Reactivity of the insecticide chlorpyrifos-methyl toward hydroxyl and perhydroxyl ion. Effect of cyclodextrins

The reactivity of Chlorpyrifos-Methyl (1) toward hydroxyl ion and the alpha-nucleophile, perhydroxyl ion was investigated in aqueous basic media. The hydrolysis of 1 was studied at 25 degrees C in water containing 10% ACN or 7% 1,4-dioxane at NaOH concentrations between 0.01 and 0.6 M; the second-order rate constant is 1.88 x 10(-2) M-1 S-1 in 10% ACN and 1.70 x 10(-2) M-1 S-1 in 7% 1,4-dioxane. The reaction with H2O2 was studied in a pH range from 9.14 to 12.40 in 7% 1,4-dioxane/H2O;the second-order rate constant for the reaction of HOO- ion is 7.9 M-1 S-1 whereas neutral H2O2 does not compete as nucleophile. In all cases quantitative formation of 3,5,6-trichloro-2-pyridinol (3) was observed indicating an S(N)2(P) pathway. The hydrolysis reaction is inhibited by alpha-, beta-, and gamma-cyclodextrin showing saturation kinetics; the greater inhibition is produced by gamma-cyclodextrin. The reaction with hydrogen peroxide is weakly inhibited by alpha- and beta-cyclodextrin (beta-CD), whereas gamma-cyclodextrin produces a greater inhibition and saturation kinetics. The kinetic data obtained in the presence of beta- or gamma-cyclodextrin for the reaction with hydroxyl or perhydroxyl ion indicate that the main reaction pathway for the cycloclextrin-mediated reaction is the reaction of HO- or HOO- ion with the substrate complexed with the anion of the cyclodextrin. The inhibition is attributed to the inclusion of the substrate with the reaction center far from the ionized secondary OH groups of the cyclodextrin and protected from external attack of the nucleophile. Sucrose also inhibits the hydrolysis reaction but the effect is independent of its concentration. Copyright (C) 2008 John Wiley & Sons, Ltd.