Solvent effect on the alpha-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 alpha-nucleophile and p-chlorophenoxide (ClPhO(-)) as corresponding normal nucleophile, in MeCN-H2O mixtures of varying compositions at 25.0 +/- 0.1 degrees C. The reactivity of Ox(-) toward the aryl acetates decreases upon addition of MeCN to the reaction medium up to ca. 30 mol % 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 alpha-effect (k(Ox)-/k(ClPhO)-) whose magnitude increases as the mol % MeCN in the reaction medium increases. The relative basicities (DeltapK(a)) of Ox(-) and ClPhO(-) have been determined spectrophotometrically using piperazine as a reference base. The DeltapK(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 (DeltaDeltapK(a)) becomes larger with increasing mol % MeCN. The plots of log k(Ox)-/k(ClPhO)- vs DeltaDeltapK(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 alpha-effect with medium composition in this system.