188 CONJUGATED ACIDS. 



HIPPURIC ACID. C 1S H 8 NO 5 .HO. 



Chemical Relations. 



Properties. Hippuric acid, known also as uro-benzoic acid, 

 separates from hot solutions on cooling, in the form of minute 

 spangles, or of larger, obliquely-striated, four-sided prisms, termi- 

 nating at the ends in two flat surfaces. The elementary form of 

 the crystals is a vertical rhombic prism, which is best studied in 

 microscopical crystals obtained by the slow evaporation of a solu- 

 tion of hippuric acid, which are similar to those of phosphate of 

 ammonia and magnesia, even in their most varied combinations. 

 (C. Schmidt.*) This acid is devoid of smell, has a slightly bitter 

 but not an acid taste, dissolves in 400 parts of cold water, and 

 very freely in hot water ; it is moreover readily soluble in alcohol, 

 but difficult of solution in ether. Even the cold aqueous solution 

 reddens litmus powerfully. 



When gently heated, hippuric acid fuses, without loss of water, 

 into an oily liquid, which, on cooling, solidifies into a crystalline 

 milk-white mass ; on the application of a stronger heat, there is 

 produced a crystalline sublimate of benzoic acid and benzoate of 

 ammonia, while a few oily drops are at the same time formed, 

 which evolve an odour of cumarin (the oil of the Tonka bean,) 

 or fresh hay, solidify on cooling, and are soluble in alcohol and 

 ammonia, but not in water. On exposing the acid to a more rapid 

 and stronger heat, an intense odour of hydrocyanic acid is developed, 

 and a porous coal is left as a residue. 



Hippuric acid is unaffected by chlorine, chlorous, and dilute 

 mineral acids ; but when heated with concentrated hydrochloric or 

 nitric acid, or even with oxalic acid, it becomes decomposed (as 

 already mentioned in page 152,) into benzoic acid and glycine 

 (Dessaignef). When heated with peroxide of manganese and sul- 

 phuric acid it is decomposed into carbonic acid, ammonia, and 

 benzoic acid (Pelouze) ; boiled with freshly prepared peroxide of 

 lead it yields benzamide, carbonic acid, and water (Fehling) ; and 

 finally, if it be dissolved in nitric acid, and a stream of nitric oxide 

 gas be passed through the solution, there is a development of 

 ammonia, whilst there remains in solution a new non-nitrogenous 

 acid which = C 18 H 7 O 7 .HO. (Strecker.) 



* Entwurf u. s. w. S. 3640. 



t Compt. rend. T. 21, pp. 1224-1227. 



