VIII] AROMATIC COMPOUNDS 



Aromatic Acids, Aldehydes and Alcohols. 



103 



There are two important series of these compounds found in the plant 

 which can be represented respectively by benzoic acid and cinnamic acid 

 and their derivatives : 



COOH 



V 



Benzoic acid 



CH=CHCOOH 



Cinnamic acid 



Salicylic acid, or o-hydroxy-benzoic acid, occurs, both in the form of 

 esters and in the free state, in various plants. The corresponding alcohol, 

 saligenin or salicylic alcohol, in the form of the glucoside, salicin, occurs 

 in the bark of certain species of Willow (Salix), and in the flower buds of 

 the Meadow-sweet (Spiraea Ulmaria). Salicin is hydrolyzed by an en- 

 zyme contained in the plant in which it occurs into saligenin and 

 glucose (see also p. 167). Salicylic aldehyde occurs in species of Spiraea 

 and other plants. 



COOH 



OH 



V 



CH=CH-COOH 



OH 



V 



o-Coumaric acid 



Salicylic acid 



The corresponding derivative of cinnamic acid, i.e. o-coumaric acid 

 is widely distributed as the anhydride, coiimarin (see p. 165). 



The relationship of cinnamic acid to phenylalanine and of ^-coumaric 

 acid to tyrosine (see p. 135) is important. 



Protocatechuic acid is a dihydroxy-benzoic acid. It has been found 

 in the free state in a few plants, but is more widely distributed as a 

 constituent of many plant products. As will be shown later it forms the 

 basis of the series of tannins. 



COOH 



OH 



CH=CHCOOH 





OH 



OH 



Protocatechuic acid 



OH 



Caffeic acid 



