EFFECT OF SOLVENT ON THE ALPHA-EFFECT - NUCLEOPHILIC-SUBSTITUTION REACTIONS OF P-NITROPHENYL ACETATE WITH M-CHLOROPHENOXIDE AND BENZOHYDROXAMATES IN MECN-H2O MIXTURES OF VARYING COMPOSITIONS

Second-order rate constants have been measured spectrophotometrically for the reactions of p-nitrophenyl acetate (PNPA) with three alpha-eff ect nucleophiles, benzohydroxamate (BHA(-)), p-methylbenzohydroxamate (MBHA(-)), and p-methyl-N-methylbenzohydroxamate (M(2)BHA(-)), and a c orresponding normal nucleophile, m-chlorophenoxide (CIPhO-), in MeCN-H 2O mixtures of varying compositions at 25.0 degrees C. The reactivity of CIPhO- and M(2)BHA(-) toward PNPA decreases upon additions of MeCN into the reaction medium up to near 30-40 mol % MeCN and is followed b y 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 CIPhO- and M(2)BKA(-) systems, the rate enhancement beyond 40 mol % MeCN is negligible for the BHA(- ) and MBHA(-) systems. The present benzohydroxamates exert a large alp ha-effect H2O. Interestingly, BHA(-) and MBHA(-) show a decreasing alp ha-effect trend with increasing mol % MeCN, while M(2)BHA(-) exhibits an increasing alpha-effect trend, indicating that the magnitude of the alpha-effect is significantly solvent dependent. Based on the results of the kinetic study and relative basicity measurements, the decreasi ng alpha-effect trend shown by BHA(-) and MBHA(-) has been attributed to an equilibrium shift of these hydroxamates (I) toward their isomeri c structures (II or III) upon the addition of MeCN. The solvent depend ent alpha-effect has led a conclusion that the solvent effect on the a lpha-effect is significant; however, the ground state contribution is not solely responsible for the alpha-effect in the present system.