198 . TANNINS 



2. The dihydrox)' acid — rrotocatechuic acid. 



OH 



OH 



COOH 

 Protocatechuic acid 



3. The Trihydric phenols — Pyi'ogallol and phloroglucinol. 

 OH OH 







jOH 



'OH HOxylOH 



Pyrogallol Phloroglucinol * 



4. The trihydroxy acid — Gallic acid. 

 OH 



HOOCljoH 

 Gallic 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. 



PYROCATECHOL, CATECHOL OR PYROCATECHIN. CuH,(OH).,. 



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

 by the destructive distillation of catechu, the sap 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 Ptero- 

 ca?-pus, 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 140°. 



* Although behaving normally as a trihydric phenol, phloroglucinol reacts 

 with certain ketonic reagents as though it had the following constitution : — 

 CO 



/\ 

 CH3 CHj 



CO CO 



CH., 



