapor with atmospheric oxygen in the presence of a catalyst, a mixture of maleic acid and its anhydride is obtained:
Chemical properties of benzene homologues.
Benzene homologues have a number of special chemical properties associated with the mutual influence of the alkyl radical on the benzene ring, and vice versa.
Reactions in the side chain. In terms of chemical properties, alkyl radicals are similar to alkanes. Hydrogen atoms in them are replaced by halogens by a free radical mechanism. Therefore, in the absence of a catalyst during heating or UV irradiation, a radical substitution reaction occurs in the side chain. The effect of the benzene ring on alkyl substituents always results in the replacement of the hydrogen atom at the carbon atom directly bonded to the benzene ring (a-carbon atom).
Substitution in the benzene ring is possible only by the mechanism in the presence of a catalyst:
Below you will find out which of the three isomers of chlorotoluene are formed in this reaction.
Under the action of potassium permanganate and other strong oxidants on the homologues of benzene, the side chains are oxidized. No matter how complex the substituent chain is, it is destroyed, with the exception of the -carbon atom, which is oxidized into a carboxyl group.
Homologues of benzene with one side chain give benzoic acid:
Orientation (substitution) rules in the benzene ring.
The most important factor determining the chemical properties of a molecule is the distribution of electron density in it. The nature of the distribution depends on the mutual influence of the atoms.
In molecules that have only -bonds, the mutual influence of atoms is carried out through the inductive effect (see § 17). In molecules that are conjugated systems, the action of the mesomeric effect is manifested.
The influence of substituents, transmitted through a conjugated system of -bonds, is called the mesomeric (M) effect.
In a benzene molecule, the -electron cloud is distributed evenly over all carbon atoms due to conjugation.
If, however, some substituent is introduced into the benzene ring, this uniform distribution is disturbed, and the electron density in the ring is redistributed. The place of entry of the second substituent into the benzene ring is determined by the nature of the already existing substituent.
Substituents are divided into two groups depending on the effect they exhibit (mesomeric or inductive): electron support and electron acceptor.
Electron-donor substituents exhibit an effect and increase the electron density in the conjugated system. These include the hydroxyl group -OH and the amino group. The lone pair of electrons in these groups enters into general conjugation with the -electronic system of the benzene ring and increases the length of the conjugated system. As a result, the electron density is concentrated in the ortho and para positions:
Alkyl groups cannot participate in general conjugation, but they exhibit an effect under which a similar redistribution of -electron density occurs.
Electron-withdrawing substituents exhibit the -M effect and reduce the electron density in the conjugated system. These include the nitro group, the sulfo group, the aldehyde group -CHO and the carboxyl group -COOH groups. These substituents form a common conjugated system with the benzene ring, but the overall electron cloud shifts towards these groups. Thus, the total electron density in the ring decreases, and it decreases least of all in the meta positions:
For example, toluene containing a substituent of the first kind is nitrated and brominated in the para and ortho positions:
Nitrobenzene containing a substituent of the second kind is nitrated and brominated in the meta position:
In addition to the orienting action, substituents also affect the reactivity of the benzene ring: orientants of the 1st kind (except for halogens) facilitate the introduction of the second substituent; orientants of the second kind (and halogens) make it difficult.
