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
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C_6H(OH)_3COOH
the other possible isomer having the composition