Mechanism of the Hydrolysis of Endosulfan Isomers

The objective of this study was to elucidate the mechanism of abiotic hydrolysis of ES isomers, i.e., Endosulfan-1 (ES-1) and Endosulfan-2 (ES-2), using a combination of experiments and density functional theory (DFT) calculations. Hydrolysis of both ES-1 and ES-2 resulted in the formation of Endosulfan Alcohol (ES-A). The rate of hydrolysis was first order in all cases and increased with both pH and temperature. Rate expressions describing the hydrolysis rates of ES-1 and ES-2 as a function of pH and temperature were obtained and validated with independent data sets. DFT calculations were performed using three functionals (M06-2X, B3LYP, and MPW1K) and both IEFPCM-UFF and SMD to introduce solvent effects. The geometry optimization of molecules ES-1 and ES-2 showed that the free energy of ES-1 was larger, and therefore, ES-2 was the more thermodynamically stable isomer. DFT calculations also supported a hydrolysis mechanism involving two successive attacks by OH^- ions on C-O bonds resulting in the attachment of OH^- and the elimination of SO₃^- from the ES molecule, but only the first attack was rate limiting. Calculations with all functionals and solvent effect combinations supported the experimentally observed result of faster hydrolysis of ES-2 than of ES-1. The MPW1K functional along with IEFPCM-UFF for solvent effect simulated the free energy of activation to be the closest for both ES-1 and ES-2 with less than 3% error with respect to the values computed from the experimental observations. The kinetic rate expression for ES hydrolysis derived on the basis of the proposed mechanism was identical to the rate expression derived from experiments. It was deduced that the hydrolysis rates of both ES isomers may vary over 3 orders of magnitude depending on the prevalent pH and temperature.