Effect of cyclodextrin on the hydrolysis of the pesticide fenitrothion [O,O-dimethyl O-(3-methyl-4-nitrophenyl)-phosphorothioate]

The hydrolysis of fenitrothion was studied at HO- concentrations between 0.099 and 0.999 M. The second-order rate constant is 2.0 x 10(-3) M-1 S-1. The reaction is inhibited by beta-cyclodextrin at HO- concentrations of 0.5, 0.1 and 0.05 m. Saturation kinetics are observed in all cases. From the kinetic data, it is concluded that the main reaction pathway for the cyclodextrin-mediated reaction is the reaction of HO- with the substrate complexed. with the anion of beta-cyclodextrin. The rate constant for this reaction, kw(3), is about four times smaller than that for the reaction of the substrate with HO- in the solvent. The inhibition is attributed to the inclusion of the substrate with the phosphate group in an orientation that is difficult to reach by the ionized secondary OH groups of the cyclodextrin and protected for the attack of an external OH. Copyright (C) 2002 John Wiley Sons, Ltd.