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 debate. They rightfully include benzene. The structure of benzene in chemistry was finally adopted only by the beginning of the 20th century, while 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 currently referred to as aromatic hydrocarbons. In our article, we will consider what are the features of the molecule of this hydrocarbon, find out the physical properties, for example, such as solubility, boiling point and density of benzene, and also indicate 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, 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 saturated hydrocarbons contain only a sigma bond, substances of a number of ethylene contain a double bond, and alkynes have 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 core. All compounds of organic nature containing one or more of these rings in the composition of their molecules belong to the class of 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 molecule of aromatic hydrocarbon
Initially, 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.
About forty years passed before the German researcher A. Kekule presented his version of the structural formula that the benzene molecule has. Three double bonds were present in it, indicating a possible unsaturated nature of the chemical properties of the hydrocarbon. This contradicted the really existing nature of the interactions of the compounds of formula C 6 H 6 with other substances, for example, bromine, nitrate acid, chlorine.
Only after clarification of the electronic configuration of the benzene molecule in its structural formula did the designation of the benzene core (ring) appear, and it is still used in the course of organic chemistry.
Electronic configuration of the C6H6 molecule
What spatial structure does benzene have? The structure of benzene was finally confirmed by two reactions: the trimerization of acetylene with the formation of benzene and its reduction with hydrogen to cyclohexane. It turned out that the carbon atoms, joining together, form a flat hexagon and are in a state of sp 2 hybridization, using three of their four valence electrons in communication with other atoms.
The remaining six free p-electrons are perpendicular to the plane of the molecule. Overlapping with each other, they form a common electron cloud called the benzene core.
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 types of chemical bonds arising between atoms. Having examined the electronic structure of benzene, we can conclude 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, a common π-electron cloud (of all six C atoms) forms a chemical type of bond, called one and a half, 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 no exception. The structure of benzene, as we recall, indicates its molecular nature. How did this fact affect its properties?
As the temperature decreases, 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 ordinary conditions, the substance is a colorless liquid with a peculiar odor. Its boiling point is 80.1 ° C.
The density of benzene varies with temperature. The higher the temperature, the lower the density. Here are some 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, therefore 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 was experimentally established that the aromatic benzene core is stable, i.e. characterized by high tensile strength. This fact explains the propensity of a substance to undergo substitution reactions, for example, with chlorine under ordinary 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 arena molecule. Rigid 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 smoking flame with the formation of soot particles. The reaction produces carbon dioxide and water. An interesting question is: can an aromatic hydrocarbon enter into addition reactions? Let us consider it in more detail below.
What causes a break in the benzene core?
Recall that one and a half bonds are present in arene molecules resulting from the overlap of six p-electrons of carbon atoms. It underlies the benzene core. To destroy it and carry out the addition reaction, a number of special conditions are necessary, for example, such as 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 nucleus in the hexachloran molecule; six chlorine atoms joined at the site of its rupture.
Areas of practical use of benzene
In various industries, the substance is widely used as a solvent, as well as as a raw material for the further production of varnishes, plastics, dyes, and as an additive to motor fuel. Derivatives of benzene and its homologs have an even wider range of applications. For example, nitrobenzene C 6 H 5 NO 2 is the main reagent for producing 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 also used in the woodworking industry to protect wood from pests. By nitration of the benzene homologue (toluene), an explosive known as trotyl or tol is obtained.
In this article, we examined such properties of an aromatic compound as addition and substitution reactions, the combustion of benzene, and also identified areas of its application in industry and agriculture.