Solvent effect on the α-effect for the reactions of aryl acetates with butane-2,3-dione monoximate and p-chlorophenoxide in MeCN-H2O mixtures

Second-order rate constants have been measured spectrophotometrically for the nucleophilic reactions of three substituted phenyl acetates with butane-2,3-dione monoximate (Ox(-)) as an a-nucleophile and p-chlorophenoxide (ClPhO-) as corresponding normal nucleophile, in MeCN-H2O mixtures of varying compositions at 25.0 +/- 0.1 degreesC. The reactivity of Ox(-) toward the aryl acetates decreases upon addition of MeCN to the reaction medium up to ca. 30 mol lo MeCN, followed by a gradual increase in rate upon further addition of MeCN. A similar result has been obtained for the reaction of ClPhO- with the aryl acetates. However, the decrease in rate is more significant for the less reactive ClPhO- than for the more reactive Ox-. Thus, for all the aryl acetates studied, Ox(-) exhibits a sizable a-effect (k(Ox-)/k(ClPhO-)) whose magnitude increases as the mol % MeCN in the reaction medium increases. The relative basicities (Delta pK(a)) of Ox(-) and ClPhO- have been determined spectrophotometrically using piperazine as a reference base. The Delta pK(a) values increase on increasing the mol % MeCN in the medium for both Ox- and ClPhO-. The difference in the relative basicities of these nucleophiles (Delta Delta pK(a)) becomes larger with increasing mol % MeCN. The plots of log k(Ox-)/ k(ClPhO-) vs Delta Delta pK(a) for the three substrates are linear with near-unit slope, indicating that the difference in the relative basicity of the nucleophiles is largely responsible for the increasing a-effect with medium composition in this system.