Reactivity of the pesticide chlorpyrifos-methyl in aqueous media. Effect of cyclodextrin.

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

Florianopolis

Conferencia; 8th Latin American Conference on Physical Organic Chemistry; 2005

Universidade Federal de Santa Catarina

Chlorpyrifos-Methyl (1) is a widely used organophosphorous pesticide but little is known about its chemistry. Cyclodextrins (CD) are cyclic oligomers of α-D-glucopyranose that have the ability to form complexes with organic and inorganic compounds and to change the physicochemical properties of the included guest. The CD themselves- being non-toxic, biodegradable enzyme modified starch derivatives- are not noxious for the environment. Chlorpyrifos has a very low solubility in water. Its hydrolysis reaction was studied in water solutions containing 10.2 % ACN at 25 ºC. The reaction is strongly affected by the ionic strength (I) of the media. The observed rate constant, kobs at I = 0.6 M, is linearly dependent on [HO-] and from the slope kHO = 0.02 M-1s-1 is calculated. This value is about 10 times higher than the corresponding kHO for Fenitrothion, another phosphorothioate pesticide. The difference in reactivity is attributed, in part, to the lower pKa of the leaving group of 1. CD increases the solubility of 1 about 3 times. When β-CD is added to an alkaline solution of 1 a strong inhibition in the hydrolysis reaction is observed. This can be a consecuence of the inclusion of the phosphorous inside the host cavity making more difficult for the HO- to reach the reaction center. The formation of the complex was confirmed through an induced circular dichroism spectrum.