AROMATIC COMPOUNDS 271 



Orcinol is dihydroxy-toluene. Thymol and carvacrol are deri- 

 vatives of cymene : 



CH, CH, 



ii i <OH 



HO k JOH 



Orcinol. 



Protocatechuic acid, or catechol-carboxylic acid, results from the 

 OH fusion of numerous resins, e.g. gum benzoin, 



catechin resin, with caustic potash. It is pre- 

 pared by heating catechol with water and am- 

 monium carbonate to 140. 



COOH It is a colourless crystalline solid melting 



at 199 and dissolves readily in water. 



With ferric chloride its solution gives a green colour which changes to 

 blue and then to red on the addition of very dilute sodium carbonate. With 

 ferrous sulphate a violet colour is given. It is precipitated from solution by 

 lead acetate. 



Veratric acid, the dimethyl ether of protocatechuic acid, occurs in the 

 seeds of Veratrum sabadilla, together with veratrine. 



Vanillin is obtained from coniferyl alcohol by oxidation with chromic 

 acid. Vanillin is the sweet-smelling constituent of the vanilla bean. 



Coniferyl alcohol occurs in the glucoside, coniferin, and is obtained from 

 it by hydrolysis. 



OH OH 



r^OCH, /NoCH, 



COOH CHO | 



CH = CH . CH 2 OH 

 Veratric acid. Vanillin. Coniferyl alcohol. 



Homogentisic acid, or quinol-acetic acid, is found in the urine in 

 the rare disorder known as alkaptonuria. 

 :H 2 .COOH Its presence is first shown by the urine 



turning brown and black on standing, or by 

 its reducing power. 



It is a white crystalline solid melting at 146-147. 



As a derivative of hydroquinone it reacts with 



(1) ferric chloride ; 



(2) Fehling's solution or ammoniacal silver nitrate ; 



(3) turns black in the air in alkaline solution. 



