What is benzene? The structure of benzene, the formula, properties, application

Among the huge arsenal of organic substances can be identified several compounds, the discovery and study of which was accompanied by years of scientific debate. To them, benzene is the right. The structure of benzene in chemistry was finally adopted only at the beginning of the 20th century, whereas the elemental composition of the substance was determined as early as 1825, separating it from coal tar, which was obtained as a by-product of coal coking.

Benzene together with toluene, anthracene, phenol, naphthalene is now referred to as aromatic hydrocarbons. In this article we will consider what are the characteristics of the molecule of this hydrocarbon, find out the physical properties, for example, such as solubility, boiling point and benzene density, and also denote the application areas of the compound in industry and agriculture.

What are arenas?

The chemistry of organic compounds classifies all known substances into several groups, for example, such as alkanes, alkynes, alcohols, aldehydes, etc. The main distinguishing feature of each class of substances is the presence of certain types of bonds. The molecules of the ultimate hydrocarbons contain only a sigma-bond, the substances of the ethylene series are double, and the alkynes have a triple bond. To what class does benzene belong?

The structure of benzene indicates the presence in its molecule of an aromatic ring, called the benzene nucleus. All compounds of organic nature, containing one or more of these 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 the aromatic hydrocarbon molecule

First, scientists established the qualitative and quantitative composition of the compound, expressing it with the formula C ₆ H ₆ , according to which the relative molecular weight of benzene is 78. Then several variants of structural formulas were proposed, but none of them corresponded to the real physical and chemical properties of benzene observed by chemists In laboratory experiments.

It took about forty years before the German researcher A. Kekule presented his version of the structural formula, which has a benzene molecule. It was attended by three double bonds, indicating a possible unlimited character of the chemical properties of the hydrocarbon. This was in conflict with the actual nature of the interactions of the compound of the formula C ₆ H ₆ with other substances, for example, with bromine, nitrate acid, chlorine.

Only after clarifying the electronic configuration of the benzene molecule in its structural formula did the designation of the benzene ring (ring) appear, and it itself is still used in the course of organic chemistry.

The electronic configuration of the C6H6 molecule

What is the spatial structure of benzene? The structure of benzene was finally confirmed by two reactions: the trimerization of acetylene with the formation of benzene and its reduction by hydrogen to cyclohexane. It turned out that the carbon atoms, joining together, form a flat hexagon and are in the state of sp ^2- hybridization, using three of its four valence electrons in connection with other atoms.

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

Nature of the one and a half chemical bonds

It is well known that the physical and chemical properties of compounds depend primarily on their internal structure and the types of chemical bonds that arise between atoms. Having considered the electronic structure of benzene, one can come to the conclusion that its molecule has neither simple nor double bonds, which can be seen in the Kekule formula. On the contrary, between carbon atoms, all chemical bonds are equivalent. Moreover, the total π-electron cloud (of all six C atoms) forms a chemical type of bond, called a sesquioxide, 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

Arenas containing a small number of carbon atoms in the molecule are mainly liquids. Benzene is not an exception. The structure of benzene, as we recall, points to its molecular nature. How did this fact affect its properties?

As the temperature is lowered, the liquid passes into the solid phase, and benzene turns into a white crystalline mass. It easily melts 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 varies depending on the temperature change. The higher the temperature, the lower the density. Let's give some examples. At a temperature of 10 °, the density is 0.8884 g / ml, and at 20 ° the density is 0.8786 g / ml. The molecules of benzene are non-polar, so the substance is insoluble in water. But the compound itself is a good organic solvent, for example, for fats.

Features of the chemical properties of benzene

It has been experimentally established that the aromatic benzene nucleus is stable, i.e. Characterized by high resistance to rupture. This fact serves as an explanation for the propensity of the substance to undergo reactions of the type of substitution, for example, with chlorine under ordinary conditions, with bromine, with nitrate acid in the presence of a catalyst. It should be noted the high stability of benzene to the action of oxidants, such as potassium permanganate and bromine water. This again confirms the fact that there are no double bonds in the molecule. Rigid oxidation, otherwise known as combustion, is characteristic of all aromatic hydrocarbons. Since the percentage of carbon in the C ₆ H ₆ molecule is large, burning of benzene is accompanied by a smoldering 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 enter the addition reaction? Let us consider it in more detail.

What causes the breakdown of the benzene nucleus?

Recall that in the arene molecules there is a one-and-a-half bond, which arises as a result of overlapping six p-electrons of carbon atoms. It is the basis of the benzene nucleus. In order to destroy it and carry out the addition reaction, a number of special conditions are needed, for example, such as light irradiation, high temperature and pressure, catalysts. A mixture of benzene and chlorine reacts under the action of ultraviolet radiation. The product of this interaction will be hexachlorocyclohexane, a toxic crystalline substance used in agriculture as an insecticide. There is no benzene nucleus in the hexachlorane molecule, and six chlorine atoms have joined the site of its break.

Fields of practical application of benzene

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

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