PURINES 293 



Xanthine. 



Xanthine was discovered in an urinary calculus by Marcet in 

 1817 and in 1859 Scherer found it in meat and pancreas. It has 

 since been shown to be present in other animal organs and to be 

 widely distributed in plants. 



Its constitution was indicated by its products of oxidation, alloxan 

 and urea ; Fischer proved it by synthesis. 



Preparation. 



Xanthine is most easily prepared by the action of nitrous acid upon guan- 

 ine. 



Its preparation from extracts of tissues involves a complicated process of 

 separation from other purines (p. 584). 



Properties. 



Xanthine is a colourless powder which assumes a waxy appearance on rub- 

 bing. It separates slowly from its solution in alkali on the addition of acetic 

 acid in the form of colourless nodules consisting of microscopic rhombic 

 shining platelets containing i molecule of water of crystallisation. 



It is very slightly soluble in water i part in 14,151 parts at 16, i part 

 in 1300-1500 parts at 100. It is insoluble in alcohol and ether. It is 

 easily soluble in caustic alkalies and in 2 per cent, ammonia. On evapora- 

 tion of the ammoniacal solution, xanthine separates in groups of platelets. 

 It is soluble with difficulty in cold dilute nitric and hydrochloric acids, but 

 more easily on warming. It dissolves with difficulty in 5 per cent, sul- 

 phuric acid even on boiling. Xanthine hydrochloride and other salts are 

 decomposed by water. 



Ammoniacal silver nitrate precipitates it from solution as C 5 H 4 N 4 O 2 . 

 Ag2O ; the precipitate is soluble in nitric acid and from this solution 

 xanthine silver nitrate, C 5 H 4 N 4 O2 . AgNO 3 , slowly separates in aggregates of 

 tiny needles. This compound is soluble with difficulty in nitric acid. 



Reactions. 



(1) On evaporation with nitric acid, xanthine leaves a yellow residue, 

 which is coloured red by caustic soda and becomes purple on heating. 



(2) On boiling a small quantity of xanthine with chlorine water, or with 

 dilute hydrochloric acid and a small crystal of potassium chlorate, and evapor- 

 ating the solution to dryness, a white or pale yellow residue is left. On bring- 

 ing this into contact with ammonia vapour under a glass cover, it changes to 

 a rose red colour (murexide). 



(3) On adding a specimen of xanthine to a little bleaching powder in caustic 

 soda in a watch glass, the surface of the specimen becomes dark green, then 

 brown ; the colour ultimately disappears. 



(4) A solution of xanthine in dilute sodium hydroxide gives a red colour 

 .on the addition of diazobenzene sulphonic acid (see p. 285). 



(5) Xanthine is precipitated, like uric acid, by silver nitrate and ammonia 

 .and by copper sulphate and sodium bisulphite. 



