296 



TANNINS 



Acacatechin (a) is the catechin derived from acacia 

 catechu ; it melts at 203-205° and is regarded by Perkin as 

 being isomeric with gambier catechin. 



It has been suggested * that the two catechins, a and b, 

 are reduction products of quercetin, as may be seen from the 

 formulae given below and that the isomerism between the two 

 catechins is dependent upon the position of the hydroxy! 

 group marked with a star. 



OH OH 



HO— 



—OH 



HO— 



CH- 



CHOH 



OH 



OH 



Catechin, CigHi^Og 



OH 

 Quercetin, C^jH^oO, 



All the above catechins when boiled with dilute mineral 

 acids readily yield red substances of a phlobaphene character. 



Catechu tannic acid is the name given to the chief tanning 

 constituents of cutch. 



Little is known as to its constitution,! but it is beheved to 

 be an anhydride of catechin. 



THE CONSTITUTION OF THE CATECHU TANNINS. 



The catechu tannins as may be seen from the formula 

 given above for catechin contain a potential phloroglucinol 

 grouping — 



HO 



O^/OH 



OH 



and in fact on hydrolysis with dilute acid they yield proto- 



catechuic acid and phloroglucinol which latter presumably 



gives rise by oxidation to the red phlobaphenes (see p. 298) 



which are ultimately obtained. The pyrogallol tannins, on 



* Perkin : " J. Chem. Soc," 1905, 87, 398. 



t For a theoretical discussion of the chemistry of this substance, see 

 V. Euler : "Chem. Zentr.," 1921, 731. 



