Thermal decomposition of 2-methoxy-4H-1,3,2-benzodioxaphosphorin-2-sulfide (salithion) [insecticide]

Salithion rearranges thermally into two kinds of isomers, S-methyl-salithion and S-benzyl-salithion, which generate, by further heating, corresponding sulfides such as gaseous dimethyl sulfide and benzyl sulfide derivatives. Thermal degradation of salithion in a closed system proceeds more easily than that in an open system. It was found that these sulfides catalyzed isomerization reaction, and dimethyl sulfide was more effective. Autocatalytic degradation mechanism in which dimethyl sulfide gas is produced from S-methyl-salithion, plays the most important role in a closed system. Successful methods to stabilize salithion against thermal degradation are shown. Their properties are discussed on the basis of their assumed mechanism of stabilization. (i) Storage in a paper bag rather than in a gas-barrier film is suitable, possibly as a result of reduction of dimethyl sulfide accumulation. (ii) Inclusion with β-cyclodextrin is also effective to hinder bimolecular reaction of salithion to produce isomers. (iii) β-Naphtylphenylamine, a stabilizer of salithion, is assumed to be a scavenger of the isomers, which are sources of the active sulfides.