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The effect of medium on reactivity has been dissected into the ground state and transition state contributions through combination of the kinetic data with the transfer enthalpies (ΔΔ H tr) from H 2O to DMSO–H 2O mixtures for OH – ion. However, destabilization of OH – in the ground state is not solely responsible for the remarkably enhanced reactivity upon addition of DMSO to the medium. Addition of DMSO to H 2O would destabilize OH – through electronic repulsion between the anion and the negative-dipole end in DMSO. This implies that expulsion of the leaving group is not advanced in the rate-determining transition state, i.e., the reactions of 1 and 2 with OH – proceed through a stepwise mechanism, in which expulsion of the leaving group from the addition intermediate occurs after the rate-determining step.
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Although 2 possesses a better leaving group than 1, the former is less reactive than the latter by a factor of 2 in H 2O. The k N value increases from 11.6 to 32 800 M –1 s –1 for the reactions of 1 and from 5.90 to 190 000 M –1 s –1 for those of 2 as the reaction medium changes from H 2O to 80 mol % DMSO, indicating that the effect of medium on reactivity is more remarkable for the reactions of 2 than for those of 1. Second-order rate constants ( k N) for reactions of p-nitrophenyl acetate ( 1) and S- p-nitrophenyl thioacetate ( 2) with OH – have been measured spectrophotometrically in DMSO–H 2O mixtures of varying compositions at 25.0 ± 0.1 ☌.