What are aromatic hydrocarbons: the formula, properties

Consider aromatic hydrocarbons. The formula for the representatives of this homologous series is CnH2n-6.

Class Features

In the early nineteenth century, Faraday opened benzene - C6H6. Compared with saturated hydrocarbons, the structural formulas of aromatic hydrocarbons are represented in the form of cycles. Given that the molecule contains an insufficient amount of hydrogen, an aromatic ring is formed inside the ring.

How to record aromatic hydrocarbons? The formula proposed by Kekule explains the structure of this class of hydrocarbons. The presence of double bonds is a confirmation of the aromatic nature of benzene and its homologues.

Chemical properties

The general formula of aromatic hydrocarbons assumes the existence of all compounds of this class of addition reactions: hydrogenation, halogenation, hydration. The results of numerous experiments demonstrated an insignificant chemical activity of benzene.

It exhibits increased resistance to oxidation, is capable of joining only in the presence of ultraviolet irradiation or elevated temperature.

Features of the structure of benzene

The molecular formula of the aromatic hydrocarbon is C6H6. All carbon atoms are in the cp2-hybrid state, located in the same plane. Each of them has one non-hybrid C atom, which combine into a common electronic cloud, which is perpendicular to the plane of the ring. This cyclic system of conjugated n-bonds also determines the chemical passivity of benzene.

American chemist L. Pauling proposed to consider benzene in the form of two interrelated structures, which differ in the arrangement of the electron density, passing into each other.

Nomenclature and isomerism

How can you name aromatic hydrocarbons? The formula of all compounds that belong to a series of aromatic hydrocarbons must correspond to the proposed molecular structure. The simplest homologue of benzene is toluene. The difference between it and the simplest aromatic hydrocarbon is CH2.

The name of the representatives of this class is based on benzene. The numbering of carbon atoms is clockwise, starting from the senior to the junior deputy. Even (2 and 6) positions are considered ortho-positions, and 3 and 5 (odd) are meta-variants.

Characteristics of physical properties

What are the physical characteristics of aromatic hydrocarbons, the class formula of which corresponds to CnH2n-6?

Benzene, as well as its closest homologs under normal conditions, are toxic liquids having an unpleasant characteristic odor. For all arenas, insignificant solubility in water is characteristic. In unlimited quantities, they are able to dissolve in organic solvents.

Options for obtaining

As an industrial option for the production of benzene and other representatives of the aromatic hydrocarbon class, processing of coal tar or oil can be considered. A synthetic variant of obtaining representatives of this class is in the following variants:

• Cleavage of hydrogen molecules from cycloparaffins (dehydrogenation);
• Aromatization of saturated hydrocarbons (alkanes).

Both proposed methods for converting compounds into an aromatic variant involve the use of an elevated temperature and a catalyst.

Among the common methods of laboratory obtaining arenes can be mentioned the synthesis of Wurz. It is characterized by the interaction of the halogenated alkane with metallic sodium.

Features of homologues of benzene

Toluene, in which a methyl group is present, enters into chemical reactions faster than benzene. Since CH3 is a first-order orientant, the incoming substituents will be oriented in ortho (even) positions. Toluene is capable of halogenation (chlorination, bromination, iodination), and also to nitration.

Conclusion

All aromatic hydrocarbons correspond to the general formula CnH2n-6. When burning them in air oxygen, a sufficient amount of soot is released, which is easily explained by the increased content of carbon in them.