Kinetics and mechanism of the carboxylic acid catalysed ethanolysis of p-chlorophenyl isocyanate in diethyl ether

Abstract

The kinetics of carboxylic acid catalysis of the ethanolysis of p-chlorophenyl isocyanate in diethyl ether at 25° have been investigated. The results suggest that an intermediate 1 : 1-acid–ethanol adduct is rapidly formed, and that this intermediate then attacks the isocyanate in a slow step. Equilibrium constants for intermediate formation and rate constants for its reaction with the isocyanate have been deduced. Whereas the values of the former are independent of the acid's strength, the latter are inversely dependent on it. It is argued that the intermediate has a cyclic structure, and that the probable mechanism of the slow step is a simultaneous attack by the intermediate on the carbon and nitrogen atoms of the isocyanate with the formation of a cyclic transition state. This mechanism is analogous to that for the corresponding reaction of ketens. The dimerisation of carboxylic acid catalysts and their interaction with ethanol in ether have been studied using i.r. spectroscopy.