A kinetic study of the methanolysis of the sulfonylureas bensulfuron methyl and sulfometuron methyl using capillary electrophoresis

The instability of sulfonylureas in solution in methanol has led us to a kinetic study of methanolysis of two sulfonylureas using capillary electrophoresis. In a preliminary experiment, solutions of the seven compounds, bensulfuron methyl, sulfometuron methyl, nicosulfuron, chlorimuron ethyl, thifensulfuron methyl, metsulfuron methyl, and chlorsulfuron were prepared in methanol and in acetonitrile. After six weeks at room temperature the compounds dissolved in acetonitrile were quite stable as shown by electropherograms obtained with free zone capillary electrophoresis (CE). All seven of the compounds dissolved in methanol had undergone extensive degradation and in some cases the sulfonylurea could not be detected. Bensulfuron methyl and sulfometuron methyl were selected for further detailed study. Solutions of these two compounds in methanol were heated at 5, 40, 50, 57, and 65 degrees C. No decomposition of either compound was observed in similar solutions of acetonitrile incubated at 57 degrees C. The rates of decomposition in methanolic solutions were estimated from the decrease in instrumental response representing the peaks of the two sulfonylureas. The methanolysis of both compounds proceeded with pseudo first-order kinetics as evidenced by the fact that semilogarithmic plots of the concentration of the substrate as a function of time were linear (correlation coefficients of 0.99). Rate constants and half-lives for the reactions at 40, 50, 57, and 65 degrees C were determined. These varied by factors of 18 to 22 depending upon the reaction temperature. The energies of activation for these reactions were estimated from the appropriate Arrhenius plots and found to be 26 kcal per mole for bensulfuron methyl and 24.5 kcal per mole for sulfometuron methyl. The kinetics of appearance of the products of methanolysis were directly related to and accurately predicted by the pseudo first-order rate constants observed for disappearance of the sulfonylureas. Kinetic study of the reaction mixture by negative and positive-ion mass spectrometry indicated that the products of methanolysis of sulfometuron methyl were 2-carboxymethyl (N-carboxymethyl)benzsulfonamide and 2-amino-4,6-dimethyl pyrimidine and suggested a general mechanism for methanolysis of each of the seven compounds studied.