Indirect measurement of the rate constants for the diazotisation of substituted anilines by nitrosyl chloride and nitrosyl thiocyanate

Abstract

By uting, an indirect method described earlier, the relative rate constants for the diazotisation of a series of para-substituted anilines in 4.75M hydrochloric acid and also in 3.45M-sulphuric acid containing potassium thiocyanate (4 × 10^–3M) have been determined. The results are consistent with a mechanism involving rate-determining electrophilic nitrosation at the amino-nitrogen atom of the free base. para-Methoxy-substitution increases the reactivity by a factor of ca. 9, whereas a para-nitro-group deactivates by ca. 180 in the chloride-catalysed reaction; similar results were obtained for the thiocyanate-catalysed reaction. A good correlation was found between log (relative rate constant) and σ_p in both cases with a slope of ca.–3. These results argue against the suggestion that these reaction rates are diffusion-controlled, as is predicted by the early work of Schmid and his co-workers for diazotisation in 0.4M-hydrochloric acid. The kinetic substituent effects are similar to those obtained by Ridd and his co-workers for the diazotisation of para-substituted anilines under conditions where reaction occurs via the protonated form of the base. This suggests that the same factor (π-electron donation) operates in both systems.