Kinetics of ester imidazolysis in benzene

Abstract

The reaction of imidazole (IM) and its 1-methyl (1-MIM) and 4-methyl (4-MIM) derivatives with p-nitrophenyl propionate has been investigated in benzene at 25°. Under pseudo-first-order conditions, the observed rate law is k_ψ=k₂[4-MIM]² and k_ψ=k₂[IM]²+k₃[IM]³; 1-MIM is a catalyst of the reaction between 4-MIM or IM and the ester. The state of aggregation of IM and 4-MIM and the high kinetic order are taken as evidence of a main mechanism involving associated imidazole and ester to give a tetrahedral anion (paired to an imidazolium cation) as the intermediate whose breakdown is rate-limiting.