Acid and base catalysis of urea synthesis: nonlinear Brønsted plots consistent with a diffusion-controlled proton-transfer mechanism and the reactions of imidazole and N-methylimidazole with cyanic acid

Abstract

Brønsted plots for general acid and general base catalysis of the synthesis of phenylurea from aniline and cyanic acid are nonlinear. The maximum values of the rate constants for general acid and for general base catalysis are similar and the value of β_nuc for the buffer catalysed reaction is ca. 1·0. The data are consistent with a mechanism involving a zwitterionic intermediate and a rate-limiting proton-transfer step that is close to diffusion controlled in the thermodynamically favourable direction. Proof that proton transfer to the leaving nitrogen is complete before C–N bond breaking takes place in the cleavage of a substituted urea is provided by the similar rate and equilibrium constants for the reversible reaction of imidazole and N-methylimidazole with cyanic acid.