Typical chemical properties of arenes. Aromatic hydrocarbons (arenes): classification, nomenclature and isomerism, physical properties

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.

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:

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.

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):

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 matter. 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 destruction aromatic system and require large amounts of energy, so they occur only in 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 mutual influence alkyl radical onto a 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. Influence benzene ring to alkyl substituents leads to the fact that the hydrogen atom is always replaced at the carbon atom directly bonded to the benzene ring (a-carbon atom).

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 Chemical properties 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 reactivity 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.

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 and pharmaceutical preparations, explosives, synthetic fibers, motor fuel, etc. The main source of obtaining aluminum. 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. Great importance has production of A. u. from fatty petroleum hydrocarbons. For some A.u. have practical significance purely synthetic methods. 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.

Aromatic hydrocarbons, also called arenes, are represented by organic substances. Their molecules contain one or more benzene nuclei (rings). Benzene, also called benzene, is the first member of the homologous series of arenes. The chemical properties, structure of the molecule and the types of chemical bonds in its molecule have a number of features. We will look at them in our article, and also get acquainted with other compounds included in the group of aromatic hydrocarbons.

How to establish the structural formula of arenes

In 1865, the German scientist F. Kekule proposed a spatial model of the simplest arene - benzene. It looked like a flat hexagon, at the vertices of which there were carbon atoms, which were connected to each other by three simple and double bonds, alternating with each other. However, the experimentally identified chemical properties of arenes did not correspond to the formula proposed by F. Kekule. For example, benzene did not discolor a solution of potassium permanganate and bromine water, which indicated the absence of pi bonds in the arene molecules. What is the actual structure of benzene? Aromatic hydrocarbons have neither single nor double bonds. It has been experimentally established that these compounds contain an equivalent type between carbon atoms chemical bond, called one-and-a-half, or aromatic. That is why they do not enter into oxidation reactions with solutions of KMnO4 and Br2. The general formula of arenes is derived - CnH2n-6. All specific properties aromatic compounds can be explained by their electronic structure, which we will study further.

Electronic formula

Using benzene as an example, we will establish how carbon atoms are connected to each other. It turned out that all six carbon atoms are in the form of sp2 hybridization. Carbon is connected to a hydrogen atom and two neighboring carbon atoms by three sigma bonds. This is what creates the flat, hexagonal shape of the molecule. However, each carbon atom still has one more negatively charged particle that is not involved in hybridization. Its electron cloud is shaped like a dumbbell and is located above and below the plane of a hexagon called the benzene ring. Next, all six dumbbells overlap and form a common aromatic (one and a half) bond. It is she who determines all physical and chemical characteristics substances. This is the electronic structure of arenas.

What is benzene?

Getting to know the first representative of this class, benzene, will help you better understand the characteristics of aromatic hydrocarbons. An easily mobile, flammable, colorless liquid with a peculiar odor, insoluble in water, is benzene. Both the compound itself and its vapors are toxic. According to the general formula of arenes, quantitative and high-quality composition molecules of a substance can be expressed in this form: C6H6. As with other aromatic hydrocarbons - toluene, anthracene or naphthalene, combustion reactions and substitution of hydrogen atoms of the benzene ring will be typical for benzene. A feature of the severe oxidation of all aromatic compounds is a highly smoky flame. A mixture of benzene vapor and air is explosive, so all experiments with the substance in the laboratory are carried out only in a fume hood. Benzene, like other aromatic substances, does not add either water or hydrogen halides. It also does not discolor potassium permanganate solution and bromine water. Homologues of benzene, such as toluene or cumene, can be oxidized; in this case, it is not the benzene ring itself that undergoes the reaction, but only the radical.

Chemical properties of arenes

What reactions are capable of compounds containing benzene rings and a one-and-a-half bond between carbon atoms? These are, first of all, substitution reactions, which occur much more easily in them than in alkanes. Let's imagine a recording of the catalytic reaction between benzene and bromine with the participation of ferric bromide, leading to the formation of bromobenzene, a colorless liquid insoluble in water:

C6H6+ Br2→ C6H5Br +HBr

If aluminum chloride is used as a catalyst in the process, it is possible to achieve complete replacement of all hydrogen atoms in the benzene molecule. In this case, hexachlorobenzene is formed, colorless crystals of which are used in methods of protecting seeds of cultivated plants and in wood processing processes to extend its shelf life. For more full characteristics arenov let's add some facts. In order for aromatic compounds to attach other substances, such as chlorine, special conditions are needed. In our case, this will be ultraviolet irradiation of the reacting mixture. The reaction product will be hexachlorocyclohexane, or, as it is also called, hexachlorane. This is known in agriculture product - an insecticide used to control insect pests.

How and why is nitrobenzene obtained?

Let us continue our review of the chemical properties of arenes. By using concentrated nitric and sulfate acids (nitrating mixture) in one reaction, it is possible to obtain from benzene a product important for organic synthesis - nitrobenzene. This liquid is pale yellow in color, oily in appearance, and has an almond odor. It is insoluble in water, but is often used as a solvent for many organic substances: varnishes, fats, etc. Nitrobenzene is a high-tonnage product, as it is used as a raw material for the production of aniline. This substance is so significant for the chemical industry that it is worth dwelling on it in more detail. The famous Russian chemist N.N. In 1842, Zinin obtained aniline from nitrobenzene by reduction reaction with ammonium sulfide. IN modern conditions The contact method has become widespread, in which a mixture of hydrogen and nitrobenzene vapor is passed at a temperature of 300 °C over a catalyst. The resulting aromatic amine is subsequently used for the production of explosives, dyes, and medicines.

What are aromatic hydrocarbons extracted from?

The most promising is the production of arenes from the product of coking coal and during oil refining. Cycloparaffins contained in coal tar are hydrogenated over a catalyst at temperatures up to 300 °C, the reaction product is benzene. Dehydrogenation of alkanes also leads to the formation of aromatic hydrocarbons. Using the Zelinsky-Kazansky reaction, benzene is obtained from ethyne by passing it through a tube with activated carbon heated to 600 °C. The preparation of arenes, for example toluene, is carried out using the Friedel-Crafts reaction. Methylbenzene (toluene) can also be produced using heptane. The obtained types of arenes are used as solvents and additives to motor fuel, in the production of aniline dyes and pesticides.

Naphthalene

In the 50-70s of the last century, one of the favorite means of protecting fur and wool products from moths in everyday life was naphthalene. With its prolonged use, clothes acquired a characteristic, very persistent odor. However, more important is the use of naphthalene as a raw material for the synthesis of medicines, dyes, and explosives. The main methods for its production are based on the processing of petroleum distillation products and ethylene production waste - pyrolysis resin. The substance, unlike benzene, contains two benzene nuclei, so nitration and halogenation reactions occur faster in it. Continuing to give examples of arenes, we will focus on another aromatic hydrocarbon that is important for industry - vinylbenzene.

Styrene

Modern industry building materials impossible without polymer materials: easy to process, durable and wear-resistant. Polymers obtained from vinylbenzene, for example, such as polystyrene foam (foamed polystyrene), SAN and ABS plastics, are used in the production of suspended ceilings, floor coverings, and wall insulation. Styrene is obtained from ethylbenzene in the form of a colorless, flammable liquid with a peculiar odor. Subsequently, it is subjected to polymerization and a solid glassy mass is extracted - polystyrene. It serves as the starting product in the production of the above-mentioned building materials. Vinylbenzene is used as a solvent, used along with butadiene in the polymerization reaction leading to the synthesis of styrene-butadiene rubbers.

Nomenclature of aromatic compounds

The name of arenes according to the international IUPAC classification includes the designation of a substituent, to which the word “benzene” is added. For example, C6H5CH3 is methylbenzene, C6H5C2H3 is vinylbenzene. These connections have trivial names, so, the first compound is called toluene, the second - styrene. Arenes may contain two substituents, for example two methyl radicals. They are capable of joining the carbon ring in three positions: at 1 and 2 carbon atoms, then they speak of the ortho position of the substituents. If the radicals are located at 1 and 3 carbon particles, then we are talking about the meta position of the substituents; at 1 and 4 carbon atoms, this is a para-substitution. Higher homologs of benzene can be represented as derivatives of saturated hydrocarbons, in the molecules of which one hydrogen atom is replaced by a phenyl radical C6H5-. For example, a compound with the formula C6H5C6H13 would be called "phenylhexane".

In our article, we studied the chemical properties of arenes, and also characterized their properties and applications in industry.

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 in structural formulas not specified).

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.

Physical properties

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

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

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

Table. Physical properties of some arenas

Name	Formula	t°.pl., °C	t°.b.p., °C
Benzene	C6H6	5,5	80,1
Toluene (methylbenzene)	C 6 H 5 CH 3	95,0	110,6
Ethylbenzene	C 6 H 5 C 2 H 5	95,0	136,2
Xylene (dimethylbenzene)	C 6 H 4 (CH 3) 2
ortho-		25,18	144,41
meta-		47,87	139,10
pair-		13,26	138,35
Propylbenzene	C 6 H 5 (CH 2) 2 CH 3	99,0	159,20
Cumene (isopropylbenzene)	C 6 H 5 CH(CH 3) 2	96,0	152,39
Styrene (vinylbenzene)	C 6 H 5 CH=CH 2	30,6	145,2

Benzene – low boiling ( tbale= 80.1°C), colorless liquid, insoluble in water

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

Most aromatic hydrocarbons are life-threatening and toxic.

Preparation of arenes (benzene and its homologues)

In the laboratory

1. Fusion of benzoic acid salts with solid alkalis

C6H5-COONa + NaOH t → C 6 H 6 + Na 2 CO 3

sodium benzoate

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

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

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

In industry

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

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

C6H14 t , kat→C 6 H 6 + 4H 2

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

3С 2 H 2 600°C, Act. coal→C 6 H 6

3. Dehydrogenation cyclohexane and its homologues:

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

C6H12 t, kat→C 6 H 6 + 3H 2

C6H11-CH3 t , kat→C 6 H 5 -CH 3 + 3H 2

methylcyclohexantoluene

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

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

chloroethane ethylbenzene

Chemical properties of arenes

I. OXIDATION REACTIONS

1. Combustion (smoking flame):

2C6H6 + 15O2 t→12CO 2 + 6H 2 O + Q

2. Under normal conditions, benzene does not discolor bromine water and water solution potassium permanganate

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

A) in an acidic environment to benzoic acid

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 a-carbon atom, which is oxidized into a carboxyl group.

Homologues of benzene with one side chain give benzoic acid:

Homologues containing two side chains give dibasic acids:

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

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

Simplified :

C6H5-CH3+3O KMnO4→C 6 H 5 COOH + H 2 O

B) in neutral and slightly alkaline to benzoic acid salts

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

II. ADDITION REACTIONS (harder than alkenes)

1. Halogenation

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

2. Hydrogenation

C6H6 + 3H2 t , PtorNi→C 6 H 12 (cyclohexane)

3. Polymerization

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

b) benzene homologues upon irradiation or heating

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 is always replaced at the carbon atom directly bonded to the benzene ring (a-carbon atom).

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

c) benzene homologues in the presence of a catalyst

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

2. Nitration (with nitric acid)

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

nitrobenzene - smell almonds!

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

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

Application of benzene and its homologues

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

Toluene C 6 H 5 –CH 3 – solvent, used in the production of dyes, medicinal and explosives (TNT (TNT), or 2,4,6-trinitrotoluene TNT).

Xylenes C6H4(CH3)2. Technical xylene is a mixture of three isomers ( ortho-, meta- And pair-xylenes) – used as a solvent and starting product for the synthesis of many organic compounds.

Isopropylbenzene C 6 H 5 –CH(CH 3) 2 is used to produce phenol and acetone.

Chlorinated derivatives of benzene used for plant protection. Thus, the product of replacement of H atoms in benzene with chlorine atoms is hexachlorobenzene C 6 Cl 6 - a fungicide; it is used for dry treatment of wheat and rye seeds against smut. The product of the addition of chlorine to benzene is hexachlorocyclohexane (hexachlorane) C 6 H 6 Cl 6 - an insecticide; it is used to control harmful insects. The substances mentioned belong to pesticides - chemical means of combating microorganisms, plants and animals.

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

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