Solvent effect on the kinetics of ring opening of isatin in water-ethanol mixtures

Abstract

The kinetics of amide bond cleavage of isatin and N-methylisatin in the presence of ethanol was followed spectrophotometrically in the ranges of solvent composition (0-70%v/v) and temperature (40-60°C) using piperidine as a nucleophile. The reaction proceeds according to pseudo first-order kinetics. The rate of reaction strongly decreased with increasing organic solvent content. The thermodynamic activation parameters were calculated and discussed in terms of solvation. No linearity was observed between log (rate constant) and reciprocal dielectric constant for the solvent used, suggesting that there is selective solvation by the more polar water. The reactivity was analysed in the light of various simple and multiple regression equations using Kamlet - Taft solvatochromic parameters which were applied successfully to the mixed aqueous - ethanol mixtures. Finally, a mechanism for the ring opening is proposed. © Science Reviews 2000 Ltd.