Dissection of Activation Parameters in the Bell-Shaped α-Effect Following Solvent Modulation (DMSO-H2O Media)

This paper comprises results of our investigation of the alpha-effect phenomenon for the reaction of O-p-nitrophenyl thionobenzoate (PNPTB) with butane-2,3-dione monoximate (Ox(-), alpha-nucleophile) and p-chlorophenoxide (p-ClPhO-, normal-nucleophile) in DMSO-H2O mixtures of varying compositions at 15.0 degrees C, 25.0 degrees C, and 35.0 degrees C. The reactivity of Ox(-) and p-ClPhO- increases significantly as the DMSO content in the medium increases, although the effects of medium on reactivity are not the same for the reactions with Ox(-) and p-ClPhO-, Ox(-) exhibits the alpha-effect in all solvent compositions and temperatures. The alpha-effect increases up to 50 mol % DMSO and then decreases thereafter, resulting in a bell-shaped alpha-effect profile. Dissection of the activation parameters (i.e., Delta H double dagger and T Delta S double dagger) has revealed that the bell-shaped alpha-effect behavior is due to entropy of activation differences rather than enthalpy terms, although the enthalpy term controls almost entirely the solvent dependence of the reaction rate. Differences in the transition-state (TS) structures for the reactions with Ox(-) (a six-membered cyclic TS) and p-ClPhO- (an acyclic TS) are consistent with the entropy-dependent alpha-effect behavior.