Kinetics and mechanism of the diazotisation of substituted anilines by nitrosyl halides. The question of diffusion-controlled reactions

Abstract

Rate measurements have been carried out in water at 25 °C, using a stopped-flow spectrophotometer, on the diazotisation of a number of substituted anilines, under conditions where the effective nitrosating agents are thought to be nitrosyl chloride and nitrosyl bromide. Analysis of the rate data yielded values of k₂, the true rate coefficient for the N-nitrosation step. For both nitrosyl chloride and nitrosyl bromide reactions k₂ increased rapidly with increasing pK_a of the amines, but tended to level off at a value of 4–5 × 10⁹ l mol^–1 s^–1, i.e. close to the diffusion-controlled limit of 7.4 × 10⁹ l mol^–1 s^–1. For the amines of low pK_a, nitrosyl chloride was, as expected, the more effective reagent. The reversibility of the N-nitrosation step (previously noted for reactions in methanol solvent) explained the apparently large substituent selectivity observed earlier for reactions at high [HCl], measured by an indirect method. For anilines containing electron-withdrawing groups, reaction occurred in two parts and the rate constant for each determined: it is suggested that the first reaction is the initial N-nitrosation of the aniline, and the second a tautomerisation of the reactive N-nitroso intermediate.