Synthetic Tannins | Page 7

molecules of gallic acid; it is also possible, though not probable, that tannin would contain a polysaccharide instead of glucose itself. Similarly to sugar, the true glucosides can be coupled with hydroxybenzoic acids, which is proved by the preparation of tetra-galloyl-[Greek: a]-methyl glucoside; this substance, also, exhibits tannoid character.
2. DIGALLIC ACID
Whereas, until recently, tannin had been considered to be gallic acid anhydride, or digallic acid, closer investigations have revealed that neither is tannin digallic acid nor is the synthetically prepared digallic acid identical with tannin. Schiff [Footnote: _Ber._, 1871, 231 and 967.] prepared digallic acid by the interaction of phosphorus oxychloride and gallic acid, and believed the product obtained to be identical with tannin; to this latter he first ascribed an ether formula (I.), later an ester formula (II.)--
(OH)_2 (OH)_2 || || C_6H_2---0---C_6H_2 | | COOH COOH (I.)
(OH)_2 || C_6H_2(OH)_3--C--O.C_6H_2 || | O COOH (II.)
Froda [Footnote: _Gasz. chim._, 1878, 9.] held that Schiff's condensation product contained phosphorus or arsenic acid and ascribed its tanning properties to the latter; according to this investigator, digallic acid, when completely freed from arsenic acid, does not react with gelatine or quinine. Biginelli [Footnote: _Ibid._, 1909, 39, ii. 268 and 283.] did not consider the action of arsenic acid that of a catalyst, but held that it entered into reaction; according to his investigations products containing arsenic (C_7H_7O_8As and C_14H_11O_12As) are obtained when gallic acid is heated with arsenic acid.
In his preparation of digallic acid, Iljin [Footnote: _Jour. f. prakt. Chem._, 1911, 82, 451.] could only obtain gallic acid, and the ethyl ether of gallic acid showing no characteristics of the tannins; when, however, he heated gallic acid with arsenic pentoxide, he obtained bodies exhibiting the reactions given by tannins.
Bottinger [Foonote: _Ber._, 1884, 1503.] made the first attempt at synthesising tannin; he heated gallic acid or its ethyl ester with glyoxylic acid or pyroracemic acid, and obtained a substance of the composition C_14H_10O_9.2H_2O, which certainly showed some of the characteristics exhibited by tannin, but which by no means was identical with the latter. Bottinger's preparation is probably identical with [Greek: b]-digallic acid, one of two dibasic isomers having the composition--
C_6H_2(OH)_2COOH | C_6H(OH)_3COOH
the other possible isomer having the composition
C_6H(OH)_3COOH CO | C_6H_2(OH)_3
Fischer [Footnote: Ber., 1908, 41, 2875.] obtained a digallic acid (M.P. 275��-280�� C) by coupling tricarbomethoxygalloyl chloride with dicarbomethoxygallic acid.
Nierenstein [Footnote: Ibid., 1910, 43, 628.] obtained, from the carbethoxy compound of tannin, a crystalline, optically active digallic acid, M.P. 268��-270�� C. The pentacetate of this substance, obtained by reduction and acetylisation, yielded hexacetylleucotannin. A pentamethyldigallic acid methyl ester of the composition
((O.CH_3)_3)C_6H_2----COO-----C_6H_2((OCH_3)_2)COO.CH_3
was obtained by Mauthner [Footnote: _Jour. f. prakt. Chem_., 1911, 84, 140.] from the chloride of trimethylgallic acid and the methyl ester of the acid from the glucoside of syringin; on saponification with caustic potash the former compound yielded trimethylgallic acid and syringic acid.
Fischer [Footnote: Ber., 1913, 46, 1116.] synthesised the so-called _m_-digallic acid by coupling tricarbomethoxygalloyl chloride with carbonylgallic acid and subsequent splitting off of CO_2. The _m_-digallic acid appears as rather thick, colourless, microscopic needles containing about 16 per cent. water of crystallisation, M.P. 271�� C. They are slightly soluble in cold, soluble in hot water, and very soluble in methyl and ethyl alcohols. Their aqueous solution gives dark blue coloration with ferric chloride, and precipitates gelatine and quinine.
Fischer and his students [Footnote 5: Ibid., 1912, 45, 915, 2709; 1913, 46, 1116.] prepared quite a number of digallic acid derivatives, amongst which are the following:--
Pentamethyl-_m_-digallic acid methyl ester, C_20H_22O_9. Pentacetyl-_m_-digallic acid, C_24H_20O_14. Pentamethyl-_m_-digallic acid, C_19H_20O_9. Pentamethyl-_m_-digalloyl chloride, C_19H_19O_8Cl. Pentamethyl-_p_-digallic acid, C_19H_20O_9. Pentamethyl-_p_-digallic acid methyl ester, C_20H_22O_9.
Hydrolysis of digallic acid yields gallic acid; oxidation, on the other hand, ellagic acid and luteic acid (Luteo S?ure), which can be separated by shaking with pyridine. The reduction of digallic acid yields, by different methods, the same reduction compound, [Footnote: Nierenstein, Abderhalden's "Handb. d. biochem. Arbeitsm.," vi. 154.] viz., the racemic leucodigallic acid, which differs from digallic acid by being devoid of any tannoid properties; the latter distinction may be ascribed to the transformation of the tannophor group--CO.O--, to the tannoid-inactive group CH(OH)--O--.
The successful resolving of racemic leucodigallic acid into both of its optically active components can only be brought about through the _d_- or _l_-hexacarbethoxyleucodigallic acid on introducing the latter into a 1 per cent. pyridine solution and heating to 45��-50�� C., whereby the _d_- or _l_-acid is formed accompanied by a strong evolution of carbon dioxide.
Hydrolysis of leucogallic acid yields gallic acid and gallic aldehyde; oxidation by means of hydrogen peroxide yields ellagic acid and luteic acid, and oxidation with potassium persulphate and sulphuric acid, in acetic acid solution, yields purpurotannin (see below) [Footnote: Liebig's Ann., 1912, 386, 318.].
Another distinct difference between digallic acid and leucodigallic acid is the fact that the formaldehyde condensation product of the former resembles gallic acid, whereas that of the