96 CHEMISTRY OF PLANT LIFE 



These phenols themselves do not occur as constituents of tannins, 

 although they are often found in other glucosides, gums, etc. 

 The following mono-carboxyl acid derivatives of these phenols 

 are, however, found both free and in glucoside formation as con- 

 stituents of many of the common tannins. 



Pyrocatechuic add, derived from pyrocatechol, represented by 

 the formula, 



OH 



1OH 



COOH 



Gallic acid, derived from pyrogallol, and represented by the 

 formula, 



OH 



cOOH 



In most of the common tannins, however, the characteristic acids 

 are oxy-derivatives of the so-called " tannon " group, represented 

 by the formula, CeHs-CO-O-CeHs. For example, digallic acid, 

 which is a constituent of many common tannins, is a tetra-oxy, 

 mono-carboxyl derivative of this group, having the structural 

 formula, 



Ho/V- co O-/NCOOH 



H( 

 OH OH 



Ellagic add, which is an hydrolysis product of many of the pyro- 

 gallol tannins (see below) and which produces the characteristic 

 " bloom " on leather tanned by this type of tannins, has the fol- 

 lowing formula, 



