Benzene

Benzene, a compound of carbon and hydrogen, $C\_6H\_6$, discovered by Faraday in 1825, in a tarry liquid resulting from the distillation of oil. It must not be confounded with Benzine or Benzoyl, which names have at different times been used for benzene. Benzine is the name given to a distillate from American petroleum, which is much used as a substitute for turpentine, and for dissolving oils and fats. Benzoyl is the commercial name applied to a mixture of substances, including benzene and its homologues. Benzol is synonymous with benzene, while benzoline is a name applied to benzene and impure benzene indiscriminately. Benzene is found amongst the products of the destructive distillation of a great many organic bodies. The most abundant source of benzene is coal-tar (see GAS, COAL). On distilling coal-tar, the more volatile liquid hydrocarbons pass over first, mixed with acid and basic compounds, and constitute what is known as light oil or coal naphtha. When the crude naphtha is purified by redistillation and subsequent agitation, first with sulphuric acid, and then with caustic soda, an oil is obtained which consists mainly of benzene and its homologues. By submitting this oil to a process of fractional distillation, a portion is obtained, boiling at $176^\{\backslash circ\}$–$212^\{\backslash circ\}$ ($80^\{\backslash circ\}$ C.–$100^\{\backslash circ\}$ C.), from which benzene crystallises out on cooling the liquid to $32^\{\backslash circ\}$ ($0^\{\backslash circ\}$ C.). The benzene is freed by pressure from the substances remaining liquid at this temperature. Commercial benzene is, however, always impure. Pure benzene is most readily obtained by cautiously distilling a mixture of one part benzoic acid with three parts of slaked lime. The mixture of benzene and water which passes over is shaken up with a little potash, the benzene decanted, treated with calcium chloride to take up the water, and the dried benzene thus obtained is rectified on the water-bath. At ordinary temperatures benzene is a thin, limpid, colourless liquid, evolving a characteristic and pleasant odour. At $32^\{\backslash circ\}$ ($0^\{\backslash circ\}$ C.) it crystallises in beautiful fern-like forms, which liquefy at $40^\{\backslash circ\}$ ($4^\{\backslash circ\}$–$5^\{\backslash circ\}$ C.); and at $176^\{\backslash circ\}$ ($80^\{\backslash circ\}$ C.) it boils, evolving a gas which is very inflammable, burning with a smoky flame. It readily dissolves in alcohol, ether, turpentine, and wood-spirit, but is insoluble in water. It is valuable to the chemist from the great power it possesses of dissolving caoutchouc, gutta-percha, wax, camphor, and fatty substances. Impure benzene is thus much used in removing grease-stains from woolen or silken articles of clothing. When heated, benzene also dissolves sulphur, phosphorus, and iodine. Although commercially of importance, from the chemical point of view benzene is a compound of surpassing interest. It consists of 12 parts of carbon by weight, with 1 part of hydrogen, and might therefore be represented by the formula $CH$; but it has been found that the molecule weighs six times as much as this, and requires the formula $C\_6H\_6$. The manner in which the atoms are arranged in the molecule has already been referred to in the article on the AROMATIC SERIES, and it will be seen by a reference to the graphic formula there given, that the number of compounds derivable from benzene is practically unlimited. The so-called coal-tar colours are all derivations of benzene (see DYE-STUFFS, and ANILINE).