Effect of Solvent on the α-Effect: Nucleophilic Substitution Reactions of p-Nitrophenyl Acetate with m-Chlorophenoxide and Benzohydroxamates in MeCN−H2O Mixtures of Varying Compositions

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Second-order rate constants have been measured spectrophotometrically for the reactions of p-nitrophenyl acetate (PNPA) with three α-effect nucleophiles, benzohydroxamate (BHA-), p-methylbenzohydroxamate (MBHA-), and p-methyl-N-methylbenzohydroxamate (M2BHA-), and a corresponding normal nucleophile, m-chlorophenoxide (ClPhO-), in MeCN−H2O mixtures of varying compositions at 25.0 °C. The reactivity of ClPhO- and M2BHA- toward PNPA decreases upon additions of MeCN into the reaction medium up to near 30−40 mol % MeCN and is followed by a gradual increase upon further additions of MeCN. BHA- and MBHA- also exhibit initial rate decreases upon the addition of MeCN up to near 40 mol % MeCN. However, unlike the ClPhO- and M2BHA- systems, the rate enhancement beyond 40 mol % MeCN is negligible for the BHA- and MBHA- systems. The present benzohydroxamates exert a large α-effect in H2O. Interestingly, BHA- and MBHA- show a decreasing α-effect trend with increasing mol % MeCN, while M2BHA- exhibits an increasing α-effect trend, indicating that the magnitude of the α-effect is significantly solvent dependent. Based on the results of the kinetic study and relative basicity measurements, the decreasing α-effect trend shown by BHA- and MBHA- has been attributed to an equilibrium shift of these hydroxamates (I) toward their isomeric structures (II or III) upon the addition of MeCN. The solvent dependent α-effect has led a conclusion that the solvent effect on the α-effect is significant; however, the ground state contribution is not solely responsible for the α-effect in the present system.