Kinetic study of the phenylurea–nitrous acid reaction: evidence for an O-nitrosation initial step

The kinetics of the reaction between phenylurea (PhU) and nitrous acid in aqueous perchloric acid solution at 25 °C have been studied spectrophotometrically over the acidity range pH 2.2–4.0 (µ 0.20 mol dm–3) by the initial-rate method and over the range [H+] 0.50–3.00 mol dm–3(µ 2.0 mol dm–3, except for [H+] 3.0 mol dm–3) by the integration method. The proposed reaction mechanism for both weakly and strongly acidic media involves the rapid formation of an O-nitroso compound (I) followed by two separate reaction paths, the rate-controlling step in both cases being the loss of a proton by (I). In this step (I) is transformed to either of two conjugate bases, one of which subsequently undergoes rearrangement to secondary N-nitrosophenylurea (PhUNO) and the other to the unstable primary N-nitrosophenylurea, which a series of fast steps converts into benzenediazonium ion (PhN2+). These fast steps involve the nitrosating agent, which explains the stoicheiometry of the reaction ([PhU]:[HNO2] 1 : 2). The ratio between the formation constants of benzenediazonium ion and N-nitrosophenylurea is 0.3 in the absence of base catalysis. In keeping with the nature of the rate-controlling step, general base catalysis was found to be exerted in weakly acid media by both nitrite ion and the carboxylate anions of the buffers used, but not by halide ions.