TOLUENE or Methylbenzene, a constituent of the light oils distilled from coal tar (q.v.), is a colourless mobile liquid boiling at III° C/760 mm. and having a density of 0.8812 at 4° C. It is insoluble in water but dissolves readily in alcohol or ether. Toluene (Ger. Toluol), an aromatic hydrocarbon (see CHEMISTRY, Organic), is an important starting point in the manu facture of intermediates and synthetic colouring matters. (See BENZALDEHYDE ; BENZOIC ACID; DYES, SYNTHETIC.) Its mono nitro-compounds furnish ortho- and para-toluidines, two valuable bases employed in the production of magenta and other aniline dyes. During the World War toluene was in such great demand for the manufacture of the high explosive trinitrotoluene (see T.N.T.) that its output from coal tar had to be supplemented by supplies from other sources such as Borneo petroleum.
On sulphonation toluene yields mainly two sulphonic acids, of which toluene-ortho-sulphonic acid is employed in the production of saccharin (q.v.).
Originally discovered by Pelletier (5838) in an oil obtained from the resin of Pinus maritima it was subsequently prepared by Sainte-Claire Deville (1841) on distilling tolu balsam. It is to the latter circumstance that the hydrocarbon owes its name. Its re lationship to benzene was established by acting with sodium on a mixture of methyl iodide and bromobenzene and by condensing benzene and methyl chloride in presence of dry aluminium chloride. Oxidation of toluene leads successively to benzaldehyde and benzoic acid, whereas its hydrogenation furnishes hexahydro toluene. p-tolualdehyde, a pungent oil (b.p. C), results from the interaction of toluene with carbon monoxide and hydrogen chloride in the presence of aluminium and cuprous chloride, or from hydrogen cyanide and hydrogen chloride with aluminium chloride. (G. T. M.)