Reactivity of the insecticide Chlorpyrifos-Methyl towards hydroxyl and perhydroxyl ion. Effect of cyclodextrins.

RAQUEL V. VICO; RITA H. DE ROSSI; ELBA I. BUJÁN

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY

John Wiley & Sons, Ltd.

Lugar: New York; Año: 2009 vol. 22 p. 691 - 702

0894-3230

The reactivity of Chlorpyrifos-Methyl (1) toward hydroxyl ion and the a-nucleophile, perhydroxyl ion was investigated in aqueous basic media. The hydrolysis of 1 was studied at 25 8C 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_10_2 M_1 s_1 in 10% ACN and 1.70_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 SN2(P) pathway. The hydrolysis reaction is inhibited by a-, b-, and g-cyclodextrin showing saturation kinetics; the greater inhibition is produced by g-cyclodextrin. The reaction with hydrogen peroxide is weakly inhibited by a- and b-cyclodextrin (b-CD), whereas g-cyclodextrin produces a greater inhibition and saturation kinetics. The kinetic data obtained in the presence of b- or g-cyclodextrin for the reaction with hydroxyl or perhydroxyl ion indicate that the main reaction pathway for the cyclodextrin-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.