Effective charge development in the transfer of the acetyl group between nucleophiles in acetonitrile solution: acetolysis and butylaminolysis of substituted phenyl esters

Abstract

Equilibrium and rate constants have been measured for the phenolyses of acetic anhydride in acetonitrile solution. Acetolysis of substituted phenyl acetates by acetate ion possesses a Brønsted β_lg value of -1.50 which, together with a β_eq value of 2.86, indicates substantial fission of the C–OAr bond in the transition structure. The value of β_eq is employed to identify the rate-limiting steps in aminolyses in acetonitrile. Butylaminolysis of substituted phenyl acetates in acetonitrile solution yields amide and substituted phenolate anion and the kinetics obey the general rate law:Free energy plots of log k₁ and log k₂ exhibit breaks near pK_a^ArOH values of 9 and 8, respectively, and these can be interpreted by a mechanism which involves a common zwitterionic adduct T^±, which partitions to give the product by two routes: A involving direct expulsion of the phenolate ion leaving group (k₁ parameter) and B involving proton transfer prior to phenolate ion expulsion (k₂ parameter). The formation of T^± is rate-limiting for the A path and C–OAr bond fission is rate-limiting for the B mechanism