Kinetics of the reactions between phenylureas and nitrous acid. Part 2. Nitrosation of 2,4,6-trimethyl- and 4-bromo-phenylurea

Abstract

The kinetics of the reactions between nitrous acid and the 2,4,6-trimethyl and 4-bromo derivatives of phenylurea have been studied at 25 °C in aqueous perchloric acid solution ([H⁺] 0.5–2.0 mol dm^–3 and µ 2.0 mol dm^–3). The mechanism suggested for the trimethyl derivative is completely analogous to that proposed for phenylurea itself: an initial fast equilibrium step in which the electrophile NO⁺ attacks the urea oxygen atom is followed by two parallel reaction paths, both of which begin with the rate-controlling loss of a proton by the protonated O-nitroso compound to produce both of its two possible conjugate bases. One of these bases undergoes internal rearrangement to the N-nitroso compound, and the other, after a series of fast steps, yields the corresponding benzenediazonium ion. This is the only final product, since the formation of the N-nitroso compound is reversible. In the case of 4-bromophenylurea the loss of proton by the protonated O-nitroso compound no longer determines the reaction rate of the second of the above reaction paths. The observed kinetics and the suggested reaction mechanism for various ureas allow a value of the order of 0.15 mol^–1 dm³ to be calculated for the equilibrium constant between NO⁺ and substrate on the one hand and the corresponding protonated O-nitroso compound on the other. This value implies that for 2,4,6-trimethylphenylurea and phenylurea proton loss is not diffusion-controlled, which is in keeping with the general base catalysis found previously for phenylurea.