Kinetics and mechanism of the ninhydrin reaction with DL-methionine in the absence and the presence of organic solvents

Kinetic studies of the title reaction have been carried out in the absence and the presence of organic solvents ( 1propanol, dimethylsulFoxide, acetonitrile and methy l cellosolve) as a function of pH, te mperature and [reactant] in sodium ace tate-aceti c acid buffer. Addition of each organic solvent increases the absorbance as well as the rate constant of the RlIhell/allil 's purple formation . Firstl y, as the conccntration(v/v) of solvent increases, the water conter.! decreases in the reaction mixture which, in turn , blocks the ox idati ve side reaction . Secondly, the solubility of Ruhemanll's purple is higher in these organi c solvents as compared to wate r. The react ion follows a first o rder and frac ti onal order kinetics with respect to r ninhydrin] and [H +], respectively , in the excess of ninhydrin over methionine. On the basis of observed kinetic data a probable mechan ism is proposed.