280 PRACTICAL ORGANIC AND BlO-CHEMlStRY 



A series of ureides is derived from acids containing three carbon 

 atoms in their molecule : 



Diethylbarbituric acid, Carbamidomalonyl urea, 



or or 



veronal. pseudo-uric acid. 



Uric acid, the principal ureide biologically, is derived from a 

 hypothetical acid, C(OH) 2 = C(OH) COOH, containing 3 carbon 

 atoms and 2, molecules of urea ; it has the formula : 



NH C=0 



CO C NH X 



I II >co. 



NH C NH/ 



Alloxan. 



Alloxan is the most interesting of this series of compounds as it 

 can be transformed into the others ; it forms the central point in our 

 knowledge of the constitution of uric acid and other purines (p. 286). 



Preparation. 



Alloxan is produced by the careful oxidation of uric acid with nitric acid, 

 bromine or chlorine ; also by the oxidation of xanthine. 



It is easy to prepare alloxantin from uric acid and to prepare alloxan 

 from alloxantin. 



Alloxantin may be prepared as follows : io gm. of uric acid are covered 

 with 20 c.c. of water and 20 gm. of concentrated hydrochloric acid and 

 heated to 35 ; 2-5 gm. of powdered potassium chlorate are gradually added 

 with continuous stirring. The uric acid dissolves and a pale yellow liquid 

 results. This is diluted with about 75 c.c. of water, allowed to stand and 

 filtered. The filtrate, which contains alloxan, is saturated with hydrogen 

 sulphide and allowed to stand for 12-16 hours. Alloxantin mixed with sulphur 

 separates out ; it is filtered off and washed with water. It is separated from 

 sulphur by solution in a small quantity of boiling water from which it separates 

 on cooling in colourless crystals. These crystals are sometimes tinged with 

 pink. 



