If DIPHENYLMETHANE, (C_6H_5)_2CH_2, is heated with excess sulphuric acid, a dark blue mass, easily soluble in water, is obtained. The product gently heated with formaldehyde yields a brown, water-soluble condensation product; once condensation is complete, the product will stand stronger heat. If, on the other hand, more formaldehyde is added, brown, water-insoluble bakelites are formed. The water-soluble condensation product precipitates gelatine, but not aniline hydrochloride. Dissolved in water, it possesses tannoid properties: the pelt is, however, tanned on the surface only, the intermediary layers being merely pickled; after four days in the solution, the pelt after drying was found to be converted into a greyish-brown, badly coloured leather, which was empty, hard, and possessed but little tensile strength.
CARBAZOLE (dibenzopyrrole),
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N_3
on the other hand, was found a suitable base for the commercial production of synthetic tannins; its use is protected by Ger. Pat, 290,965.
TRIPHENYLMETHANE, (C_6H_5)_3CH, heated with excess sulphuric acid, yields a nearly black mass which, when condensed with formaldehyde in the cold, and subsequently heated, yields a mass which is soluble in water. With gelatine and aniline hydrochloride it exhibits reactions similar to those given by the diphenylmethane condensation products; its tanning properties also are similar to those of the latter. The resultant leather is black, but is soft and full and possesses good tensile strength.
Baeyer's observation, [Footnote: Ber., 1872, 5, 280, 1096.] that pyrogallol on condensation with formaldehyde yields an amorphous body soluble in water, which precipitates gelatine and is very similar to tannin, was confirmed by Caro [Footnote: Ibid., 1892, 25, 947.] and Kahl. [Footnote: Ibid., 1898, 31, 114.] These investigators found that by the condensation of phenols and hydroxybenzoic acids with formaldehyde, diphenylmethane derivatives were formed; pyrogallol yields hexahydroxydiphenylmethane—
C_6H_2(OH)_3 CH_2{ C_6H_2(OH)_3
Nierenstein [Footnote: Collegium, 1905, 221.] repeated these experiments, and found that in addition to the insoluble diphenylmethanes, water-soluble bodies were formed, which latter precipitate gelatine. The condensation product yielded by gallic acid was identified as hexahydroxyaurinecarboxylic acid—
_C_6H(OH)_3COOH C{-C_6H(OH)_3COOH | }C_6H(OH)_2COOH O
which is formed in addition to hexahydroxydiphenylmethane-dicarboxylic acid—