BENZENE

(C6H6), the simplest aromatic hydrocarbon, widely used in industry. Discovered by the English scientist Michael Faraday in 1825 in illuminating gas made from whale oil, it was named benzene in 1845 by A.W. von Hofmann, the German chemist, who detected it in coal tar. Benzene is obtained chiefly from coke-oven gas, which yields various grades of benzol sufficiently pure for most markets. Since 1950 benzene has also been prepared from petroleum. It is highly toxic, and long exposure to it may cause leukemia. The structure of benzene has been of interest since its discovery. In 1865 the German chemist August Kekule proposed the hexagonal formula with alternate single and double bonds; it is still used despite two objections: benzene does not show the properties normally associated with double bonds, and the number of isomeric disubstituted benzenes is smaller than would be predicted. Kekule, therefore, postulated an oscillation of the double bonds, giving the ring the symmetry demanded by the number of known isomers. In 1931 the American chemist Linus Pauling, applying quantum mechanics, described a hybrid state resulting from resonance of the two structures that Kekule had assumed to be in equilibrium. This hybrid state explains the low reactivity of benzene by giving the carbon-carbon bond in the ring properties intermediate between those of normal double and single bonds. X-ray data support this view: the distance between the carbon atoms of a normal double bond is 1.34 angstroms, that of a single bond 1.54, and that of a benzene bond 1.39. A similar symmetrical structure is indicated by molecular-orbital considerations. The parent substance of a large class of chemical compounds, benzene is used for the commercial synthesis of ethyl benzene, phenol, and maleic anhydride (used to make plastics); aniline (for dyestuffs); dodecylbenzene (for detergents); and chlorobenzenes (for insecticides). With sulfuric acid and with nitric acid, benzene gives monosubstituted, disubstituted, and trisubstituted products. With chlorine and bromine, substitution or addition takes place. Benzene is remarkably resistant to oxidation, but, with vanadium oxide at 450 C (842 F), benzoquinone and maleic anhydride are formed. Reduction to cyclohexane takes place in the presence of hydrogen and nickel at 180 C (356 F). Passed through a red-hot tube, benzene yields diphenyl, diphenylbenzenes, acetylene, and hydrogen. A colourless, limpid liquid with a characteristic odour, benzene boils at 80.1 C (176.2 F) and solidifies at 5.5 C (41.9 F). It is only slightly soluble in water. It is an excellent solvent for rubber, gums, fats, and a number of resins.