Kinetics and mechanism of the alkaline hydrolysis of guanidine, hexamethylguanidinium perchlorate, and tetramethylurea

Abstract

The kinetics of the alkaline hydrolysis of tetramethylurea and hexamethylguanidinium perchlorate show terms second order in hydroxide ion. The mechanism probably involves the hydroxide ion catalysed decomposition of a tetrahedral intermediate. The rate of the alkaline hydrolysis of guanidine reaches a plateau at alkali concentrations >1M. Mechanisms involving either attack of hydroxide ion on the guanidinium cation or unimolecular decomposition of the free base are consistent with the kinetic data but unimolecular decomposition of the guanidinium anion, as observed for some carbamic acid esters and a guanidine derivative, is not. The attack of hydroxide ion on the guanidinium cation is thought unlikely as it cannot explain the 750-fold increase in rate over the hexamethyl derivative. This rate difference can be accounted for if hydrolysis proceeds through the free base as this route is inaccessible to the hexamethyl derivative. A unimolecular decomposition of guanidine free base would result in the production of cyanamide but this could not be detected, consequently a mechanism involving the attack of water on the free base is preferred.