Kinetics and mechanism of degradation of aqueous promecarb insecticide studied by UV spectrophotometry and HPLC

The present paper deals with the degradation mechanism of the insecticide promecarb, or 3-isopropyl-5-methylphenyl-N-methylcarbamate, in aqueous media. The reaction kinetics have been investigated using UV spectrophotometry and reversed phase HPLC. The determination of 3-isopropyl-5-methylphenol as the main product of promecarb hydrolysis gives evidence for the significant reactivity of this insecticide in alkaline solution. The rate constants were determined following a proposed first-order kinetic model. The positive activation entropy obtained Delta s(not equal) = +66.82 J mol(-1) K-1 and the absence of general basic catalysis indicates an E1cB mechanism involving unimolecular collapse of the promecarb via a methylisocyanate intermediate. These results were confirmed by the fact that promecarb fits well into Bronsted and Hammett plots obtained for a series of substituted N-methylcarbamates for which the decomposition in aqueous media was established to follow an E1cB mechanism.