Thermal Decomposition of O, O-Dimethyl O-(3-Methyl-4-Nitrophenyl) Phosphorothioate-Autocatalytic Decomposition Mechanism

It was observed that the apparent degradation rate of fenitrothion (Sumithion®) in a closed system is significantly greater than that in an open system. For this observation, there is proposed the autocatalytic degradation mechanism by dimethylsulfide, which is evolved through the decomposition of S-methyl-Sumithion and catalyzes the rearrangement of Sumithion into S-methyl-Sumithion. It is supposed that decomposition not only at high temperature but also at lower temperature such as room temperature, would be subject to this mechanism. As far as the catalytic potential of dimethylsulfide is concerned, it was found that additions of 0.4 to 1.6w/w% of the thioether to Sumithion accelerated decomposition by more than 20 to 50 times. Some kinds of metal salt also facilitated the isomerization of Sumithion, but calcium dodecylbenzene sulfonate (surfactant) showed almost no effect. Kinetic studies were also carried out. Decomposition rate of Sumithion, S-methyl-Sumithion and Sumioxon were measured.