Effects on the reactivity by changing the electrophilic center from C==O to C==S: contrasting reactivity of hydroxide, p-chlorophenoxide, and butan-2,3-dione monoximate in DMSO/H2O mixtures.

Second-order rate constants have been measured spectrophotometrically for the reactions of O-p-nitrophenyl thionobenzoate (1, PNPTB) with HO(-), butan-2,3-dione monoximate (Ox(-), alpha-nucleophile), and p-chlorophenoxide (p-ClPhO(-), normal nucleophile) in DMSO/H(2)O of varying mixtures at (25.0+/-0.1) degrees C. Reactivity of these nucleophiles significantly increases with increasing DMSO content. HO(-) is less reactive than p-ClPhO(-) toward 1 up to 70 mol % DMSO although HO(-) is over six pK(a) units more basic in these media. Ox(-) is more reactive than p-ClPhO(-) in all media studied, indicating that the alpha-effect is in effect. The magnitude of the alpha-effect (i.e., k(Ox(-) )/k(p) (-ClPhO(-) )) increases with the DMSO content up to 50 mol % DMSO and decreases beyond that point. However, the dependency of the alpha-effect profile on the solvent for reactions of 1 contrasts to that reported previously for the corresponding reactions of p-nitrophenyl benzoate (2, PNPB); reactions of 1 result in much smaller alpha-effects than those of 2. Breakdown of the alpha-effect into ground-state (GS) and transition-state (TS) effects shows that the GS effect is not responsible for the alpha-effect across the solvent mixtures. The role of the solvent has been discussed on the basis of the bell-shaped alpha-effect profiles found in the current study as well as in our previous studies, that is, a GS effect in the H(2)O-rich region through H-bonding interactions and a TS effect in the DMSO-rich media through mutual polarizability interactions.