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 (1-propanol, dimethylsulfoxide, acetonitrile and methyl cellosolve) as a function of pH, temperature and [reactant] in sodium acetate-acetic acid buffer. Addition of each organic solvent increases the absorbance as well as the rate constant of the Ruhemann's purple formation. Firstly, as the concentration(v/v) of solvent increases, the water content decreases in the reaction mixture which, in turn, blocks the oxidative side reaction. Secondly, the solubility of Ruhemann's purple is higher in these organic solvents as compared to water. The reaction follows a first order and fractional order kinetics with respect to [ninhydrin] and [H+], respectively, in the excess of ninhydrin over methionine. On the basis of observed kinetic data a probable mechanism is proposed.