THE NINHYDRIN REACTION 



PAUL E. HOWE 



The recent use of the characteristic reaction between triketo- 

 hydrindene hydrate (ninhydrin) and amino acids, peptones, etc. 

 (blue coloration), for the detection of products of protein hydroly- 

 sis in tests for pregnancy/ and its further appHcation to other tests, 

 have elicited considerable interest in the nature of the reaction, the 

 Hmits of its appHcabihty, and the factors which may modify its 

 sensitiveness. Ruhemann^ has studied this reaction carefully and 

 was the first^ to prepare triketohydrindene hydrate in sufficient 

 quantity to analyze it, and to determine its chemical and physical 

 properties.* He recognized the characteristic reaction with amino 



1 Abderhalden : Münch. med. Woch., 1912, lix, p. 1939. 



2 Ruhemann : Jour. Chem. Soc. Trans., 1910, xcvii, pp. 1438 and 2025 ; ibid., 

 1911, xcix, p. 792; ibid., 1912, ci, p. 780. 



3 Beilstein mentions triketohydrindene in his third edition, Vol. iii, p. 314 

 (1897), and in the subsequent "Aufgangs Band," iii, p. 242, mentions the method 

 of preparation outlined by Kaufmann (B. 1897, xxx, p. 387), who could not 

 obtain sufficient material to make an analysis. 



CO 



* Triketohydrindene hydrate, QHj C(OH)o, crystallizes from water 



CO 



in colorless prisms, which on heating lose water and at 125° C. turn red, froth at 

 139° C. and melt at 239-240° C. The Compound is moderately soluble in cold 

 water, readily so in hot water, sparingly soluble in ether, and quite readily soluble 

 in ethyl acetate. It is decomposed by ammonium Compounds, yielding scarlet 

 prisms, reduces Fehling sol. and ammoniacal silver nitrate sol., and forms hydra- 

 zones and addition Compounds. (Ruhemann.) 



Triketohydrindene hydrate can be prepared from cinnamic acid through a 

 series of reactions involving its reduction, chlorination (phenyl-propionyl chlo- 



CH2 



rid) and the removal of hydrochloric acid, giving a-hydrindone C6H4 CH2. 



\ y 



CO 

 (Kipping: Jour. Chem. Soc. Trans., 1894, Ixv, p. 48.) This latter Compound, 

 when treated with /'-nitrosodimethylanilin (Ruhemann: loc. cit.), in the presence 



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