Kinetics of abiotic hydrolysis of isoxaflutole: Influence of pH and temperature in aqueous mineral buffered solutions

A kinetic study of the chemical hydrolysis of isoxaflutole [5-cyclopropyl-4-(2-methanesulfonyl-4-trifluoromethylbenzoyl)isoxazole (IFT)],a new herbicide recently developed by Rhone-Poulenc Agro, in buffered, sterile aqueous solutions was carried out in the dark at 295, 308, and 323 K and at line pH values between 1.8 and 10.1. Samples were analyzed by HPLC-UV. The decrease In IFT concentration was accompanied by an increase in the concentration of its diketonitrile derivative (DKN). Obeying pseudo-first-order kinetics, isoxaflutole hydrolysis increased with increasing pH and temperature: for 295 K and pH 9.3 the rate of degradation was 100-fold faster than at pH 3.8. Using the Arrhenius equation, the rate constants K-obsd, activation energies E-a, and entropies Delta S double dagger were calculated, and plotting log(K-obsd) against pH showed that the effect of pH varied with temperature. According to Delta S double dagger values the mechanism of the reaction was found to be different with respect to pH range. The benzoic acid derivative, known as a degradation product of DKN in plants, was not detected in the present study.