ACYL SUBSTITUENT EFFECTS IN GENERAL BASE CATALYZED AMMONOLYSIS REACTIONS OF ESTERS

The reactions of ammonia with series of p-chlorophenyl and of. p-nitrophenvl benzoates have been investigated in 33% acetonitrile-water (v/v) at 25˚ and ionic strength 0.3. The rates of reaction of the latter series of esters with ammonia are dependent only upon the first power of the ammonia concentration, k1, while those of the p-chlorophenyl benzoates have an additional term in the rate equation dependent upon [NH3]2, k2. Under the conditions of these experiments terms dependent on [OH-][NH3] or [NH4 +][NH3] did not contribute to the observable kinetics. The values of p for the k1 and k2 terms for the reactions of the p-chlorophenyl benzoates are 1.08 ± 0.12 and 1.876 ± 0.011, respectively, indicating that the general base catalyzed reactions of these compounds are more sensitive to electron withdrawal in the acyl position than are the uncatalyzed. The p value for the k1 term for the reactions of the p-nitrophenyl esters is 1.426 ± 0.015, demonstrating that the uncatalyzed ammonolysis reactions of these compounds are more sensitive to acyl electron withdrawal than are the corresponding reactions of the p-chlorophenyl esters. The integration of these observations into the currently held position that the transition state for the rate-determining step in these reactions resembles amine attack upon the ester rather than alcohol expulsion is accomplished by consideration of both the developing charge on the nucleophilic atom and the contribution to the overall free energy of the reaction made by carbon-oxygen bond loosening in the transition state. Increasing the stability of the leaving group and the negative charge on the nucleophile both lead to larger p values for acyl substituents. © 1969, American Chemical Society. All rights reserved.