Reduction of the carbamate pesticides oxamyl and methomyl by dissolved Fe^II and Cu^I

The degradation of two oxime carbamate pesticides, oxamyl and methomyl, was investigated in anoxic solutions containing various metal ions and reducing agents. In reagent-free solutions, these carbamates degrade slowly via base-catalyzed elimination. Rates of carbamate degradation are accelerated by Fe^II, Cu^I, and Cu^II, but not by several other metal ions and reducing agents. In the presence of Fe^II, carbamates undergo a net two-electron reduction that is coupled to the sequential one-electron oxidation of two Fe^II ions. The observed products are a substituted nitrile, methanethiol, and methylamine. A radical intermediate is inferred by polymerization of the radical scavenger acrylonitrile. Redox kinetics (i) vary with carbamate identity, (ii) exhibit first-order dependence on both Fe^II and carbamate concentration, (iii) are relatively independent of pH, (iv) follow Arrhenius temperature dependence, and (v) are only indirectly influenced by the presence of O₂. Coordinatively saturated Fe^II complexes (Fe^IIEDTA^2- and Fe^II(CN)₆^4-) react with oxamyl at rates equal to and greater than hexaquo Fe^II, respectively, indicating that an inner-sphere Fe^II-carbamate coordination complex is not required for electron transfer. Experimental results indicate that Cu^I reduces the carbamates by the same mechanism as Fe^II but at much higher rates. In contrast, Cu^II acts as a catalyst for both elimination and reduction reactions.