Activation energies of aminolysis of aliphatic esters in aprotic media

Activation enthalpies and entropies for the third-order aminolysis reaction of n-butyl formate, n-butyl-trichloroacetate and sec-butyl trifluoroacetat e were determined in solvents of various polarity and pi-basicity. The impo rtance of the steric requirements of the reagents together with high activa tion entropy values support the assumption of cyclic transition states for the reaction. Electrophilicity of the ester and the solvent properties pred etermine the reaction mechanism in the range between the pathway through a zwitterionic intermediate for the formate and the nearly concerted reaction for the trichloroacetate. The negative activation energy of an aminolysis reaction should not necessarily be attributed to a good leaving group, but can be determined by the electrophilicity of the ester and the solvent effe cts.