1911 Encyclopædia Britannica/Toluene
TOLUENE, or Methylbenzene, C7H8 or C6H5·CH3, an aromatic hydrocarbon; the first homologue of benzene. Discovered by Pelletier (Ann. chim. pays., 1838, 67, p. 269) in the oil obtained in the manufacture of gas from the resin of Pinus maritima, and named retinnaphte, it was prepared from the same gas by Couerbe (ibid., 69, p. 184) and named heptacarbure quadrihydrique, C7H4 (C=6); Sainte-Claire Deville (ibid. 1841 [3] 3, p. 168) obtained it by distilling Tolu balsam, naming it benzoéne, and Glénard and Bouldault obtained a substance by the dry distillation of dragon's blood which they called dracyl. The complete identity of these substances was established by A. W. Hofmann and Muspratt, and they adopted the name toluol (anglicized to toluene), which was proposed by Berzelius. Its derivatives and its relation to benzene had been previously studied by the above and other experimenters, its relation to benzene being first proved experimentally by Cannizzaro and its constitution settled by Fittig and Tollens’s synthesis from sodium and a mixture of methyl iodide and brombenzene.
The hydrocarbon occurs in wood-tar and in petroleum, and is prepared commercially by fractional distillation of the light oil fraction of the coal-tar distillate (see Coal Tar). It may be obtained synthetically by Fittig and Tollens’s method (above); by Friedel and Craft’s process, devised in 1877, of acting with aluminium chloride on a mixture of benzene and methyl chloride; this reaction leads to the production of higher homologues which may, however, break down under the continued action of the aluminium chloride; or by heating the toluene carboxylic acids obtained by oxidizing the higher homologues of benzene. It forms a colourless mobile liquid, boiling at 110·3° C. and having a specific gravity of 0·8708 (13·1/4°). It is insoluble in water, but dissolves readily in alcohol and ether. On reduction it yields hexahydrotoluene; oxidation with dilute nitric acid or chromic acid gives benzoic acid; whilst chromyl chloride and water give benzaldehyde. On nitration it gives ortho- and para-nitrotoluenes—which on reduction yield the valuable toluidines, C6H4(CH3)(NH2)—and on sulphonation the parasulphonic acid is formed with a little of the ortho acid. Chlorination in the cold gives ortho- and para-chlortoluenes, but at the boiling point the side chain is substituted (see Benzaldehyde).