Preparation of reaction arenes. Organic Chemistry: Arenas

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 easily dissolve 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 of their molecules.

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

Table. Physical properties of some arenas

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

FROM 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 water solution 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!

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 - 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. The substances mentioned are pesticides - chemicals control of 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.

VIDEO EXPERIENCES

Chemistry is a very fascinating science. It studies all substances that exist in nature, and there are a lot of them. They are divided into inorganic and organic. In this article, we will look at aromatic hydrocarbons, which belong to the last group.

What it is?

These are organic substances that have one or more benzene nuclei in their composition - stable structures of six carbon atoms connected in a polygon. These chemical compounds have a specific smell, which can be understood from their name. Hydrocarbons of this group are cyclic, in contrast to alkanes, alkynes, etc.

aromatic hydrocarbons. Benzene

This is the simplest chemical compound from this group of substances. The composition of its molecules includes six carbon atoms and the same amount of hydrogen. All other aromatic hydrocarbons are derivatives of benzene and can be obtained using it. This substance under normal conditions is in a liquid state, it is colorless, has a specific sweet smell, and does not dissolve in water. It begins to boil at a temperature of +80 degrees Celsius, and freeze - at +5.

Chemical properties of benzene and other aromatic hydrocarbons

The first thing you need to pay attention to is halogenation and nitration.

Substitution reactions

The first of these is halogenation. In this case, in order for the chemical reaction to take place, a catalyst, namely iron trichloride, must be used. Thus, if we add chlorine (Cl 2) to benzene (C 6 H 6), we will get chlorobenzene (C 6 H 5 Cl) and hydrogen chloride (HCl), which will be released as a clear gas with a pungent odor. That is, as a result of this reaction, one hydrogen atom is replaced by a chlorine atom. The same thing can happen when other halogens (iodine, bromine, etc.) are added to benzene. The second substitution reaction - nitration - proceeds according to a similar principle. Here, a concentrated solution of sulfuric acid acts as a catalyst. To carry out this kind of chemical reaction, it is necessary to add nitrate acid (HNO 3), also concentrated, to benzene, as a result of which nitrobenzene (C 6 H 5 NO 2) and water are formed. In this case, the hydrogen atom is replaced by a group of a nitrogen atom and two oxygens.

Addition reactions

This is the second type of chemical interactions that aromatic hydrocarbons are capable of entering into. They also exist in two forms: halogenation and hydrogenation. The first occurs only in the presence of solar energy, which acts as a catalyst. To carry out this reaction, chlorine must also be added to benzene, but in a larger amount than for substitution. There should be three chlorine per molecule of benzene. As a result, we get hexachlorocyclohexane (C 6 H 6 Cl 6), that is, six more chlorine will join the existing atoms.

Hydrogenation occurs only in the presence of nickel. To do this, mix benzene and hydrogen (H 2). The proportions are the same as in the previous reaction. As a result, cyclohexane (C 6 H 12) is formed. All other aromatic hydrocarbons can also enter into this type of reaction. They occur according to the same principle as in the case of benzene, only with the formation of more complex substances.

Obtaining chemicals of this group

Let's start with benzene. It can be obtained using a reagent such as acetylene (C 2 H 2). Of the three molecules of this substance under the influence high temperature and catalyst, one molecule of the desired chemical compound is formed.

Also, benzene and some other aromatic hydrocarbons can be extracted from coal tar, which is formed during the production of metallurgical coke. Toluene, o-xylene, m-xylene, phenanthrene, naphthalene, anthracene, fluorene, chrysene, diphenyl and others can be attributed to those obtained in this way. In addition, substances of this group are often extracted from petroleum products.

What do the various chemical compounds of this class look like?

Styrene is a colorless liquid with a pleasant odor, slightly soluble in water, the boiling point is +145 degrees Celsius. Naphthalene is a crystalline substance, also slightly soluble in water, melts at a temperature of +80 degrees, and boils at +217. Anthracene under normal conditions is also presented in the form of crystals, however, no longer colorless, but yellow in color. This substance is insoluble neither in water nor in organic solvents. Melting point - +216 degrees Celsius, boiling point - +342. Phenantrene looks like shiny crystals that dissolve only in organic solvents. Melting point - +101 degrees, boiling point - +340 degrees. Fluorene, as the name implies, is capable of fluorescence. This, like many other substances of this group, are colorless crystals, insoluble in water. Melting point - +116, boiling point - +294.

Application of aromatic hydrocarbons

Benzene is used in the production of dyes as a raw material. It is also used in the production of explosives, pesticides, and some drugs. Styrene is used in the production of polystyrene (polystyrene) by polymerization of the starting material. The latter is widely used in construction: as a heat and sound insulating, electrical insulating material. Naphthalene, like benzene, is involved in the production of pesticides, dyes, and drugs. In addition, it is used in the chemical industry to produce many organic compounds. Anthracene is also used in the manufacture of dyes. Fluorene plays the role of a polymer stabilizer. Phenantrene, like the previous substance and many other aromatic hydrocarbons, is one of the components of dyes. Toluene is widely used in the chemical industry for the extraction of organic substances, as well as for the production of explosives.

Characterization and use of substances extracted with aromatic hydrocarbons

These include, first of all, the products of the considered chemical reactions benzene. Chlorobenzene, for example, is an organic solvent, also used in the production of phenol, pesticides, organic substances. Nitrobenzene is a component of metal polishing agents, is used in the manufacture of some dyes and flavors, and can play the role of a solvent and oxidizing agent. Hexachlorocyclohexane is used as a poison for pest control and also in the chemical industry. Cyclohexane is used in the production of paints and varnishes, in the production of many organic compounds, in the pharmaceutical industry.

Conclusion

After reading this article, we can conclude that all aromatic hydrocarbons have the same chemical structure, which allows us to combine them into one class of compounds. In addition, their physical and Chemical properties are also very similar. Appearance, the boiling and melting points of all chemicals in this group do not differ much. Many aromatic hydrocarbons find their use in the same industries. Substances that can be obtained as a result of halogenation, nitration, hydrogenation reactions also have similar properties and are used for similar purposes.

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 arene series. Chemical properties, molecular structure and types of chemical bonds in its molecule have a number of features. We will consider them in our article, and also get acquainted with other compounds that are part of the group of aromatic hydrocarbons.

How the structural formula of arenes was established

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 single and double bonds, alternating with each other. However, the experimentally revealed chemical properties of arenes did not correspond to the formula proposed by F. Kekule. For example, benzene did not decolorize a solution of potassium permanganate and bromine water, which indicated the absence of pi bonds in the arene molecules. What is the real structure of benzene? Aromatic hydrocarbons have neither single nor double bonds. It has been experimentally established that these compounds contain an equivalent type of chemical bond between carbon atoms, called one and a half, or aromatic. That is why they do not enter into an oxidation reaction with solutions of KMnO4 and Br2. The general formula of arenes is derived - CnH2n-6. All the specific properties of aromatic compounds can be explained by their electronic structure, which we will study further.

Electronic formula

Using the example of benzene, we will establish how the carbon atoms are interconnected. It turned out that all six carbon atoms are in the form of sp2 hybridization. The carbon is connected to a hydrogen atom and two adjacent carbon atoms by three sigma bonds. This is what forms the flat hexagonal shape of the molecule. However, each carbon atom has one more negatively charged particle that is not involved in hybridization. Its electron cloud is dumbbell-shaped and sits above and below the plane of a hexagon called the benzene ring. Further, 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 arenes.

What is benzene?

A better understanding of the features of aromatic hydrocarbons will help acquaintance with the first representative of this class - benzene. 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, the quantitative and qualitative composition of a substance molecule can be expressed as follows: C6H6. As for other aromatic hydrocarbons - toluene, anthracene or naphthalene, for benzene, combustion reactions and substitution of hydrogen atoms of the benzene ring will be typical. A feature of the hard oxidation of all aromatic compounds is a strongly sooty flame. A mixture of benzene vapor with 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 water or hydrogen halides. It also does not discolour potassium permanganate solution and bromine water. Benzene homologues, such as toluene or cumene, can be oxidized, in which case it is not the benzene ring itself that undergoes the reaction, but only the radical.

Chemical properties of arenes

What kind of reactions are compounds containing benzene rings and a one-and-a-half bond between carbon atoms capable of? These are, first of all, substitution reactions that take place in them much easier than in alkanes. Imagine a record of a catalytic reaction between benzene and bromine involving 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 substitution of all hydrogen atoms in the benzene molecule. In this case, hexachlorobenzene is formed, the colorless crystals of which are used in methods for protecting seeds of cultivated plants and in wood processing processes to extend its shelf life. For more complete characteristics arenes let's add some facts. In order for aromatic compounds to be able 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 means - an insecticide used to control insect pests.

How and why is nitrobenzene obtained?

Let us continue our review of the chemical properties of arenes. Using concentrated nitric and sulfate acids (nitrating mixture) in one reaction, it is possible to obtain from benzene an important product for organic synthesis - nitrobenzene. It is a pale yellow liquid, 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 large-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. Zinin in 1842 obtained aniline from nitrobenzene by the reduction reaction with ammonium sulfide. In modern conditions, the contact method has become widespread, in which a mixture of hydrogen vapor and nitrobenzene is passed at a temperature of 300 ° C over a catalyst. The resulting aromatic amine is further used for the production of explosives, dyes, medicines.

Where are aromatic hydrocarbons extracted from?

The most promising is the production of arenes from the coking product of coal and in the process of oil refining. Cycloparaffins contained in coal tar are subjected to hydrogenation over a catalyst at temperatures up to 300 ° C, the reaction product will be benzene. The dehydrogenation of alkanes also leads to the formation of aromatic hydrocarbons. By the Zelinsky-Kazansky reaction, benzene is obtained from ethine by passing it through a tube with activated carbon heated to 600 °C. The preparation of arenes, such as toluene, is carried out using the Friedel-Crafts reaction. It is also possible to extract methylbenzene (toluene) 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 woolen products from moths in everyday life was naphthalene. With its prolonged use, clothes acquired a characteristic, very persistent smell. However, more important is the use of naphthalene as a raw material for the synthesis medicines, dyes, explosives. The main methods of its production are based on the processing of oil distillation products and ethylene production waste - pyrolysis resin. The substance, unlike benzene, contains two benzene nuclei, so the nitration and halogenation reactions take place faster in it. Continuing to give examples of arenes, let us dwell on one more aromatic hydrocarbon important for industry - vinylbenzene.

Styrene

The modern building materials industry is impossible without polymer materials: easy to process, durable and wear-resistant. Polymers derived from vinylbenzene, such as polystyrene (expanded polystyrene), SAN and ABS plastics, are used in the manufacture of stretch ceilings, floor coverings, wall insulation. Styrene is obtained from ethylbenzene as a colorless, flammable liquid with a peculiar odor. In the future, it is subjected to polymerization and a solid vitreous mass is obtained - polystyrene. It also serves as the initial product in the production of the above 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

Name of arenas international classification IUPAC includes a substituent designation followed by the word "benzene". For example, C6H5CH3 is methylbenzene, C6H5C2H3 is vinylbenzene. These compounds also have trivial names, for example, the first compound is called toluene, the second - styrene. Arenes may contain two substituents, for example two methyl radicals. They are able to join the carbon cycle in three positions: at 1 and 2 carbon atoms, then they speak of the ortho position of substituents. If the radicals are located at 1 and 3 carboxylic particles, then we are talking about the meta position of the substituents, at 1 and 4 carbon atoms - this is a parasubstitution. The higher homologues of benzene can be represented as derivatives of saturated hydrocarbons, in the molecules of which one hydrogen atom is replaced by the 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.

BUTRENA

Aromatic hydrocarbons (arenes) - cyclic hydrocarbons, united by the concept of aromaticity, which determines common features in the structure and chemical properties.

Classification

According to the number of benzene rings in the molecule, arenas are subdivided on the:

mononuclear

multi-core

Nomenclature and isomerism

The structural ancestor of the hydrocarbons of the benzene series is benzene C 6 H 6 from which the systematic names of homologues are built.

For monocyclic compounds, the following non-systematic (trivial) names are retained:

The position of the substituents is indicated by the smallest digits (the direction of the numbering does not matter),

and for disubstituted compounds, the notation can be used ortho, meta, pair.

If there are three substituents in the ring, then they should receive the smallest numbers, i.e. the series "1,2,4" takes precedence over "1,3,4".

1,2-dimethyl-4-ethylbenzene (correct) 3,4-dimethyl-1-ethylbenzene (incorrect)

The isomerism of monosubstituted arenes is due to the structure of the carbon skeleton of the substituent; for di- and polysubstituted benzene homologues, more isomerism is added, caused by the different arrangement of substituents in the nucleus.

Isomerism of aromatic hydrocarbons of the composition C 9 H 12:

Physical Properties

The boiling and melting points of arenes are higher than those of alkanes, alkenes, alkynes, they are low-polar, insoluble in water and readily soluble in non-polar organic solvents. Arenas are liquids or solids that have specific odors. Benzenes and many condensed arenes are toxic, some of them exhibit carcinogenic properties. Intermediate products of the oxidation of condensed arenes in the body are epoxides, which either directly cause cancer themselves or are precursors of carcinogens.

Getting arenas

Many aromatic hydrocarbons have an important practical value and are produced on a large industrial scale. Row industrial ways based on the processing of coal and oil.

Oil consists mainly of aliphatic and alicyclic hydrocarbons; for the conversion of aliphatic or acyclic hydrocarbons into aromatic, oil aromatization methods have been developed, the chemical bases of which have been developed by N.D. Zelinsky, B.A. Kazansky.

1. Cyclization and dehydrogenation:

2. Hydrodesmethylation:

3. Benzene homologues are obtained by alkylation or acylation followed by reduction of the carbonyl group.

a) Alkylation according to Friedel-Crafts:

b) Friedel-Crafts acylation:

4. Obtaining biphenyl by the Wurtz-Fitting reaction:

5. Obtaining diphenylmethane by the Friedel-Crafts reaction:

Structure and chemical properties.

Aromaticity Criteria:

Based on theoretical calculations and experimental study of cyclic conjugated systems, it was found that a compound is aromatic if it has:

• Flat cyclic σ-skeleton;
• A conjugated closed π-electron system, covering all atoms of the cycle and containing 4n + 2, where n = 0, 1, 2, 3, etc. This formulation is known as Hückel's rule. Aromaticity criteria make it possible to distinguish conjugated aromatic systems from all others. Benzene contains a sextet of π electrons and follows Hückel's rule at n = 1.

What gives aromaticity:

In spite of a high degree unsaturation, aromatic compounds are resistant to oxidizing agents and temperature, they are more likely to enter into substitution reactions rather than addition. These compounds have increased thermodynamic stability, which is ensured by the high conjugation energy of the aromatic system of the ring (150 kJ/mol); therefore, arenes preferentially enter into substitution reactions, as a result of which they retain aromaticity.

The mechanism of electrophilic substitution reactions in the aromatic ring:

The electron density of the π-conjugated system of the benzene ring is a convenient target for attack by electrophilic reagents.

As a rule, electrophilic reagents are generated during the reaction with the help of catalysts and appropriate conditions.

E - Y → E δ + - Y δ - → E + + Y -

Formation of a π-complex. The initial attack by the electrophile on the π-electron cloud of the ring leads to the coordination of the reactant with the π-system and the formation of a donor-acceptor type complex called π-complex. The aromatic system is not disturbed:

Formation of a σ-complex. The limiting stage, at which the electrophile forms a covalent bond with a carbon atom due to two electrons of the π-system of the ring, which is accompanied by the transition of this carbon atom from sp2- in sp3- hybrid state and disruption of the aromatic, the molecule turns into a carbocation.

Stabilization of the σ-complex. It is carried out by splitting off a proton from the σ-complex with the help of a base. In this case, the closed π-system of the ring is restored due to two electrons of the breaking C – H covalent bond, i.e. the molecule returns to the aromatic state:

Effect of substituents on the reactivity and orientation of electrophilic substitution

Substituents in the benzene ring break the uniformity in the distribution π- electron cloud of the ring and thereby affect the reactivity of the ring.

• Electron donor substituents (D) increase the electron density of the ring and increase the rate of electrophilic substitution, such substituents are called activating.
• Electron-withdrawing substituents (A) lower the electron density of the ring and reduce the reaction rate, called deactivating.

AROMATIC HYDROCARBONS

For aromatic compounds or arenas, applies large group compounds whose molecules contain a stable cyclic group (benzene ring) with special physical and chemical properties.

These compounds primarily include benzene and its numerous derivatives.

The term "aromatic" was originally used in relation to products of natural origin, which had an aromatic smell. Since among these compounds there were many that included benzene rings, the term "aromatic" began to apply to any compounds (including those with an unpleasant odor) containing a benzene ring.

Benzene, its electronic structure

According to the benzene formula C 6 H 6, it can be assumed that benzene is a highly unsaturated compound, similar, for example, to acetylene. However, the chemical properties of benzene do not support this assumption. So, under normal conditions, benzene does not give reactions characteristic of unsaturated hydrocarbons: it does not enter into addition reactions with hydrogen halides, it does not decolorize a solution of potassium permanganate. At the same time, benzene enters into substitution reactions similarly to saturated hydrocarbons.

These facts indicate that benzene is partly similar to saturated, partly to unsaturated hydrocarbons and at the same time differs from both. Therefore, for a long time, there were lively discussions between scientists on the question of the structure of benzene.

In the 60s. of the last century, most chemists accepted the theory of the cyclic structure of benzene based on the fact that monosubstituted benzene derivatives (for example, bromobenzene) do not have isomers.

The most recognized formula of benzene, proposed in 1865 by the German chemist Kekule, in which double bonds in the ring of carbon atoms of benzene alternate with simple ones, and, according to Kekule's hypothesis, simple and double bonds move continuously:

However, the Kekule formula cannot explain why benzene does not exhibit the properties of unsaturated compounds.

According to modern concepts, the benzene molecule has the structure of a flat hexagon, the sides of which are equal to each other and are 0.140 nm. This distance is an average between 0.154 nm (single bond length) and 0.134 nm (double bond length). Not only the carbon atoms, but also the six hydrogen atoms associated with them lie in the same plane. The angles formed by the bonds H - C - C and C - C - C are 120 °.

The carbon atoms in benzene are in sp 2 hybridization, i.e. of the four orbitals of the carbon atom, only three are hybridized (one 2s- and two 2p-), which take part in the formation of σ-bonds between carbon atoms. The fourth 2 p-orbital overlaps with the 2 p-orbitals of two neighboring carbon atoms (right and left), six delocalized π-electrons located in dumbbell-shaped orbitals, the axes of which are perpendicular to the plane of the benzene ring, form a single stable closed electronic system.

As a result of the formation of a closed electronic system by all six carbon atoms, the "alignment" of single and double bonds occurs, i.e. in the benzene molecule there are no classical double and single bonds. The uniform distribution of the π-electron density between all carbon atoms is the reason for the high stability of the benzene molecule. To emphasize the uniformity of the π-electron density in the benzene molecule, one resorts to the following formula:

Nomenclature and isomerism of aromatic hydrocarbons of the benzene series

The general formula for the homologous series of benzene C n H 2 n -6.

The first homologue of benzene is methylbenzene, or toluene, C 7 H 8

has no position isomers, like all other monosubstituted derivatives.

The second homologue C 8 H 10 can exist in four isomeric forms: ethylbenzene C 6 H 5 -C 2 H 5 and three dimethylbenzenes, or xylene, C b H 4 (CH 3) 2 (ortho-, meta- and pair-xylenes, or 1,2-, 1,3- and 1,4-dimethylbenzenes):

The radical (residue) of benzene C 6 H 5 - is called phenyl; the names of radicals of benzene homologues are derived from the names of the corresponding hydrocarbons by adding the suffix to the root -silt(tolyl, xylyl, etc.) and lettering (o-, m-, p-) or digits the position of the side chains. Generic name for all aromatic radicals aryls similar to the title alkyls for alkane radicals. The radical C 6 H 5 -CH 2 - is called benzyl.

When naming more complex benzene derivatives, from the possible numbering orders, one is chosen in which the sum of the digits of the substituent numbers will be the smallest. For example, dimethyl ethyl benzene of the structure

should be called 1,4-dimethyl-2-ethylbenzene (the sum of the digits is 7), not 1,4-dimethyl-6-ethylbenzene (the sum of the digits is 11).

The names of the higher homologues of benzene are often derived not from the name of the aromatic nucleus, but from the name of the side chain, that is, they are considered as derivatives of alkanes:

Physical properties of aromatic hydrocarbons of the benzene series

The lower members of the benzene homologous series are colorless liquids with a characteristic odor. Their density and refractive index are much higher than those of alkanes and alkenes. The melting point is also noticeably higher. Due to the high carbon content, all aromatic compounds burn with a very smoky flame. All aromatic hydrocarbons are insoluble in water and highly soluble in most organic solvents: many of them are readily steam distillable.

Chemical properties of aromatic hydrocarbons of the benzene series

For aromatic hydrocarbons, the most typical reactions are the substitution of hydrogen in the aromatic ring. Aromatic hydrocarbons enter into addition reactions with great difficulty under harsh conditions. A distinctive feature of benzene is its significant resistance to oxidizing agents.

Addition reactions

Addition of hydrogen

In some rare cases, benzene is capable of addition reactions. Hydrogenation, i.e., the addition of hydrogen, occurs under the action of hydrogen under harsh conditions in the presence of catalysts (Ni, Pt, Pd). In this case, a benzene molecule adds three hydrogen molecules to form cyclohexane:

Addition of halogens

If a solution of chlorine in benzene is exposed to sunlight or ultraviolet rays, then three halogen molecules are radically added to form a complex mixture of stereoisomers of hexachlorocyclohexane:

Hexachlorocyclohexai (trade name hexachloran) is currently used as an insecticide - substances that destroy insects that are pests of agriculture.

Oxidation reactions

Benzene is even more resistant to oxidizing agents than saturated hydrocarbons. It is not oxidized by dilute nitric acid, KMnO 4 solution, etc. Benzene homologues are oxidized much more easily. But even in them, the benzene core is relatively more resistant to the action of oxidizing agents than the hydrocarbon radicals associated with it. There is a rule: any benzene homologue with one side chain is oxidized to a monobasic (benzoic) acid:

Benzene homologues with multiple side chains of any complexity are oxidized to form polybasic aromatic acids:

Substitution reactions

1. Halogenation

Under normal conditions, aromatic hydrocarbons practically do not react with halogens; benzene does not decolorize bromine water, but in the presence of catalysts (FeCl 3, FeBr 3, AlCl 3) in an anhydrous medium, chlorine and bromine vigorously react with benzene at room temperature:

Nitration reaction

For the reaction, concentrated nitric acid is used, often mixed with concentrated sulfuric acid (catalyst):

In unsubstituted benzene, the reactivity of all six carbon atoms in substitution reactions is the same; substituents may attach to any carbon atom. If there is already a substituent in the benzene nucleus, then under its influence the state of the nucleus changes, and the position into which any new substituent enters depends on the nature of the first substituent. It follows from this that each substituent in the benzene nucleus exhibits a certain guiding (orienting) effect and contributes to the introduction of new substituents only in certain positions in relation to itself.

According to the guiding influence, various substituents are divided into two groups:

a) substituents of the first kind:

They direct any new substituent into ortho and para positions with respect to themselves. At the same time, almost all of them reduce the stability of the aromatic group and facilitate both substitution reactions and reactions of the benzene ring:

b) substituents of the second kind:

They direct any new substitute to a meta position in relation to themselves. They increase the stability of the aromatic group and hinder substitution reactions:

Thus, the aromatic nature of benzene (and other arenes) is expressed in the fact that this compound, being unsaturated in composition, in a number of chemical reactions manifests itself as a limiting compound, it is characterized by chemical stability, the difficulty of addition reactions. Only under special conditions (catalysts, irradiation) does benzene behave as if it had three double bonds in its molecule.