UREIDES 275 



Properties. 



Allantoine forms shining colourless prisms which have no taste or smell 

 and are neutral in reaction to litmus. It is not easily soluble in cold water 

 (i in 1 60 parts) or cold alcohol but easily soluble in hot water and hot 

 alcohol; on heating it turns brown at 220 and melts with decomposition at 



23 i. 



Allantoine forms compounds with metals : it is precipitated by ammoniacal 

 silver solutions : the precipitate is soluble in ammonia ; it is also precipitated 

 by salts of lead, copper and mercury (see above). It reduces Fehling's 

 solution on prolonged boiling. 



Allantoine is decomposed by hydrolysis with acids or alkalies, giving urea 

 (or ammonia and carbon dioxide) and acetic acid and oxalic acid. It is also 

 decomposed by hypobromite solution with evolution of nitrogen. 



Tests and Identification. 



In order to identify allantoine it must be isolated from solution, either as 

 such or as its silver compound, and analysed ; the silver compound contains 

 4073 per cent, of silver. The presence of glyoxylic acid may be shown (i) 

 by boiling with alkali and testing the solution by adding some dilute indole 

 solution and pouring sulphuric acid under the mixture ; a red ring is formed 

 at the junction of the liquids. (2) By boiling with about 15 per cent, soda 

 for 1-2 minutes, cooling, acidifying with acetic acid and testing for oxalic 

 acid with calcium chloride. 



C. UREIDES OF DIBASIC ACIDS. 



Some of these ureides were first obtained by the oxidation of uric 

 acid and were the fundamental substances from which its constitution 

 was determined. Others were prepared synthetically in the study of 

 these ureides and in the attempts to prepare uric acid synthetically. 



Parabanic Acid and Oxaluric Acid, 



CO NH, CO NH, 



)CO CO NH 2 . 



CO NH/ C 



OOH 



Parabanic acid, or Oxalylurea, is formed on oxidising uric acid 

 with nitric acid. It is prepared synthetically by the action of phosphorus 

 oxychloride upon a mixture of urea and oxalic acid. 



Parabanic acid is a white crystalline substance, which is soluble in water 

 and alcohol. 



Oxaluric acid is obtained by the action of bromine upon parabanic 

 acid and by the action of water upon salts of parabanic acid. 



Oxaluric acid is a crystalline powder soluble in water with difficulty and 

 is present in small quantities in urine. 



Both parabanic acid and oxaluric acid are decomposed by hydrolysis by 

 boiling with water, acids, or alkalies into urea and oxalic acid. 



