Synthetic Tannins | Page 5

of his own preparation as 746, Turkish tannin as 521 and Chinese tannin as 899. In this connection it should be noted that the calculated molecular weight of pentagalloyl glucose, which in E. Fischer's opinion forms a substantial part of the tannin molecule, is 940, but Fischer also thinks that this compound possesses a much higher molecular weight.
STRUCTURE OF TANNIN--The oldest structural formula of tannin is Schiff's digallic acid formula:--[Footnote 1: Ber., 1871, 4, 231.]
---------CO.O.---------- ^ ^ OH | | | | 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.). K?nig 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 ^ ______ ^ | | | | | | | | V V
Nierenstein gave the name "Tannophor" to the mother-substance of tannin, phenylbenzoate, C_6H_5-COO-C_6H_5.
Dekker [Footnote: "De Looistoffen," vol. ii, p. 30 (1908).] was, however, unable to detect diphenylmethane on distilling with zinc dust, and did, therefore, not accept Nierenstein's views. In proposing the formula--
O || HO ^ _ __C | | | | | }O | | | __OH | |____|_C_/ \OH HO V \__/ OH OH OH
Dekker [Footnote: _Ber._, 1906, 34, 2497.] was enabled to account for most of the details in the behaviour of tannin, viz.: (1) the empirical constitution, C_14H_10O_9; (2) the almost complete hydrolysis into gallic acid (the dotted line indicates the decomposition of the molecule into 2 molecules gallic acid by taking up water); (3) the formation of diphenylmethane as a result of distillation with zinc dust; and (4) the electrical non-conductivity. Since tannin on acetylating yields a considerable amount of triacetylgallic acid, it should, according to Dekker, contain at least six acetylisable hydroxyls.
Nierenstein [Footnote: _Chem. Ztg._, 1906, 31, 880.] objected to this formula on account of its containing seven hydroxyl groups, whereas Dekker found six, Nierenstein five, and Herzig still fewer hydroxyl groups. The formula would also favour the conception of tinctorial properties which could hardly be ascribed to tannin. Lloyd [Footnote: Chemical News, 1908, 97, 133.] proposed a very intricate formula containing three digallic acid groups joined into one six-ring system, which would then explain the optical activity; it would, on the other hand, also require an inactive cis-form.
Iljin [Footnote: _Jour. of the Russian phys. chem. Soc._, 1908, 39, 470.] prepared two phenylhydrazine derivatives of tannin (C_74 H_58 N_8 O_30 and C_98 H_82 N_14 O_96) and proposed the formula, C_58 H_40 O_33, the constitution of which would be--
R_1 R_1 | | }C--O--O--C{ | | | R_2 | R_2 O R_1 | R_1 | | | }C--O--O--C{ | | R_2 R_2
where R1= CO C_6 H_2 (OH)_3 and R2= C_6 H_2 (OH)_2
Nierenstein [Footnote: _Ber._, 1905, 38, 3841; 1907, 40, 917; 1908, 41, 77 and 3015; 1909, 42, 1122 and 3552; _Chem. Ztg._, 1907, 31, 72; 1909, 34, 15.] considers tannin to be a mixture of digallic acid