Alkaline and Neutral Hydrolysis of Four Phenylurea Herbicides

The kinetics of hydrolysis of four phenylurea herbicides - fenuron. monuron, diuron and chloroxuron - were measured in aqueous alkaline solutions at temperatures of 64°C and 84°C and extrapolated to 25°C. At hydroxide concentrations ≥ 8.6 × 10−4 mol 1−1, alkaline hydrolysis dominates but the reaction does not obey second-order kinetics and approaches a maximum value at concentrations > 0.1 mol 1−1. The kinetic data support a mechanism of reaction proposed previously for the alkaline hydrolysis of trichloro- and trifluoroacetanilides. The mechanism is a hydroxide-ion-promoted equilibrium formation of a reactive tetrahedral intermediate anion that can either revert to the starting compound, decompose to products, or react with a second hydroxide to form a dianion prior to decomposition to products. Statistical analysis of the data suggests that the break-down of the reactive intermediate proceeds entirely via the path in which a second hydroxide removes a proton from the intermediate to yield a dianion that decomposes, whereas the path in which the intermediate breaks down without the assistance of OH− is of no kinetic importance, even at hydroxide concentrations as low as 0.00086 mol 1−1. Our hydrolysis rate constant measurements suggest that neutral hydrolysis is a major route of environmental degradation of the herbicides. The half-lives in years of the four ureas at 25°C and pH 7 are estimated to be 89 for fenuron. 66 for monuron, 41 for diuron, and 41 for chloroxuron.