278 TANNINS 



term Tannin are so numerous as to make it improbable that 

 they all have the same physiological significance. 



THE PHENOLIC CONSTITUENTS OF THE TANNINS. 



The classification and properties of the tannins will be more 

 easily understood if preceded by a brief description of certain 

 relatively simple phenolic substances from which the complex 

 tannins are built up (p. 291). 



The substances include the following : — 



1. The dihydric phenols — pyrocatechol, resorcinol and 

 hydroquinone. 



2. The dihydroxy acid — protocatechuic acid. 



3. The trihydric phenols — pyrogallol and phloroglucinol. 



4. The trihydroxy acid — gallic acid. 



5. Ellagic acid. 



6. Digallic acid. 



The above substances occur in varying proportions among 

 the products obtained by subjecting different tannins to fusion 

 with caustic potash or other chemical treatment ; and upon 

 their occurrence is based the chemical classification of the 

 tannins. 



CATECHOL (Syn. PYROCATECHOL) CgH^IOH),. 



OH 



OH 



Catechol 



This substance is so called from the fact that it is obtained 

 by the destructive distillation of catechu, an extract of the 

 bark of Mimosa Catechu ; it is also obtained by the fusion 

 with potash of other tanno-resins such as kino, the sap of 

 various species of Pterocarpus, Biitea or Eucalyptus ; also it 

 occurs in small quantities combined with sulphuric acid in the 

 urine of horses and of human beings. It crystallizes from 

 benzene in colourless glistening plates and melts at 146°. 



Reactiojis. 



I. Catechol is precipitated from aqueous solution by lead 

 acetate. (Distinction from resorcin and hydroquinone.) 



