Kinetics of ester imidazolysis in benzene

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 kψ=k2[4-MIM]2 and kψ=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.