Aromatic hydrocarbons– compounds of carbon and hydrogen, the molecule of which contains a benzene ring. The most important representatives of aromatic hydrocarbons are benzene and its homologues - products of the replacement of one or more hydrogen atoms in a benzene molecule with 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 of a saturated hydrocarbon containing the same number of carbon atoms - hexane (C 6 H 14), then we can see that benzene contains eight less 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 its structural formula as cyclohexanthriene - 1, 3, 5.

Thus, the molecule corresponding Kekule's formula, contains double bonds, therefore, benzene must be unsaturated, i.e., it must easily undergo addition reactions: hydrogenation, bromination, hydration, etc.

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

Trying to explain these discrepancies, 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 reality, 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 special about the structure of benzene? Based on the researchers' data and calculations, it was concluded that all six carbon atoms are in a 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 each other, forming a single π-system. Thus, the system of alternating double bonds depicted in Kekulé’s formula is a cyclic system of conjugated, overlapping bonds. This system consists of two toroidal (donut-like) regions of electron density lying on either side of the benzene ring. Thus, it is more logical to depict benzene as a regular hexagon with a circle in the center (π-system) than as cyclohexatriene-1,3,5.

The American scientist L. Pauling proposed to represent benzene in the form of two boundary structures that differ in the distribution of electron density and constantly transform into each other, i.e., consider it an intermediate compound, “averaging” of two structures.

Bond length measurements confirm these assumptions. It was found that all C-C bonds in benzene have the same length (0.139 nm). They are slightly shorter than single C-C bonds (0.154 nm) and longer than double bonds (0.132 nm).

There are also compounds whose molecules contain several cyclic structures.

Isomerism and nomenclature

Benzene homologues are characterized by isomerism of the position of several substituents. The simplest homolog of benzene - 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. The atoms in the aromatic ring are numbered from highest to lowest substituent:

According to the old nomenclature, positions 2 and 6 are called orthopositions, 4 - pair-, and 3 and 5 - meta-provisions.

Physical properties
Under normal conditions, benzene and its simplest homologues are very toxic liquids with a characteristic unpleasant odor. They dissolve poorly in water, but well in organic solvents.

Chemical properties of benzene

Substitution reactions. Aromatic hydrocarbons undergo 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), the hydrogen atom is replaced by a nitro group -NO2:

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 undergo addition reactions to the benzene ring. In this case, cyclohexane or its derivatives are formed.
1. Hydrogenation. Catalytic hydrogenation of benzene occurs at a higher temperature than the hydrogenation of alkenes:

2. Chlorination. The reaction occurs when illuminated with ultraviolet light and is 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 homologues of benzene following toluene have isomers. Isomerism can be associated both with the number and structure of the substituent (1, 2), and with the position of the substituent in the benzene ring (2, 3, 4). Compounds of the general formula C 8 H 10:

According to the old nomenclature used to indicate the relative location of two identical or different substituents on the benzene ring, the prefixes are used ortho- (abbreviated o-) - substituents are located at neighboring carbon atoms, meta-(m-) – through one carbon atom and pair— (P-) – substituents 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 substitutions ( bromination, nitration). Toluene is oxidized by permanganate when heated:

Benzene homologues are used as solvents to produce dyes, plant protection products, plastics, and medicines.

Benzene

The simplest representative of arenes is benzene. Let's take a closer look at its properties.

Physical properties

Benzene is a transparent, colorless, highly volatile liquid with a characteristic odor (it is because of the strong odor that aromatic compounds got their name). Melting point 5.5°C, boiling point - 80°C. Does not mix with water, but mixes well with most organic solvents. It is a solvent for non-polar organic substances. Burns with a smoky flame (incomplete combustion) with the formation, in addition to carbon dioxide and water, of a significant amount of soot. Poisonous both as a liquid and as a vapor if inhaled.

Obtaining benzene

1. In industry, benzene is produced by oil reforming, which is essentially the dehydrogenation of oil alkanes with the formation of a cyclic skeleton. In its “pure” form, the main reforming reaction is the dehydrogenation of hexane:

In addition, benzene is one of the volatile products of coking. Coking is heating coal to 1000°C without air access. This also produces many other valuable reagents for organic synthesis and coke used in metallurgy. Benzene can also be obtained by trimerization of acetylene over activated carbon at 100°C.

2. Of course, benzene is not produced in the laboratory, but theoretically there are methods for its synthesis (they are used to obtain its derivatives). Both industrial and laboratory methods are reflected in the diagram below.

Scheme methods for producing benzene

Industrial methods.

Chemical properties of benzene

The chemical properties of benzene are determined, of course, by its p-system. Just as in the case of alkenes, it can be attacked by an electrophilic species. However, in the case of aromatic compounds, the result of such an attack will be completely different. The high stability of the p-system leads to the fact that at the end of the reaction it is, as a rule, restored and the result of the reaction is not addition (which would destroy

p-system), but electrophilic substitution. Let's take a closer look at its mechanism.

In the first stage, the attack of the AB molecule containing the electrophilic center A leads to the formation of an extremely unstable p-complex (stage 1). In this case, the aromatic system is not disrupted. Next, a covalent bond is formed between one of the atoms of the ring and particle A (stage 2). In this case, firstly, the A-B bond is broken, and secondly, the p-system is destroyed. The resulting unstable positively charged molecule is called an s-complex. As already mentioned, the restoration of the p-system is energetically very favorable, and this leads to the rupture of either the C-A bond (and then the molecule returns to its original state) or the C-H bond (stage 3). In the latter case, the reaction ends, and the product is the replacement of hydrogen by A.

Most reactions of aromatic compounds have this mechanism (electrophilic substitution, abbreviated S E). Let's look at some of them.

1. Halogenation. Occurs only in the presence of catalysts - Lewis acids (see "Lewis Theory"). The task of the catalyst is to polarize the halogen molecule to form a good electrophilic center:

| AlCl 3 + Cl 2 “Cl + [AlCl 4 ] - The resulting particle has an electrophilic chlorine atom, and

reaction occurs:

TO Nitration. It is carried out with a mixture of nitric and sulfuric acids (nitrating mixture). The following reaction occurs in the nitrating mixture:

HNO 3 +H 2 SO 4 “NO + 2 +H 2 O

The resulting nitronium hydrosulfate has a powerful electrophilic center - the nitronium ion NO + 2. Accordingly, a reaction takes place, the general equation of which is:

3. Sulfonation. In concentrated sulfuric acid there is an equilibrium:

2H 2 SO 4 “SO 3 H + - +H 2 O

The molecule on the right side of the equilibrium has a strong electrophile SO 3 H +, which reacts with benzene. Resulting reaction:

Alkylation according to Friedel-Crafts. When benzene reacts with alkyl chlorides or alkenes in the presence of Lewis acids (usually aluminum halides), alkyl-substituted benzenes are obtained. In the case of alkyl halides, the first stage of the process is:

RСl + АlСl 3 «R + [АlСl 4 ] - In the second stage, the electrophilic particle R + attacks the p-system:

In the case of alkenes, the Lewis acid polarizes the double bond of the alkene, and again an electrophilic center is formed on the carbon:

Non-electrophilic reactions include:

1. Hydrogenation of benzene. This reaction involves the destruction of the p-system and requires harsh conditions (high pressure, temperature, catalyst - platinum metals):

2. Radical chlorination. In the absence of Lewis acids and under harsh ultraviolet irradiation, benzene can react with chlorine by a radical mechanism. In this case, the p-system is destroyed and the product of chlorine addition is formed - the solid substance hexachlorane, which was previously used as an insecticide:

Benzene homologues

Nomenclature and isomerism of arenes

All arenas can be roughly divided into two rows. The first row is benzene derivatives (toluene, biphenyl): the second row is condensed (polynuclear) arenes (naphthalene, anthracene).

Let's consider the homologous series of benzene; compounds of this series have the general formula C n H 2 n. 6. Structural isomerism in the homologous series of benzene is due to the mutual arrangement of substituents in the nucleus. Monosubstituted benzene derivatives do not have positional isomers, since all atoms in the benzene ring are equivalent,

I Group C 6 H 5 is called phenyl. The phenyl and substituted phenyl groups are called aryl. Some benzene derivatives are shown below:

Benzene reaction scheme

Isomers with two substituents at positions 1,2; 1,3 and 1,4 are called ortho-, meta- and para-isomers:

Nomenclature of aromatic compounds

Below are the names of some aromatic compounds:

C 6 H 5 NH 3 + Cl - Phenylammonium chloride (anilinium chloride)

C b H 5 CO 2 H Benzenecarboxylic acid (benzoic acid)

C 6 H 5 CO 2 C 2 H 5 Benzene carboxylic acid ethyl ester (ethyl benzoate)

C 6 H 5 COCl Benzenecarbonyl chloride (benzoyl chloride)

C 6 H 5 CONH 2 Benzenecarboxamide (benzamide)

C 6 H 5 CN Benzenecarbonitrile (benzonitrile)

C6H5CHO Benzenecarbaldehyde (benzaldehyde)

C 6 H 5 COCH 3 Acetophenone

C6H5OH Phenol

C 6 H 5 NH 2 Phenylamine (aniline)

C 6 H 5 OCH 3 Methoxybenzene (anisole)

These names follow IUPAC nomenclature. In parentheses are traditional names that are still widespread and quite acceptable.

Arena nomenclature

The name of a benzene derivative with two or more substituents on the benzene ring is constructed in this way. The carbon atom of the benzene ring to which the substituent closest to the beginning of the above list is attached receives the number 1. Next, the carbon atoms of the benzene ring are numbered so that the locant - the number of the second substituent - is the smallest.

3-Hydroxybenzenecarboxylic acid (3-hydroxybenzoic acid)

The carboxyl group is treated as the main group and is assigned a locant of "1". The ring numbering is constructed so that the hydroxyl group receives a smaller (“3” rather than “5”) locant.

2-aminobenzenecarb aldehyde (2-aminobenzaldehyde)

The -CHO group is considered as the main one. She receives a locant of "1". Group-NH 2 is in position "2" rather than "6". In addition, the name o-aminobenzaldehyde is acceptable.

1-bromo-2-nitro-4-chlorobenzene These groups are listed in alphabetical order.

Getting arenas

Preparation from aliphatic hydrocarbons. When straight-chain alkanes with at least 6 carbon atoms per molecule are passed over heated platinum or chromium (III) oxide, dehydrocyclization occurs - the formation of an arene with the release of hydrogen. For example:

2. Dehydrogenation of cycloalkanes. The reaction occurs by passing vapors of cyclohexane and its homologues over heated platinum:

|. Preparation of benzene by trimerization of acetylene. According to the method of N.D. Zelinsky and B.A. Kazansky, benzene can be obtained by passing acetylene through a tube with activated carbon heated to 100°C. The whole process can be represented by a diagram:

4. Preparation of benzene homologues using the Friedel-Crafts reaction(see Chemical properties of benzene).

5. Fusion of salts of aromatic acids with alkali: C 6 H 6 -COONa+NaOH ®C 6 H 6 +Na 2 CO 3

Application of arenas

Arenas are used as chemical raw materials for the production of medicines, plastics, dyes, pesticides and many other organic substances. Arenes are widely used as solvents.

Dehydrogenation reactions make it possible to use petroleum hydrocarbons to produce hydrocarbons of the benzene series. They indicate the connection between different groups of hydrocarbons and their mutual transformation into each other.

Arenas(aromatic hydrocarbons) - compounds whose molecules contain one or more benzene rings - cyclic groups of carbon atoms with a specific nature of bonds.

Benzene - molecular formula C 6 H 6. It was first proposed by A. Kekule:

Arena structure.

All 6 carbon atoms are in sp 2-hybridization. Each carbon atom forms 2 σ -bonds with two neighboring carbon atoms and one hydrogen atom, which are in the same plane. The angles are 120°. Those. All carbon atoms lie in the same plane and form a hexagon. Each atom has a non-hybrid R-the habitation on which the unpaired electron is located. This orbital is perpendicular to the plane, and therefore π -the electron cloud is “spread” over all carbon atoms:

All connections are equal. Conjugation energy is the amount of energy that must be expended to destroy an aromatic system.

This is what determines the specific properties of benzene - the manifestation of aromaticity. This phenomenon was discovered by Hückel, and is called Hückel's rule.

Arene isomerism.

Arenas can be divided into 2 groups:

• benzene derivatives:

• condensed arenas:

The general formula of arenes is WITHnH 2 n -6 .

Arenes are characterized by structural isomerism, which is explained by the mutual arrangement of substituents in the ring. If there are 2 substituents in the ring, then they can be in 3 different positions - ortho (o-), meta (m-), para (p-):

If one proton is “taken away” from benzene, a radical is formed - C 6 H 5, which is called the aryl radical. Protozoa:

Arenes are called the word “benzene”, indicating the substituents in the ring and their positions:

Physical properties of arenas.

The first members of the series are colorless liquids with a characteristic odor. They are highly soluble in organic solvents, but insoluble in water. Benzene is toxic, but has a pleasant smell. Causes headaches and dizziness; inhalation of large quantities of vapor can cause loss of consciousness. Irritating to mucous membranes and eyes.

Getting arenas.

1. From aliphatic hydrocarbons using the “aromatization” of saturated hydrocarbons that make up the oil. When passed over platinum or chromium oxide, dihydrocyclization occurs:

2. Dehydrogenation of cycloalkanes:

3. From acetylene (trimerization) when passing over hot coal at 600°C:

4. Friedel-Crafts reaction in the presence of aluminum chloride:

5. Fusion of salts of aromatic acids with alkali:

Chemical properties of arenes.

Arene substitution reactions.

The arene core has a mobile π - a system that is affected by electrophilic reagents. Arenes are characterized by electrophilic substitution, which can be represented as follows:

An electrophilic particle is attracted to π -ring system, then a strong bond is formed between the reagent X and one of the carbon atoms, in which case the unity of the ring is disrupted. To restore aromaticity, a proton is emitted and 2 electrons S-N pass into the π-system of the ring.

1. Halogenation occurs in the presence of catalysts - anhydrous chlorides and bromides of aluminum and iron:

2. Nitration of arenes. Benzene reacts very slowly with concentrated nitric acid when heated. But if you add sulfuric acid, the reaction proceeds very easily:

3. Sulfonation occurs under the influence of 100% sulfuric acid - oleum:

4. Alkylation with alkenes. As a result, chain elongation occurs, the reaction proceeds in the presence of a catalyst - aluminum chloride:

Arene addition reactions.

1. Hydrogenation (with catalysts) of arenes:

2. Radical halogenation due to the interaction of benzene vapor and strong UV radiation. As a result, a solid product is formed - C 6H6Cl6:

3. Oxidation by air oxygen. The reaction occurs at vanadium (V) oxide and 400°C:

Benzene homologues have a number of differences - for their products I am the original substituent in the ring:

Substitution in the ring is possible only in the presence of a catalyst (iron and aluminum chloride); the substitution occurs in the ortho- and para-positions relative to the alkyl radical:

If strong oxidizing agents (potassium permanganate) act, the alkyl chain is destroyed and benzoic acid is formed:

Additional materials on the topic: Arenas. Properties of arenas.

Definition, structure of the arene molecule.

Arenas- these are organic compounds, hydrocarbons of a carbocyclic aromatic nature, the molecules of which contain one or more benzene rings - cyclic groups of carbon atoms with the specific nature of the benzene ring bonds, and which correspond to the general formula C n H 2 n -6.

Benzene is the simplest arene with a molecular formula C 6 H 6. The formula was first proposed by A. Kekule:

The structure of arene molecules.

All 6 carbon atoms are in sp 2-hybridization. Each carbon atom forms 2 σ -bonds with two neighboring carbon atoms and one hydrogen atom, which are in the same plane. The angles are 120°. Those. All carbon atoms lie in the same plane and form a hexagon. Each atom has a non-hybrid R-the habitation on which the unpaired electron is located. This orbital is perpendicular to the plane, and therefore π -the electron cloud is “spread” over all carbon atoms:

All connections are equal. Conjugation energy is the amount of energy that must be expended to destroy an aromatic system.

This is what determines the specific properties of benzene - the manifestation of aromaticity. This phenomenon was discovered by Hückel, and is called Hückel's rule.

2. Isomerism of arenes.

Arenas can be divided into 2 groups:

benzene derivatives:

Condensed arenas:

The general formula of arenes is C n H 2n-6.

Arenes are characterized by structural isomerism, which is explained by the mutual arrangement of substituents in the ring. If there are 2 substituents in the ring, then they can be in 3 different positions - ortho (o-), meta (m-), para (p-).

TOPIC 20. Aromatic hydrocarbons of the benzene series.

Homologous series. General formula. Nomenclature. Isomerism.

Natural sources and methods of production: from aliphatic, alicyclic and aromatic compounds. Wurtz-Fittig and Friedel-Crafts reactions.

Physical properties of benzene and its homologues.

Chemical properties. Electrophilic substitution reactions: halogenation, nitration, sulfonation, alkylation and acylation. Reactions of addition of hydrogen, halogens, ozone. Benzene oxidation. Reactions of benzene homologues involving the side chain: halogenation, nitration, oxidation, dehydrogenation.

Homologous series. General formula.

The first and most important representative of monocyclic aromatic hydrocarbons (arenes) is benzene C 6 H 6 .

Hence the common name for the homologous series of aromatic hydrocarbons - benzene series.

General formula homologous series of benzene: S p N 2p-6.

Arena nomenclature.

Trivial names are often used to name arenas: benzene, toluene, styrene, cumene, etc.

According to the international substitution nomenclature arenes are considered as benzene derivatives, in which the position of the substituents is indicated by numbers, while the numbers of the carbon atoms on which the substituents are located should be the smallest:

methylbenzene ethylbenzene isopropylbenzene

(toluene)(cumene)

vinylbenzene ethynylbenzene

(styrene) (phenylacetylene)

In the case of two identical substituents, instead of numbers, you can use prefixes: 1,2- (ortho-), 1,3- (meta-), 1,4- (para-):

1,2-dimethylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene

(ortho-xylene) (meta-xylene) (para-xylene)