The kinetics of hydrolysis of methyl and phenyl lsocyanates

Abstract

The hydrolysis of phenyl isocyanate is subject to general base catalysis by tertiary amines and the point for water falls on the Brønsted plot, which indicates that the uncatalysed reaction involves two molecules of water, one acting as a nucleophile and the other as a general base. The rather small solvent isotope effect, k_w^H₂O/k_w^D₂O= 1.65, and the proton inventory, are discussed. The hydrolysis of methyl isocyanate (unlike phenyl isocyanate) is acid- catalysed, probably proceeding with pre-equilibrium protonation. Methyl isocyanate reacts with hydrogenphosphate dianion and with hydrogensulphate ion, forming mixed anhydride species. The formal reaction with hydrogensulphate ion may proceed by pre-equilibrium protonation followed by nucleophilic attack by sulphate ion.