GALLOTANNIC ACID 213 



form of a glucoside, or whether the sugar which is frequently 

 found associated with it was merely an impurity.* 



E. Fischer and Freudenberg,f on reinvestigating the ques- 

 tion, found that " tannin," even after repeated careful purifica- 

 tion, yielded about 7 to 8 per cent of glucose on hydrolysis 

 with sulphuric acid ; from this it was concluded that "tannin" 

 or gallotannic acid as it occurs in nature is not identical with 

 synthetic digallic acid, but is in reality a compound of five 

 molecules of such a digallic acid with one molecule of glucose, 

 in which the five hydroxyl groups of the sugar are esterified 

 by five molecules of acid. Such a compound would be a 

 pentadigalloyl glucose of the formula 



C 6 H 7 6 [C 6 H 2 (OH) 3 CO . O . C t5 H 2 (OH) 2 . CO] S or C 76 H S2 O 46 



Actually two isomeric substances of this formula, with a 

 molecular weight of 1700, have been synthesized. J 



The one derived from meta-digallic acid, i.e. penta- (m-dl- 

 galloyl) /3-glucose, has been found to be practically identical 

 with Chinese tannin, and to differ from it only in regard to its 

 specific rotation ; this difference is, however, of no great signifi- 

 cance considering the colloidal nature of the substance concerned 



Fischer, Bergmann, and Lipschitz have also synthesized a 

 galloyl glucose of the formula 



C 6 H 2 (OH) 3 . CO . O . CH . (CHOH) 2 . CH . CHOH . CH 2 OH 



o 



which is identical with glucogallin, a substance first isolated 

 from Chinese rhubarb by Gilson. II 



* Cf. Strecker : " Annalen," 1852, 8l, 248 ; 1854, 90, 328 ; Pottevin : " Compt. 

 rend.," 1901, 132, 704. 



t Fischer and Freudenberg : " Ber. deut. chem. Gesells.," 1912, 45, 915 and 

 2709. 



Fischer, Bergmann, and Lipschitz: id., 1918, 51, 45. 



Two possible digallic acids can be produced by the loss of one molecule of 

 water between two molecules of gallic acid (see p. 204). 

 OH 



OH / - \ OH OH 



, - v H0<r >COOH / - \ / - \ 



HO/ J)CO.O_^ _ / HO/ \CO.O. / \X)OH 



OH OH OH 



w-digallic acid. /-digallic acid. 



II Gilson : " Compt. rend.," 136, 385. For summary of work on synthesis 

 of tannins see Fischer: " Ber. deut, chem, Gesells.," 1919, 52, 13, 809. 



