142 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Hippuric Acid, C 6 H 5 CO NH . CH 2 . COOH. 



Hippuric acid, or benzoyl glycine, is formed synthetically by the 

 kidney when benzoic acid is injected into the blood-stream of animals. 

 It is a compound which is normally present in the urine of animals, 

 especially herbivora. Benzoic acid is formed from the aromatic sub- 

 stances of the food and is converted into hippuric acid during its 

 excretion. 



Preparation from Urine. 



Hippuric acid is readily obtained from herbivorous urine by adding 

 I c.c. of concentrated hydrochloric acid and 12 gm. of ammonium 

 sulphate to every 25 c.c. The compound cdmmences to crystallise 

 out in about 5 minutes and the crystallisation is complete in 10- 



1 5 minutes. The crystals are 

 filtered off and dried. They are 

 generally more or less pigmented 

 and are purified by recrystallisation 

 from boiling water with the addi- 

 tion of a small quantity of animal 

 charcoal. On filtering and cooling 

 the filtrate, hippuric acid crystal- 

 lises out in the form of needles 

 (Fig. 30). 



Properties and Reactions. 

 Hippuric acid is a white crys- 

 talline substance which melts at 

 187*5. It i s soluble with difficulty 

 in cold water, but readily in hot water, in alcohol and ethyl acetate. 

 It is only slightly soluble in ether and chloroform and insoluble in 

 petroleum ether (this distinguishes it from benzoic acid). 



(1) On hating, hippuric acid melts; the mass on further heating 

 turns reddish-brown, due to the decomposition of the glycine, and a 

 smell of bitter almonds is produced. A sublimate of benzoic acid is 

 also formed. If this be dissolved in dilute sodium carbonate solution 

 and the solution be acidified with dilute hydrochloric acid, benzoic acid 

 is precipitated. The crystals have a different appearance under the 

 microscope to hippuric acid and give the reactions for benzoic acid 



(P. 256). 



(2) Ammonia is given off on heating hippuric acid with soda fame. 



(3) Hippuric acid is hydrolysed into benzoic acid and glycine by 

 boiling with concentrated hydrochloric acid : 



C fi H 5 CO NH . CH 2 . COOH + H 2 O = 0,/^COOH + H 2 N . CH 2 . COOH. 



FIG. 30. Hippuric acid. (After Funke.) 



