The mechanism of the color reaction of aminopropylon with ninhydrin

The colorimetric determination of aminopropylonwith ninhydrin in mixed pharmaceutical preparations has been reported previously (Analysis of aminopropylon. I and II). This paper deals with the determination of chemical structure of the pigment obtained in the above procedure. The hydrolyzate of aminopropylon forms with ninhydrin a water soluble reddish violet pigment in acidic solution even at room temperature. It is different from Ruhemann Purple, because it has two absorption peaks at 415 mμ and 536 mμ in aqueous solution. The sulfuric acid hydrolyzate of aminopropylon is 4-aminoantipyrine. The pigment obtained by the reaction of 4-aminoantipyrine and ninhydrin is decomposed by heating with 20% HCl again into 4-aminoantipyrine and ninhydrin. An intermediate compound is obtained as white crystals by the reaction of 4-aminoantipyrine and ninhydrin at -15°C in a buffer solution of pH 6.6. Analytical value together with IR and NMR spectra indicate that it is 1-pheny1-2, 3-dimethy1-4-(1', 3'-dioxo-2'-hydroxy-2'-indanylamino)-pyrazoline-5-one. The compound obtained by reducing the pigment with NaBH4 agrees with 1-pheny1-2, 3-dimethy1-4-(1' 3'-dihydroxy-2'-indanylamino)-pyrazoline-5-one, from the data of elemental analysis and IR-and NMR spectra. Those facts show that the pigment is 1-pheny1-2, 3-dimethy1-4-( 3'-dioxo-2'-indanylidenimino)-pyrazoline-5-one.