Deuterium isotope effects in the ninhydrin reaction of primary amines

The rate of development of Ruhemann's purple in the ninhydrin reaction of two deuterated primary amines, aa-d2-p-tyramine and aa-d2-beta-phenylethylamine, is significantly reduced. It appears to be a primary isotope effect and indicates that the cleavage of the carbon-hydrogen bond at the a-position is involved in the rate-determining step of the color reaction.