Aromatic hydrocarbons. Benzene, structural formula, properties and preparation

METHODOLOGICAL INSTRUCTIONS

organic chemistry course

« AROMATIC HYDROCARBONS»

Rostov-on-Don

Guidelines for the course of organic chemistry “Aromatic hydrocarbons”. - Rostov n/a: Rost. state builds. univ., 2007. - 12 p.

Theoretical principles on the topic “Aromatic hydrocarbons” are presented. A definition of aromatic hydrocarbons is given, as well as the concept of “aromaticity”. The structure of the benzene molecule is described. The nomenclature and isomerism of aromatic compounds with one benzene ring are considered. The main methods for producing arenes are presented, and the physical and chemical properties of aromatic hydrocarbons are also considered.

Designed for first- and second-year students of full-time and part-time forms of study in the specialties PSM, ZChS, SSP, BTP and AS.

Compiled by: Ph.D. chem. Sciences, Associate Professor

M.N. Mitskaya,

Ph.D. chem. Sciences, Asst.

E.A. Levinskaya

Reviewer: Ph.D. chem. Sciences, Associate Professor

L.M. Astakhova

© Rostov State

Construction University, 2007

Aromatic compounds (arenes) - organic compounds with a planar cyclic structure in which all carbon atoms create a single delocalized π-electron system containing (4n+2) π-electrons.

Aromatic compounds include primarily benzene C 6 H 6 and its numerous homologues and derivatives. Aromatic compounds may contain one or more benzene rings per molecule (polynuclear aromatic compounds). But we will look at aromatic compounds with one benzene ring.

The structure of the benzene molecule

Benzene was discovered by M. Faraday in 1825 in illuminating (coke oven) gas, and the structure of the benzene molecule is most often expressed by the formula proposed by the German chemist A. Kekule (1865)

According to modern concepts, the benzene molecule has the structure of a flat hexagon, the sides of which are equal to each other and amount to 0.14 nm. This distance is the average value 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 H-C-C and C-C-C bonds are 120°:

All carbon atoms in a benzene molecule are in a state of sp 2 hybridization. Each of them is connected by its three hybrid orbitals with two of the same orbitals of two neighboring carbon atoms and one orbital of the H atom, forming three σ bonds (see figure). The fourth, unhybridized 2p orbital of the carbon atom, whose axis is perpendicular to the plane of the benzene ring, overlaps with similar orbitals of two neighboring carbon atoms located on the right and left.

Scheme of formation of σ-bonds and π-bonds in a benzene molecule

This overlap occurs above and below the plane of the benzene ring. As a result, a single closed system of π-electrons is formed. As a result of such uniform overlap of the 2p orbitals of all six carbon atoms, “alignment” of single and double bonds occurs, i.e. the benzene ring lacks classical double and single bonds. The uniform distribution of π-electron density between all carbon atoms, due to π-electron delocalization, is the reason for the high stability of the benzene molecule. Currently, there is no single way to graphically depict a benzene molecule taking into account its real properties. But to emphasize the uniformity of the π-electron density in the benzene molecule, they resort to the following formulas:

It is necessary, however, to remember that none of these formulas corresponds to the actual physical state of the molecule, much less can reflect the whole variety of its properties. Kekule's formula is currently only a symbol of the benzene molecule. However, it is widely used, while keeping in mind its disadvantages.

DEFINITION

Benzene(cyclohexatriene - 1,3,5) is an organic substance, the simplest representative of a number of aromatic hydrocarbons.

Formula – C 6 H 6 (structural formula – Fig. 1). Molecular weight – 78.11.

Rice. 1. Structural and spatial formulas of benzene.

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 containing one electron. Six p-electrons form a single π-electron cloud (aromatic system), which is depicted as a circle inside a six-membered ring. The hydrocarbon radical obtained from benzene is called C 6 H 5 - - phenyl (Ph-).

Chemical properties of benzene

Benzene is characterized by substitution reactions that occur via an electrophilic mechanism:

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

C 6 H 6 + Cl 2 = C 6 H 5 -Cl + HCl;

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

- alkylation with alkenes

C 6 H 6 + CH 2 = CH-CH 3 → C 6 H 5 -CH(CH 3) 2;

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

— hydrogenation (the reaction occurs when heated, the catalyst is Pt)

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

Like any organic compound, benzene undergoes a combustion reaction with the formation of carbon dioxide and water as reaction products (burns with a smoky flame):

2C 6 H 6 +15O 2 → 12CO 2 + 6H 2 O.

Physical properties of benzene

Benzene is a colorless liquid, but has a specific pungent odor. Forms an azeotropic mixture with water, mixes well with ethers, gasoline and various organic solvents. Boiling point – 80.1C, melting point – 5.5C. Toxic, carcinogen (i.e. promotes the development of cancer).

Preparation and use of benzene

The main methods of obtaining benzene:

— dehydrocyclization of hexane (catalysts – Pt, Cr 3 O 2)

CH 3 –(CH 2) 4 -CH 3 → C 6 H 6 + 4H 2;

— dehydrogenation of cyclohexane (the reaction occurs when heated, the catalyst is Pt)

C 6 H 12 → C 6 H 6 + 4H 2;

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

3HC≡CH → C 6 H 6 .

Benzene serves as a raw material for the production of homologues (ethylbenzene, cumene), cyclohexane, nitrobenzene, chlorobenzene and other substances. Previously, benzene was used as an additive to gasoline to increase its octane number, however, now, due to its high toxicity, the benzene content in fuel is strictly regulated. Benzene is sometimes used as a solvent.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Among the huge arsenal of organic substances, several compounds can be distinguished, the discovery and study of which was accompanied by many years of scientific controversy. Benzene rightfully belongs to them. The structure of benzene in chemistry was finally accepted only at the beginning of the 20th century, while the elemental composition of the substance was determined back in 1825, isolating it from coal tar, which was obtained as a by-product of coking coal.

Benzene, together with toluene, anthracene, phenol, and naphthalene, is currently classified as aromatic hydrocarbons. In our article we will look at what this hydrocarbon is, find out its physical properties, for example, solubility, boiling point and density of benzene, and also outline the areas of application of the compound in industry and agriculture.

What are arenas?

The chemistry of organic compounds classifies all known substances into several groups, for example, alkanes, alkynes, alcohols, aldehydes, etc. The main distinguishing feature of each class of substances is the presence of certain types of bonds. Molecules of saturated hydrocarbons contain only a sigma bond, substances of the ethylene series contain a double bond, and alkynes contain a triple bond. What class does benzene belong to?

The structure of benzene indicates the presence in its molecule of an aromatic ring called the benzene ring. All organic compounds containing one or more such rings in their molecules are classified as arenes (aromatic hydrocarbons). In addition to benzene, which we are now considering, this group includes a large number of very important substances, such as toluene, aniline, phenol and others.

How to solve the problem of the structure of an aromatic hydrocarbon molecule

At first, scientists established it by expressing it with the formula C 6 H 6, according to which the relative molecular weight of benzene is 78. Then several options for structural formulas were proposed, but none of them corresponded to the real physical and chemical properties of benzene observed by chemists in laboratory experiments.

About forty years passed before the German researcher A. Kekule presented his version of the structural formula that the benzene molecule has. It contained three double bonds, indicating the possible unsaturated nature of the chemical properties of the hydrocarbon. This conflicted with the actually existing nature of the interactions of the compound of the formula C 6 H 6 with other substances, for example, with bromine, nitrate acid, and chlorine.

Only after the electronic configuration of the benzene molecule was clarified, the designation of the benzene nucleus (ring) appeared in its structural formula, and it itself is still used in organic chemistry courses.

Electronic configuration of the C6H6 molecule

What spatial structure does benzene have? The structure of benzene was finally confirmed through two reactions: the trimerization of acetylene to form benzene and its reduction with hydrogen to cyclohexane. It turned out that carbon atoms, connecting with each other, form a flat hexagon and are in a state of sp 2 hybridization, using three of their four valence electrons in connection with other atoms.

The remaining six free p-electrons are located perpendicular to the plane of the molecule. Overlapping with each other, they form a common electron cloud called the benzene nucleus.

The nature of one-and-a-half chemical bonds

It is well known that the physical and chemical properties of compounds depend, first of all, on their internal structure and the types of chemical bonds that arise between atoms. Having examined the electronic structure of benzene, we can come to the conclusion that its molecule has neither single nor double bonds, which can be seen in the Kekulé formula. On the contrary, all chemical bonds between carbon atoms are equivalent. Moreover, the common π-electron cloud (of all six C atoms) forms a chemical type of bond called sesquicentral, or aromatic. It is this fact that determines the specific properties of the benzene ring and, as a consequence, the nature of the chemical interaction of aromatic hydrocarbons with other substances.

Physical properties

As the temperature decreases, the liquid turns into a solid phase, and benzene turns into a white crystalline mass. It melts easily at a temperature of 5.5 °C. Under normal conditions, the substance is a colorless liquid with a peculiar odor. Its boiling point is 80.1 °C.

The density of benzene changes with changes in temperature. The higher the temperature, the lower the density. Let's give a few examples. At a temperature of 10°, the density is 0.8884 g/ml, and at 20° - 0.8786 g/ml. Benzene molecules are non-polar, so the substance is insoluble in water. But the compound itself is good, for example, for fats.

Features of the chemical properties of benzene

It has been experimentally established that the aromatic benzene ring is stable, i.e. characterized by high resistance to tearing. This fact explains the tendency of a substance to undergo substitution-type reactions, for example, with chlorine under normal conditions, with bromine, with nitrate acid in the presence of a catalyst. It should be noted that benzene is highly resistant to oxidizing agents such as potassium permanganate and bromine water. This once again confirms the absence of double bonds in the arene molecule. Severe oxidation, otherwise called combustion, is characteristic of all aromatic hydrocarbons. Since the percentage of carbon in the C 6 H 6 molecule is high, the combustion of benzene is accompanied by a smoky flame with the formation of soot particles. As a result of the reaction, carbon dioxide and water are formed. An interesting question is: can an aromatic hydrocarbon undergo addition reactions? Let's consider it further in more detail.

What does the rupture of the benzene ring lead to?

Let us recall that arene molecules contain a one-and-a-half bond, which arises as a result of the overlap of six p-electrons of carbon atoms. It is the basis of the benzene nucleus. To destroy it and carry out the addition reaction, a number of special conditions are required, for example, light irradiation, high temperature and pressure, and catalysts. A mixture of benzene and chlorine undergoes an addition reaction under the influence of ultraviolet radiation. The product of this interaction will be hexachlorocyclohexane, a toxic crystalline substance used in agriculture as an insecticide. There is no longer a benzene ring in the hexachlorane molecule; six chlorine atoms have been added to the site where it breaks.

Areas of practical application of benzene

In various industries, the substance is widely used as a solvent, as well as a raw material for the further production of varnishes, plastics, dyes, and as an additive to motor fuel. Benzene derivatives and its homologues have an even wider range of applications. For example, nitrobenzene C 6 H 5 NO 2 is the main reagent for the production of aniline. As a result, hexachlorobenzene is obtained with chlorine in the presence of aluminum chloride as a catalyst. It is used for pre-sowing treatment of seeds, and is also used in the woodworking industry to protect wood from pests. Nitration of a benzene homologue (toluene) produces an explosive known as TNT or tol.

In this article, we examined such properties of an aromatic compound as addition and substitution reactions, combustion of benzene, and also identified the areas of its application in industry and agriculture.

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

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 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

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 → C6H6Cl6 (hexachlorocyclohexane - hexachlorane)

2. Hydrogenation

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

3. Polymerization

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

1. Halogenation -

a ) benzene

C6H6+Cl2 AlCl 3 → C 6 H 5 -Cl + HCl (chlorobenzene)

C6H6 + 6Cl2 t ,AlCl3→C 6 Cl 6 + 6HCl( hexachlorobenzene)

C 6 H 6 + Br 2 t,FeCl3→ C 6 H 5 -Br + HBr( bromobenzene)

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!

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 the replacement of H atoms in benzene with chlorine atoms - hexachlorobenzene C 6 Cl 6 - is a fungicide; it is used for dry dressing 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.

VIDEO EXPERIENCES

The modern point of view on the structure of benzene: a flat molecule, the carbon atoms of which are in a state of sp 2 hybridization and are combined into a regular hexagon.

Picture of a benzene molecule:

Aromaticity- unusually low energy of the unexcited state caused by the delocalization of π electrons.

Aromaticity-a concept that characterizes the totality of structural, energetic properties and features of the reactivity of cyclic structures with a system of conjugated bonds

Signs of aromaticity Any compound is aromatic if it has: a) a flat closed ring; b) a conjugated π-electron system, covering all atoms of the cycle; c) if the number of electrons involved in conjugation corresponds to the Hückel formula (4n+2., where n is the number of cycles).

Isomerism

Nomenclature

ortho-, meta- And pair- substituted:

Physical properties

All aromatic compounds have a smell. Benzene, toluene, xylenes, ethylbenzene,

cumene, styrene - liquids, naphthalene, anthracene - solids.

26. Aromatic hydrocarbons of the benzene series. Nomenclature. Isomerism. Methods for obtaining benzene and its homologues: from coal tar, aromatization and dehydrocyclization of paraffins, according to the Wurtz-Fittig reaction, Friedel-Crafts alkylation with olefins, alkyl halides, alcohols, from benzoic acid salts, trimerization of alkynes.

Arenes (aromatic hydrocarbons) are cyclic compounds whose molecules contain one or more benzene nuclei. Empirical formula of benzene C6H6

Isomerism

Di-, tri- and tetra-substituted aromatic hydrocarbons are characterized by isomerism of the position of the substituent and isomerism of the side alkyl chain.

Mono-, penta- and hexa-substituted arenes do not have isomers associated with the position of the substituent in the ring.

Nomenclature

Benzene derivatives are called substituted benzenes. For many of them, either trivial names are used, or the substituent is indicated by a prefix before the word “benzene”. In the case of monosubstituted benzenes, the names do not include numbers, since all six carbon atoms of the benzene molecule are equivalent, and only one monosubstituted benzene is possible for each substituent.

If two substituents are present on a benzene molecule, then three different disubstituted benzenes can exist. They are named accordingly ortho-, meta- And pair- substituted:

If benzene has three or more substituents, then their position in the ring should be indicated only by numbers. In all cases, the names of the substituents are listed before the word “benzene” in alphabetical order. The number 1 in the name can be omitted; the substituent from which the countdown begins is in this case included in the basis of the name:

Receipt:

1. Coal tar processing, oil distillation, dry distillation

wood

2. Aromatization of oil.

3. Dehydrocyclization of hexane and heptane.

C 6 H 14 → C6H6 + 4H 2

C 7 H 16 → C6H5-CH3 + 4H2

4. Wurtz-Fittig:

Friedel–Crafts alkylation. Two possible reaction mechanisms have been accepted. In the first case, the electrophilic particle is a carbocation formed as a result of the interaction of a haloalkane with aluminum chloride (Lewis acid):

In the second case, it can be assumed that the electrophile is the alkyl group of the polar complex of AlCl 3 with an alkyl halide.

Alkylation of benzene (Friedel-Crafts reaction)

C 6 H 6 + C 2 H 5 Cl → C 6 H 5 -C 2 H 5 + HCl

27.Electrophilic substitution in the aromatic series (nitration, sulfonation, halogenation, alkylation and Friedel-Crafts acylation). The concept of - and -complexes. Mechanism of electrophilic substitution reactions