EFFECTIVE CHARGE DEVELOPMENT IN THE TRANSFER OF THE ACETYL GROUP BETWEEN NUCLEOPHILES IN ACETONITRILE SOLUTION - ACETOLYSIS AND BUTYLAMINOLYSIS OF SUBSTITUTED PHENYL-ESTERS

Equilibrium and rate constants have been measured for the phenolyses o f acetic anhydride in acetonitrile solution, Acetolysis of substituted phenyl acetates by acetate ion possesses a Bronsted beta(lg) value of -1.50 which, together with a beta(eq) value of 2.86, indicates substa ntial fission of the C-OAr bond in the transition structure, The value of beta(eq) is employed to identify the rate-limiting steps in aminol yses in acetonitrile. Butylaminolysis of substituted phenyl acetates i n acetonitrile solution yields amide and substituted phenolate anion a nd the kinetics obey the general rate law: Rate = k(1)[ester][amine] k(2)[ester][amine](2) + k(3)[ester][amine][18-crown-6] Free energy pl ots of log k(1) and log k(2) exhibit breaks near pK(a)(ArOH) values of 9 and 8, respectively, and these can be interpreted by a mechanism wh ich involves a common zwitterionic adduct T-+/-, which partitions to g ive the product by two routes: A involving direct expulsion of the phe nolate ion leaving group (k(1) parameter) and B involving proton trans fer prior to phenolate ion expulsion (k(2) parameter). The formation o f T-+/- is rate-limiting for the A path and C-OAr bond fission is rate -limiting for the B mechanism.