Kinetics of chemical degradation of isoxaflutole: influence of the nature of aqueous buffers (alkanoic acid/sodium salt vs phosphate)

A kinetic study of the chemical degradation of isoxaflutole (5-cyclopropyl- 1,2-oxazol-4-yl alpha alpha alpha -trifluoro-2-mesyl-p-tolyl ketone) into i ts diketonitrile derivative (DKN), which is its active herbicide principle, in organic buffers at different pH values was carried out using a HPLC/UV detection method. The values of the pseudo-first-order rate constants K-obs for the reaction were calculated and compared with those previously obtain ed in inorganic buffers. In both cases, K-obs was found to be dependent on pH and temperature, but at pH 5.2 the degradation of isoxaflutole in CH3COO H/CH3COONa buffers was considerably faster than in KH2PO4/Na2HPO4 buffers, indicating that the compound was sensitive to the nature of the reagents us ed to prepare buffered solutions. The influence of phosphate and acetate co ncentrations and the influence of the R-substituent in RCOOH/RCOONa buffers were investigated. For the HA/A(-) buffers studied, the values of K-obs we re linearly dependent on HA and A(-) concentrations, which meant that the d egradation of isoxaflutole was subject to general catalysis. The values of K-obs Were also found to be dependent on the number and the position of the CH4 groups of the R-substituent. The known degradation product of DKN (a b enzoic acid derivative) was not detected throughout this study. (C) 2001 So ciety of Chemical Industry.