Kinetic Study and Mechanism Hydrolysis of 4‐Bromo‐3,5 dimethylphenyl N‐methylcarbamate in Aqueous Media

Abstract

ABSTRACTDegradation via hydrolysis is among the main transformation pathways and particularly for N‐methylcarbamates. Carbamate pesticide hydrolysis is known to proceed through alkaline catalysis, with reaction of the hydroxide ion with the carbonyl function or with abstraction of hydrogen in the α position with respect to the carbonyl. This reaction leads to the formation of methylamine and corresponding phenol. In this respect, the reaction kinetics of 4‐bromo‐3,5‐dimethylphenyl N‐methylcarbamate (BDMC) hydrolysis have been investigated in alkaline solution using a spectrophotometric technique and reversed phase liquid chromatography. The kinetic constants were determined following a proposed pseudo–first‐order kinetic model. The positive activation entropy ΔS≠ = +35.73 J mol−1 K−1 and the absence of general base catalysis indicated an unimolecular elimination conjugate base (E1cB) hydrolytic mechanism involving the formation of methyl isocyanate. This result was confirmed by the fact that BDMC fits well into brönsted and Hammett lines, obtained for a series of substituted N‐methylcarbamate whose decomposition in aqueous media was established to follow an E1cB mechanism.