INDENE, C9H8, a hydrocarbon found in the fraction of the coal tar distillate boiling between 176° and 182° C., and from which it may be extracted by means of its picrate (G. Kramer, A. Spilker, Ber., 1890, 23, p. 3276). It may also be obtained by distilling the calcium salt of hydrindene carboxylic acid, C6H4(CH2)2·CH·COOH. It is an oil which boils at 179.5°-180.5°, and has a specific gravity 1.04 (15° C.). Dilute nitric acid oxidizes it to phthalic acid, and sodium reduces it in alcoholic solution to hydrindene, C9H10. A. v. Baeyer and W. H. Perkin (Ber., 1884, 17, p. 125) by the action of sodiomalonic ester on ortho-xylylene bromide obtained a hydrindene dicarboxylic ester,

C6H4(CH2Br)2 + 2CHNa(CO2C2H5)2 = 2NaBr + CH2(CO2C2H5)2 + C6H4:[CH2]2:C(CO2C2H5)2;

this ester on hydrolysis yields the corresponding acid, which on heating loses carbon dioxide and gives the monocarboxylic acid of hydrindene. The barium salt of this acid, when heated, yields indene and not hydrindene, hydrogen being liberated (W. H. Perkin, Jour. Chem. Soc., 1894, 65, p. 228). Indene vapour when passed through a red hot tube yields chrysene. It combines with nitrosyl chloride to form indene nitrosate (M. Dennstedt and C. Ahrens, Ber., 1895, 28, p. 1331) and it reacts with benzaldehyde, oxalic ester and formic ester (J. Thiele, Ber., 1900, 33, p. 3395).

On the derivatives of indene see W. v. Miller, Ber., 1890, 23, p. 1883; Th. Zincke, Ber., 1887, 20, p. 2394, 1886, 19, p. 2493; and W. Roser and E. Haselhoff, Ann., 1888, 247, p. 140.