Montmorillonite-Catalyzed Hydrolysis of Phosmet

We studied the factors that influence the catalytic hydrolysis of phosmet (0,0, dimethyl, S-(N-phthalimidomethyl) dithiophosphate) by montmorillonite and also the hydrolysis mechanism. The catalytic activity of montmorillonite depends on the nature of the interlayer exchange cation and on the total or tetrahedral layer charge of the silicate; both factors condition the accessibility of the organic molecule to the exchangeable cation for the formation of interlayer complexes. The half-life of phosmet in an aqueous solution at pH 6 is 500-fold greater in the system free of clay than in the presence of Ca-montmorillonite. We compare the catalytic activity of the cations as montmorillonite exchangeable cations and as salts. The process of hydrolysis in the presence of montmorillonite takes place in two stages, both of first-order kinetics, of different hydrolysis rates, the first stage having a high hydrolysis rate of short duration, and the second having a slow and continuous hydrolysis rate. The bidentate complex formed by phosmet and the interlayer exchangeable cations, discussed in an earlier publication by the authors, must be the cause of hydrolysis. This interaction must enhance the electrophilic nature of the phosphorus atom, thus facilitating its nucleophilic attack by the OH- ion, producing P-S bond fission.