Nucleophilic Substitution of Phosphorothionate Ester Pesticides with Bisulfide (HS^-) and Polysulfides (S_n^2-)

The reactions of five organophosphorus insecticides (OPs) (chlorpyrifos-methyl, parathion-methyl, fenchlorphos, chlorpyrifos, and parathion) with hydrogensulfide/bisulfide (H₂S/HS^-) and polysulfides (S_n^2-) were examined in well-defined aqueous solutions over a pH range from 5 to 9. The rates are first-order in the concentration of the different reduced sulfur species. Experiments at 25 °C demonstrated that the reaction of the five OPs with the reduced sulfur species follows a S_N2 mechanism. The activation parameters of the reaction of OPs with bisulfide were determined from the measured second-order rate constants over a temperature range of 5−60 °C. The determined second-order rate constants show that the reaction of an OP with polysulfides is from 15 to 50 times faster than the reaction of the same OP with bisulfide. The dominant transformation products are desalkyl OPs, which indicate that the nucleophilic substitution of reduced sulfur species occurs at the carbon atom of the alkoxy groups. And also the results show that these reduced sulfur species are much better nucleophiles, and thus degrade these pesticides faster than the well-studied base hydrolysis by OH^-. When the determined second-order rate constants are multiplied with the concentration of HS^- and S_n^2- reported in salt marshes and porewater of sediments, predicted half-lives show that abiotic degradation by sulfide species may be of comparable importance to microbially mediated degradation in anoxic environments.