Kinetics of chemical degradation of organophosphorus pesticides; hydrolysis of chlorpyrifos and chlorpyrifos‐methyl in the presence of copper(II)

The kinetics of the hydrolysis reactions of chlorpyrifos‐methyl and chlorpyrifos, in the presence of copper(II), have been measured in buffered aqueous solutions. For each reaction, a rate law of the type ‐d[pesticides]/dt= k2[pesticides][Cu2+] was observed. As the concentration of copper(II) increased, a corresponding increase in the rate of hydrolysis was observed until the concentration of copper(II) reached about 1.0 × 10−2M. At this point, the rate of hydrolysis became independent of the concentration of copper(II) in both reactions. Using the experimental data, a mechanism for each reaction is proposed, in which the copper(II) ion forms a six‐membered ring complex with the nitrogen in the ring structure and the sulphur of the phosphorothioate moiety. The rate of hydrolysis increased with increasing pH. At higher pH values, precipitation of Cu2+ was observed and general base‐hydrolysis reactions became more and more important. The differences in the rate constant observed is explained using the inductive effect theory of the alkyl group.