Kinetics and mechanism of hydrolysis of phenylureas

The hydrolysis of phenylureas has been found to be affected by temperature, pH and buffer concentration. Kinetic evidence suggests that the formation of phenylisocyanate, the initial product in the title reaction, occurs via an intermediate zwitterion. Depending on pH and buffer concentrations, the zwitterion can be produced through three parallel routes: at low pH, specific acid–general base catalysis, followed by slow deprotonation of a nitrogen atom by a general base; at high pH, specific basic–general acid catalysis, followed by slow protonation of a N atom by a general acid; at intermediate pH the reaction proceeds through a proton switch promoted by buffers. Bifunctional acid–base buffers such as HCO3−/CO32−, H2PO4−/HPO42− and CH3COOH/CH3COO− are very efficient catalysts. At high buffer concentration, as well as at pH < 3 or > 12, the breakdown of the zwitterion is rate-determining. The results are discussed in relation to recently published papers reporting different pathways.