Arenes do not react with halogens. Aromatic hydrocarbons (arenes): classification, nomenclature and isomerism, physical properties

DEFINITION

Aromatic hydrocarbons (arenes)- substances whose molecules contain one or more benzene rings. General formula of the homologous series of benzene C n H 2 n -6

The simplest representatives of aromatic hydrocarbons are benzene - C 6 H 6 and toluene - C 6 H 5 -CH 3. Hydrocarbon radicals derived from arenes are called: C 6 H 5 - - phenyl (Ph-) and C 6 H 5 -CH 2 - - benzyl.

All six carbon atoms in the benzene molecule are in the sp 2 hybrid state. Each carbon atom forms 3σ bonds with two other carbon atoms and one hydrogen atom lying in the same plane. Six carbon atoms form a regular hexagon (σ-skeleton of the benzene molecule).

Each carbon atom has one unhybridized p-orbital, which contains one electron. Six p-electrons form a single π-electron cloud (aromatic system), which is depicted as a circle inside a six-membered cycle.

Chemical properties of arenes

Benzene and its homologues are characterized by substitution reactions proceeding according to the electrophilic mechanism:

- halogenation (benzene interacts with chlorine and bromine in the presence of catalysts - anhydrous AlCl 3, FeCl 3, AlBr 3)

C 6 H 6 + Cl 2 \u003d C 6 H 5 -Cl + HCl

- nitration (benzene easily reacts with a nitrating mixture - a mixture of concentrated nitric and sulfuric acids)

- alkylation with alkenes

C 6 H 6 + CH 2 \u003d CH-CH 3 → C 6 H 5 -CH (CH 3) 2

Addition reactions to benzene lead to the destruction of the aromatic system and proceed only under harsh conditions:

- hydrogenation (the reaction proceeds when heated, the catalyst is Pt)

- addition of chlorine (occurs under the action of UV radiation with the formation of a solid product - hexachlorocyclohexane (hexachlorane) - C 6 H 6 Cl 6)

Physical properties of arenes

The first members of the benzene homologous series are colorless liquids with a specific odor. They are lighter than water and practically insoluble in it. They dissolve well in organic solvents and are themselves good solvents.

Getting arenas

The main methods for obtaining benzene and its homologues:

- dehydrocyclization of alkanes (catalysts - Pt, Cr 3 O 2)

– dehydrogenation of cycloalkanes (the reaction proceeds when heated, the catalyst is Pt)

– trimerization of acetylene (the reaction proceeds when heated to 600C, the catalyst is activated carbon)

3HC≡CH → C 6 H 6

- alkylation of benzenes (Friedel-Crafts reaction) (catalyst - aluminum chloride or phosphoric acid)

Examples of problem solving

EXAMPLE 1

Exercise

The vapor density of the substance is 3.482 g/l. Its pyrolysis yielded 6 g of soot and 5.6 liters of hydrogen. Determine the formula for this substance.

Solution

Find the amount of soot (carbon) substance: v(C) = m(C)/M(C) v(C) = 6/12 = 0.5 mol Find the amount of hydrogen substance: v (H 2) \u003d V (H 2) / V m v (H 2) \u003d 5.6 / 22.4 \u003d 0.25 mol Therefore, the amount of substance of one hydrogen atom will be equal to: v(H) \u003d 2 × 0.25 \u003d 0.5 mol Let's denote the number of carbon atoms in the hydrocarbon molecule as x, and the number of hydrogen atoms as y, then the ratio of these atoms in the molecule: x:y = 0.5: 0.5 = 1:1 The simplest formula of the hydrocarbon CH The molecular weight of a hydrocarbon is: M(C x H y) \u003d ρ × V m \u003d 3.482 × 22.4 \u003d 78 g / mol The molecular weight of a molecule of CH composition is: M(CH) = 13 g/mol n \u003d M (C x H y) / M (CH) \u003d 78/13 \u003d 6, therefore, the coefficients x and y must be multiplied by 6, then the desired hydrocarbon has the composition C 6 H 6 - this is benzene

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 emits a proton, and two electrons S-N connections go into the l-electronic 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 severe 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 maybe 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 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.

aromatic hydrocarbons- compounds of carbon and hydrogen, in the molecule of which there is a benzene ring. The most important representatives of aromatic hydrocarbons are benzene and its homologues - the products of substitution of one or more hydrogen atoms in the benzene molecule for hydrocarbon residues.

The structure of the benzene molecule

The first aromatic compound, benzene, was discovered in 1825 by M. Faraday. Its molecular formula was established - C 6 H 6. If we compare its composition with the composition saturated hydrocarbon containing the same number of carbon atoms - hexane (C 6 H 14), then you can see that benzene contains eight fewer hydrogen atoms. As is known, the appearance of multiple bonds and cycles leads to a decrease in the number of hydrogen atoms in a hydrocarbon molecule. In 1865, F. Kekule proposed it structural formula as cyclohexantriene - 1, 3, 5.

So the molecule corresponding to Kekule formula, contains double bonds, therefore, benzene must have an unsaturated character, i.e., it is easy to enter into addition reactions: hydrogenation, bromination, hydration, etc.

However, the data of numerous experiments have shown that benzene enters into addition reactions only under harsh conditions (at high temperatures and light), and is resistant to oxidation. The most characteristic of it are substitution reactions, therefore, benzene is closer in character to the marginal hydrocarbons.

Trying to explain these inconsistencies, many scientists have proposed various options for the structure of benzene. The structure of the benzene molecule was finally confirmed by the reaction of its formation from acetylene. In fact, the carbon-carbon bonds in benzene are equivalent, and their properties are not similar to those of either single or double bonds.

Currently, benzene is denoted either by the Kekule formula, or by a hexagon in which a circle is depicted.

So what is the peculiarity of the structure of benzene? Based on the researchers' data and calculations, it was concluded that all six carbon atoms are in the state sp 2 hybridization and lie in the same plane. unhybridized p-orbitals of carbon atoms that make up double bonds (Kekule formula) are perpendicular to the plane of the ring and parallel to each other.

They overlap with each other, forming a single π-system. Thus, the system of alternating double bonds depicted in the Kekule formula is a cyclic system of conjugated, overlapping α-bonds. This system consists of two toroidal (donut-like) regions of electron density lying on both sides of the benzene ring. So, represent benzene in the form regular hexagon with a circle in the center (π-system) is more logical than in the form of cyclohexatriene-1,3,5.

The American scientist L. Pauling suggested that benzene be represented as two boundary structures that differ in the distribution of electron density and constantly transform into each other, that is, consider it an intermediate compound, an "averaging" of two structures.

The measured bond lengths confirm these assumptions. It was found that all C-C bonds in benzene have the same length (0.139 nm). They are somewhat shorter than single C-C ties(0.154 nm) and longer doubles (0.132 nm).

There are also compounds whose molecules contain several cyclic structures.

Isomerism and nomenclature

The benzene homologues are characterized by position isomerism of several substituents. The simplest benzene homologue, toluene (methylbenzene), does not have such isomers; the following homologue is presented as four isomers:

The basis of the name of an aromatic hydrocarbon with small substituents is the word benzene. Atoms in an aromatic ring are numbered from the highest substituent to the youngest:

According to the old nomenclature, positions 2 and 6 are called ortho positions, 4 - pair-, and 3 and 5 - metapositions.

Physical Properties
Benzene and its simplest homologues under normal conditions are very toxic liquids with a characteristic unpleasant odor. They are poorly soluble in water, but well - in organic solvents.

Chemical properties of benzene

Substitution reactions. Aromatic hydrocarbons enter into substitution reactions.
1. Bromination. When reacting with bromine in the presence of a catalyst, iron bromide (ΙΙΙ), one of the hydrogen atoms in the benzene ring can be replaced by a bromine atom:

2. Nitration of benzene and its homologues. When an aromatic hydrocarbon interacts with nitric acid in the presence of sulfuric acid (a mixture of sulfuric and nitric acids is called a nitrating mixture), a hydrogen atom is replaced by a nitro group -NO 2:

By reducing the nitrobenzene formed in this reaction, aniline is obtained - a substance that is used to obtain aniline dyes:

This reaction is named after the Russian chemist Zinin.
Addition reactions. aromatic compounds can also enter into addition reactions to the benzene ring. In this case, cyclohexane or its derivatives are formed.
1. hydrogenation. The catalytic hydrogenation of benzene proceeds at more high temperature than the hydrogenation of alkenes:

2. Chlorination. The reaction proceeds under illumination with ultraviolet light and is a free radical:

Benzene homologues

The composition of their molecules corresponds to the formula C n H 2 n-6. The closest homologues of benzene are:

All benzene homologues following toluene have isomers. Isomerism can be associated both with the number and structure of the substituent (1, 2), and with the position of the substituent in the benzene ring (2, 3, 4). Connections general formula C 8 H 10:

According to the old nomenclature used to indicate the relative position of two identical or different substituents in the benzene ring, prefixes are used ortho- (abbreviated o-) - substituents are located at neighboring carbon atoms, meta-(m-) - through one carbon atom and pair— (P-) - substitutes against each other.
The first members of the homologous series of benzene are liquids with a specific odor. They are lighter than water. They are good solvents.

Benzene homologues react substitution ( bromination, nitration). Toluene is oxidized by permanganate when heated:

Benzene homologues are used as solvents, for the production of dyes, plant protection products, plastics, and medicines.

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, however, they easily dissolve in organic solvents - alcohol, ether, acetone.

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.

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

Table. Physical properties of some arenas

Name	Formula	t°.pl., °C	t°.bp., °C
Benzene	C 6 H 6	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 \u003d CH 2	30,6	145,2

Benzene - low-boiling ( tkip= 80.1°C), colorless liquid, insoluble in water

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

Most aromatic hydrocarbons are life threatening and toxic.

Obtaining arenes (benzene and its homologues)

In the laboratory

1. Fusion of salts of benzoic acid with solid alkalis

C 6 H 5 -COONa + NaOH t → C 6 H 6 + Na 2 CO 3

sodium benzoate

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

From 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, reforming;
• from coal tar and coke oven gas

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

C 6 H 14 t , kat→C 6 H 6 + 4H 2

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

3C 2 H2 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

C 6 H 12 t, cat→C 6 H 6 + 3H 2

C 6 H 11 -CH 3 t , kat→C 6 H 5 -CH 3 + 3H 2

methylcyclohexanetoluene

4. Alkylation of benzene(obtaining homologues of benzene) – 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 (smoky flame):

2C 6 H 6 + 15O 2 t→12CO 2 + 6H 2 O + Q

2. Benzene under normal conditions does not decolorize bromine water and an aqueous solution of potassium permanganate

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

A) in an acidic environment to benzoic acid

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 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 :

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

B) in neutral and slightly alkaline to salts of benzoic acid

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 - hexachloran)

2. Hydrogenation

C 6 H 6 + 3H 2 t , PtorNi→C 6 H 12 (cyclohexane)

3. Polymerization

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

b) benzene homologues upon irradiation or heating

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, heating or UV irradiation leads to a radical substitution reaction 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 → (mixture of orta, 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 almond!

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, trotyl)

The use 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, aniline is obtained from it), chlorobenzene C 6 H 5 Cl, phenol C 6 H 5 OH, styrene, etc.

Toluene C 6 H 5 -CH 3 - a solvent used in the manufacture of dyes, drugs and explosives (trotyl (tol), or 2,4,6-trinitrotoluene TNT).

Xylene C 6 H 4 (CH 3) 2 . Technical xylene is a mixture of three isomers ( ortho-, meta- and pair-xylenes) - is used as a solvent and starting product for the synthesis of many organic compounds.

Isopropylbenzene C 6 H 5 -CH (CH 3) 2 serves to obtain phenol and acetone.

Chlorine derivatives of benzene used for plant protection. Thus, the product of substitution of H atoms in benzene with chlorine atoms is hexachlorobenzene C 6 Cl 6 - a fungicide; it is used for dry seed dressing of wheat and rye against hard smut. The product of the addition of chlorine to benzene is hexachlorocyclohexane (hexachloran) C 6 H 6 Cl 6 - an insecticide; it is used to control harmful insects. These substances refer to pesticides - chemical means of combating microorganisms, plants and animals.

Styrene C 6 H 5 - CH \u003d CH 2 polymerizes very easily, forming polystyrene, and copolymerizing with butadiene - styrene-butadiene rubbers.

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