Aromatic hydrocarbons chemical physical properties and applications. Ways to get arenas

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

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

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

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

Electronic structure of benzene

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

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

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

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

Homologous series, isomerism, nomenclature

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

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

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

Physical properties

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

Chemical properties

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

I. Substitution reactions

1. Halogenation (with Cl 2, Br 2)

2. Nitration

3. Sulfonation

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

Alkylation of benzene also occurs when it reacts with alkenes:

Styrene (vinylbenzene) is obtained by dehydrogenation of ethylbenzene:

II. Addition reactions

1. Hydrogenation

2. Chlorination

III. Oxidation reactions

1. Combustion

2C 6 H 6 + 15O 2 → 12CO 2 + 6H 2 O

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

No chemical reaction occurs (similar to alkanes).

Properties of benzene homologues

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

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

Aromatic compounds- cyclic organic compounds that contain an aromatic system

Oil is a complex mixture of hydrocarbons. Additionally, the composition of oil includes a non-carbon part and mineral impurities. The carbon part of oil consists of paraffinic (alkanes), naphthenic (cyclanes) and aromatic (arenes) hydrocarbons.

Aromatic hydrocarbons (arenes) have monocyclic (benzene, toluene, xylenes) or bi- and polycyclic (naphthalene, anthracene, etc.) structures. They are contained in oil 10 - 20%.

There are mononuclear (one benzene group in the molecule) and polynuclear aromatic hydrocarbons containing two or more benzene groups. Arene molecules can contain hydrocarbon radicals with straight or branched carbon chains as side chains, as well as those containing double or triple bonds and cyclic groups:

The first and one of the most important representatives of the homologous series of mononuclear aromatic hydrocarbons is benzene C 6 H 6. Hence the general name of the homologous series - benzene series.

The structure of benzene

The general formula of monocyclic arenes C n H 2 n -6 shows that they are unsaturated compounds.

This structure of the benzene molecule did not explain many of the properties of benzene:

Benzene is characterized by substitution reactions rather than addition reactions, which are characteristic of unsaturated compounds. Addition reactions are possible, but they are more difficult than for alkenes.

Benzene does not enter into reactions that are qualitative reactions to unsaturated hydrocarbons (with bromine water and KMnO 4 solution).

Physical properties

The physical properties of arenes are related to the number of carbon atoms, the presence of substituents and their location in the molecule. Arenes have higher boiling points than the corresponding cycloalkanes. This is explained by the dense packing of their molecules (flat ring), as well as stronger physicochemical interactions between molecules due to the presence of π electrons.

Homologues with adjacent alkyl substituents boil at higher temperatures than n-isomers.

The melting temperatures of arenes are higher, the more symmetrically the alkyl substituents are located. This is explained by the fact that asymmetry makes it difficult to order a substance in the solid state.

An increase in the number of cycles is accompanied by an increase in the melting temperature. The appearance of side chains reduces the melting point, and chain elongation leads to its increase.

Arenes are characterized by the highest density and refractive index among other hydrocarbons, which is used for analytical purposes.

In addition, arenes differ from other hydrocarbons in their pronounced ability to selectively dissolve in certain solvents. Such selective solvents include polar liquids: sulfur dioxide, dimethyl sulfate, sulfolane, acetone, phenol, furfural, diethylene glycol, aniline, nitrobenzene, etc.

Chemical properties and use

Addition reactions. Arenas undergo addition reactions with great difficulty.

Substitution reactions most typical for arenas. They proceed in relatively mild conditions. Benzene homologues undergo substitution reactions especially easily.

Halogenation . Depending on the halogenation conditions, products of varying degrees of substitution can be obtained:

Sulfonation . Concentrated sulfuric acid easily replaces hydrogen with a sulfuric acid residue to form sulfonic acid.

This reaction occurs quantitatively and can serve as one of the methods for determining the content of arenes in oil fractions.

Phenol is obtained from benzenesulfonic acid and chlorobenzene by fusing them with alkali.

The main application of phenol is the production of phenol-formaldehyde resins.

Nitration . When benzene is treated with a mixture of concentrated nitric and sulfuric acids, nitrobenzene is obtained:

By reducing nitrobenzene, aniline is obtained:

Most aniline is used to make polyurethane foams.

When toluene is completely nitrated, the explosive substance TNT (2,4,6-trinitrotoluene) is obtained:

Alkylation . In the presence of catalysts such as AlCl 3, HF, H 2 SO 4, HCl, BF 3, arenes enter into an alkylation reaction with alkenes, alcohols, and halogenated alkanes. In this way, ethylbenzene and isopropylbenzene are produced industrially:

By catalytic dehydrogenation, styrene is obtained from ethylbenzene, and α-methylstyrene is obtained from isopropylbenzene - valuable monomers used in the production of rubbers and plastics:

Dealkylation and hydrodealkylation. Due to the fact that benzene is of greatest importance, it is currently obtained by dealkylation or hydrodealkylation of toluene:

Condensation with formaldehyde . In the presence of concentrated sulfuric acid, the arenes condense with formaldehyde to form an insoluble brown precipitate:

This reaction is used for the analytical determination of arenes in petroleum fractions.

Oxidation . Arenes (except for benzene, naphthalene and other hononuclear homologues) easily enter into oxidation reactions. In the series of alkyl derivatives of arenes, the resistance to oxidation decreases with increasing length and degree of branching of the side chain. In this case, acidic compounds are formed. These properties of arenes are widely used in industry to obtain oxygen-containing derivatives:

In order to obtain terephthalic acid, various processes for the oxidation of toluene have also been developed. The most resistant to oxidation by atmospheric oxygen are benzene and naphthalene. However, they are also in very harsh conditions ( heat

Terephthalic acid is an intermediate product for the production of synthetic polyester fiber - lavsan (terylene). Phthalic anhydride is used for the production of alkyd and polyester resins, plasticizers, and repellents. Maleic anhydride is used in the production of polyester resins and lubricating oil additives.

Formation of complexes with picric acid. Polycyclic arenes (naphthalene, anthracene and their homologues) easily form complex compounds with picric acid (2,4,6 - trinitrophenol) - picrates.

Benzene and its homologues do not form stable complexes and can serve as solvents for complex formation.

Aromatic hydrocarbon picrates are yellow crystalline solids with distinct melting points. Each polycyclic hydrocarbon corresponds to a picrate with a certain melting point. It is possible to identify a polycyclic aromatic hydrocarbon by the melting point of picrate.

Complexation with picric acid is used as a method for isolating polycyclic aromatic hydrocarbons. Picrates are easily decomposed by hot water. Picric acid dissolves in water, and polycyclic aromatic hydrocarbons are released in free form.

2.4.4. Hydrocarbons of mixed structure

High-boiling oil fractions mainly consist mainly of hydrocarbons of mixed (hybrid) structure. These are polycyclic hydrocarbons, the molecules of which contain cycloalkane structures condensed with arenes.

Kerosene-gas oil fractions contain the simplest hybrid bicyclic hydrocarbons and their homologues:

The arene rings of hybrid hydrocarbons have predominantly short (methyl or ethyl) substituents, while the cycloalkane rings have one or two rather long alkyl substituents. There are especially many hybrid hydrocarbons in oil fractions. Their structure has been little studied.

Hybrid hydrocarbons are undesirable components of lubricating oils because they impair viscosity properties and reduce their oxidation stability.

2.4.5. Oil arenas, influence on the properties of petroleum products,

application

Arenas are desirable components of carburetor fuels, as they have high octane numbers (toluene -103, ethylbenzene - 98).

The presence of arenes in significant quantities in diesel and jet fuels worsens combustion conditions and is therefore highly undesirable.

Polycyclic arenes with short side chains impair the performance properties of oils and are therefore removed from them.

Arenas are valuable raw materials for petrochemical synthesis, in the production of synthetic rubbers, plastics, synthetic fibers, aniline dyes and explosives, and pharmaceuticals. The most important are benzene, toluene, xylenes, ethylbenzene, and naphthalene.

General consideration.

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

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

The structural features 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 (the skeleton of a 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 that are not localized in pairs, as in the formation of ordinary double bonds, but are 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 length of a single bond in alkanes (0.154 nm) and the length of a double bond in alkenes (0.133 nm). The equivalence of connections is usually depicted with a circle inside the cycle (Fig. 21.1, c). Circular conjugation gives an energy gain of 150 kJ/mol. This value constitutes the conjugation energy - the amount of energy that must be expended to disrupt 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 only possible under very harsh conditions.

Nomenclature and isomerism.

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 positional isomers, since all atoms in the benzene ring are equivalent. Disubstituted derivatives exist in the form of three isomers, differing in the relative arrangement of substituents. The position of the substituents is indicated by numbers or prefixes:

The radicals of aromatic hydrocarbons 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 carbon content in their molecules.

Methods of obtaining.

1. Preparation from aliphatic hydrocarbons. When straight-chain alkanes with at least 6 carbon atoms per 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 by passing vapors of cyclohexane and its homologues over heated platinum:

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

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

5. Fusion of salts of aromatic acids with alkali:

Chemical properties.

General consideration. 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)

The most typical reactions for arenes are those that proceed through the mechanism of electrophilic substitution, denoted by the symbol (from the English substitution electrophilic).

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

Disruption of aromaticity is energetically unfavorable, therefore the structure of the β-complex is less stable than the aromatic structure. To restore aromaticity, a proton is removed from the carbon atom bound to 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 react with chlorine or bromine under normal conditions. The reaction can only take place 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 a neutral halogen molecule to form an electrophilic particle from it:

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

3. Sulfonation. The reaction easily takes place under the influence of “fuming” sulfuric acid (oleum):

4. Friedel-Crafts alkylation. As a result of the reaction, an alkyl group is introduced into the benzene ring to produce benzene homologues. The reaction occurs when benzene is exposed to haloalkanes 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 benzene homologues can be obtained:

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

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

Reactions of addition to arenes lead to the destruction of the aromatic system and require large amounts of energy, so they occur only under harsh conditions.

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

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

8. Oxidation by air oxygen. In terms of its 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.

Side chain reactions. The chemical properties of alkyl radicals are similar to alkanes. The hydrogen atoms in them are replaced by halogen by a free radical mechanism. Therefore, in the absence of a catalyst, upon heating or UV irradiation, a radical substitution reaction occurs in the side chain. The influence of the benzene ring on alkyl substituents leads to the fact that the hydrogen atom at the carbon atom directly bonded to the benzene ring (a-carbon atom) is always replaced.

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.

When benzene homologues are exposed to potassium permanganate and other strong oxidizing agents, the side chains are oxidized. No matter how complex the chain of the substituent 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:

Rules for orientation (substitution) 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 occurs through the inductive effect (see § 17). In molecules that are conjugated systems, the mesomeric effect manifests itself.

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 any substituent is introduced into the benzene ring, this uniform distribution is disrupted and a redistribution of the electron density in the ring occurs. The place where the second substituent enters the benzene ring is determined by the nature of the existing substituent.

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

Electron-donating substituents have 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 -electronic system 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 an -M effect and reduce the electron density in the conjugated system. These include the nitro group, sulfo group, aldehyde -CHO and carboxyl -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 at the meta positions:

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

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

In addition to the orienting effect, substituents also influence the reactivity of the benzene ring: orientants of the 1st kind (except halogens) facilitate the entry of the second substituent; Orientants of the 2nd kind (and halogens) make it difficult.

Methods of obtaining. 1. Preparation from aliphatic hydrocarbons. To obtain benzene and its homologues in industry, they use aromatization saturated hydrocarbons that make up oil. When straight-chain alkanes 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; To do this, vapors of cyclohexane and its homologues are passed over heated platinum.

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

4. Benzene homologues are obtained from benzene by its reaction 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 a-electron cloud on both sides of the flat a-skeleton of the molecule (see Fig. 23.1, b).

The most typical reactions for arenes are those that 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:

In the first stage, the electrophilic particle X is attracted to the n-electron cloud and forms an n-complex with it. Two of the ring's six n-electrons then form an a-bond between X and one of the carbon atoms. In this case, the aromaticity of the system is disrupted, since only four a-electrons remain in the ring, distributed between five carbon atoms (a-complex). To maintain aromaticity, the a-complex ejects a proton and two electrons S-N connections go into the l-electronic system.

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

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

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 to form an electrophilic particle from it.

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

3. Sulfonation. The reaction easily takes place with “fuming” sulfuric acid (oleum).

4. Friedel-Crafts alkylation- see above for methods of obtaining benzene homologues.
5. Alkylation with alkenes. These reactions are widely used industrially to produce ethylbenzene and isopropylbenzene (cumene). Alkylation is carried out in the presence of an AlC1 3 catalyst. The reaction mechanism is similar to the mechanism of the previous reaction.

All the reactions discussed above 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 occur only under harsh conditions.

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

Homologues of benzene upon hydrogenation give cyclohexane derivatives.

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

8. Oxidation by air oxygen. In terms of resistance to oxidizing agents, benzene resembles alkanes - the reaction requires harsh conditions. For example, the oxidation of benzene with atmospheric oxygen occurs only with strong heating (400 ° C) of its vapor in air in the presence of a V 2 0 5 catalyst; products - 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.

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

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

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

Benzene homologs with one side chain give benzoic acid.

Benzene is obtained from coal tar, formed during the coking of coal and oil, using synthetic methods.

1. Preparation from aliphatic hydrocarbons. When straight-chain alkanes with 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: method 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 is highly durable. The most typical reactions for arenes are those that proceed according to the mechanism electrophilic substitution, denoted by the symbol S E (from the English substitution electrophilic).

Chemical properties of benzene.

1. Substitution reactions:

Halogenation . Benzene does not react with chlorine or bromine under normal conditions. The reaction can only occur 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 a neutral halogen molecule to form an electrophilic particle from it:

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

Sulfonation. The reaction easily takes place under the influence of “fuming” sulfuric acid (oleum):

2. Friedel-Crafts alkylation. As a result of the reaction, an alkyl group is introduced into the benzene ring to produce benzene homologues. The reaction occurs when benzene is exposed to haloalkanes RСl 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 benzene homologues can be obtained:

Alkylation with alkenes. These reactions are widely used industrially to produce ethylbenzene and isopropylbenzene (cumene). Alkylation is carried out in the presence of an AlCl 3 catalyst. The reaction mechanism is similar to the mechanism of the previous reaction:

All the reactions discussed above proceed according to the mechanism electrophilic substitution S E . Reactions of addition to arenes lead to the destruction of the aromatic system and require large amounts of energy, so they occur only under harsh conditions.

3. Addition reactions that involve breaking bonds:

Hydrogenation. The reaction of hydrogen addition to arenes occurs under heating and high pressure in the presence of metal catalysts (Ni, Pt, Pd). Benzene is turning to cyclohexane, A benzene homologues - into cyclohexane derivatives:

Radical halogenation. The interaction of benzene vapor with chlorine proceeds according to a radical mechanism only under the influence of hard ultraviolet radiation. In this case, benzene combines with three chlorine molecules and forms solid product - hexachlorocyclohexane (hexachlorane) C 6 H 6 Cl 6:

4. Oxidation by atmospheric oxygen. In terms of its 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 burns. (View experiment) Benzene flames are smoky due to the high carbon content of the molecule.

2 C 6 H 6 + 15 O 2 → 12CO 2 + 6H 2 O

6. Application of arenas.

Benzene and its homologues are used as chemical raw materials for the production of drugs, 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 and polyethylene. It accelerates the ripening of fruits (tomatoes, citrus fruits) when small amounts are introduced into the air of greenhouses. Propylene is used for the synthesis of glycerin, alcohol, and for the production of polypropylene, which is used for the manufacture of ropes, ropes, and packaging material. Synthetic rubber is produced from 1-butene.

Acetylene is used for autogenous welding of metals. Polyethylene is used as 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 fuel, etc. The main source of aromatic hydrocarbons is The products of coking coal are used. From 1 T Kam.-Ug. resins can be released on average: 3.5 kg benzene, 1.5 kg toluene, 2 kg naphthalene. The production of A. is of great importance. from fatty petroleum hydrocarbons. For some A.u. Purely synthetic methods are of practical importance. Thus, ethylbenzene is produced from benzene and ethylene, the dehydrogenation of which leads to styrene.

SELF-CONTROL TASKS:

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.