STRUCTURE OF TANNIN—The oldest structural formula of tannin is Schiff's digallic acid formula:—[Footnote 1: Ber., 1871, 4, 231.]
————-CO.O.—————
^ ^ OH
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HO | | OH HOOC | | OH
V V
OH
A drawback to the acceptance of this formula is the absence of an asymmetrical C-atom; the formula, therefore, does not explain the optical activity exhibited by tannin. Schiff attempted to overcome this difficulty by adopting a diagonal structural formula, but even when adopting Clauss' diagonal formula for benzene the optical activity of a number of other compounds depends upon the existence of the asymmetrical C-atom. Biginelli [Footnote 2: Gazz chim. Ital., 1909, 39, 268.] also opposed the digallic acid formula, and supported his view by referring to the arsenic compounds obtained by him on heating arsenic acid and gallic acid, instead of obtaining digallic acid. Walden, [Footnote 3: Ber., 1898, 31, 3168.] on the other hand, found, on analysing the digallic acid thus prepared, only slight traces of arsenic and, by the elementary analysis, obtained figures closely corresponding to those of digallic acid.
Bottinger [Footnote 4: Ibid., 1884, 17, 1476.] prepared the so-called [Greek: b]-digallic acid by heating ethyl gallate with pyroracemic acid and sulphuric acid and proposed the so-called ketone-tannin formula:—
HO_____OH ______OH HO{_____}————CO————{______}OH COOH OH
Schiff completed this formula by a diagonal, so as to explain the optical activity observed—
HO OH ______OH
HO{_____}————CO————{______}OH
COOH OH
[Diagonal bond between HO and COOH on left.]
The ketone formula was corroborated by Nierenstein, [Footnote: Ber. 1905, 38, 3641.] who distilled tannin with zinc dust and obtained diphenylmethane (smell of benzene) and a crystalline product, M.P. 7O°-71° C. (M.P. of diphenyl = 71° C.). König and Kostanecki [Footnote: Ibid., 1906, 39, 4027.] sought to find the constitution of the tannins in the leuco-compounds of the oxyketones, to which catechin belongs. Nierenstein (see above), however, emphasises that the high molecular weight and the optical activity speak against the digallic acid formula, but in favour of this are the following points: (1) the decomposition of tannin with the formation of gallic acid; (2) the decomposition of methylotannin with the formation of di- and trimethyl esters of gallic acid; and (3) the production of diphenylmethane on distillation with zinc dust. The latter reaction especially illustrates the analogous formation of fluorene from compounds of the type—
—CO.O
^ ______ ^
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V V
Nierenstein gave the name "Tannophor" to the mother-substance of tannin, phenylbenzoate, C_6H_5-COO-C_6H_5.