PHENOLIC CONSTITUENTS 285 



Walden * to be untenable, since the physical properties of 

 the two substances are quite different, and the position was 

 subsequently cleared up by Fischer who showed that the 

 natural gallotannic acid occurring in oak galls was actually 

 a pentadigalloyl ester of glucose (see p. 291). 



ELLAGIC ACID. Cj^HeOg. 



Closely related to gallic acid is the substance known as 

 Ellagic acid, its name being obtained by the inversion of the 

 word gallic. 



Its constitution is, according to Graebe, best represented 

 by the formula — ■ 



^° No.co/ 



from which it will be seen that it may be considered to be pro- 

 duced by the abstraction of two molecules of water from two 

 molecules of gallic acid, with simultaneous oxidation or removal 

 of two atoms of hydrogen. 



Synthetically it may be prepared by the oxidation of gallic 

 acid by means of arsenic acid, or better by oxidizing gallic 

 acid in acetic acid solution with potassium persulphate and 

 sulphuric acid.f 



Whether or not this substance occurs free in nature is not 

 definitely estabhshed ; certain it is, however, that ellagic acid 

 can be readily obtained by the hydrolysis of ellagitannic acid,| 

 a substance which almost invariably accompanies gallotannic 

 acid in the numerous vegetable products in which this latter 

 occurs ; it also occurs in conjunction with tannins of the pyro- 

 gallol class, and constitutes the bloom which is produced on 

 leather by this type of tannin. 



The most convenient natural sources are " divi-divi " 

 {Ccesalpinia coriaria), " algarobilla " {Ccesalpinia hrevifolia), 

 " myrobalans " {Terminalia Chehula), etc. Aqueous extracts 



* Walden: " Ber. deut. chem. Gesells.," 1897, 30, 3153; 1898, 31, 



3167. 



t Perkin and Nierenstein : " J. Chem. Soc," 1905, 87, 1415. 

 I Sisley : " Bull. Soc. Chim.," 1909, [4], 5, 727. 



