Kinetic study on the acid hydrolysis of 1-aryl-2-phenyl-2-imidazolines. Basicity-rate of hydrolysis and structure relationships

Rate constants (k_obs) of hydrolysis in sulfuric acid-water mixtures at 100° for five 1-aryl-2-phenyl-2-imidazolines were determined. The influence of substituents in the phenyl group at N-1 upon the rate of hydrolysis was studied. When the imidazoline ring is considered to be a substituent of the benzene ring at N-1, a good correlation with the Hammett equation is found. In opposition to the behavior in alkaline media it was observed that reaction rates were enhanced by electron-withdrawing phenyl substituents and reduced by electron-releasing groups providing, similarly, a change in the mechanism of the reaction in the second case that was not observed in the first. Agreement with the Hammett equation allowed comparison between experimental and “calculated” rate constants which are fairly close. An equation relating the rate constants with the pK_a values of the imidazolinium ions is given.