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 pKa 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 pKa, 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.