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

Abstract

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-, α-nucleophile), and p-chlorophenoxide (p-ClPhO-, normal nucleophile) in DMSO/H 2O of varying mixtures at (25.0±0.1)°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 pKa units more basic in these media. Ox- is more reactive than p-ClPhO- in all media studied, indicating that the α-effect is in effect. The magnitude of the α-effect (i.e., kox /kp-ClPhO) increases with the DMSO content up to 50 mol% DMSO and decreases beyond that point. However, the dependency of the α-effect profile on the solvent for reactions of 1 contrasts to that reported previously for the corresponding reactions of p-ni-trophenyl benzoate (2, PNPB); reactions of 1 result in much smaller α-effects than those of 2. Breakdown of the α-effect into ground-state (GS) and transition-state (TS) effects shows that the GS effect is not responsible for the α-effect across the solvent mixtures. The role of the solvent has been discussed on the basis of the bell-shaped α-effect profiles found in the current study as well as in our previous studies, that is, a GS effect in the H2O-rich region through H-bonding interactions and a TS effect in the DMSO-rich media through mutual polarizability interactions. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.