Free Radicals Formed by the Reaction of Ninhydrin with Alkyl Amines

Abstract

Esr spectral studies on the reaction mixtures of ninhydrin with primary alkyl amines indicated two kinds of free radical products (radical-I and -II) were formed in alkaline side.
Esr signal of the radical-I showed a maximum intensity at pH around 10 and the characteristic hyperfine spectra were analyzed to be attributed to one nitrogen and α-proton(s) in the amine residue and moreover to two ortho and two meta-protons in the ninhydrin residue.
The radical-II formed only in the alkaline solutions above pH 10 and showed far intense esr signal than that of the former. The spectral analyses indicate the radical product might be composed of two units of indandione and one unit of alkyl amine. Effects of reaction temperature, molar ratio and oxygen on the radical formation were also examined.