Comparisons of rate constants for thiolate-disulfide interchange in water and in polar aprotic solvents using dynamic proton NMR line shape analysis

Abstract

The rate constants for representative thiolate-disulii de rcactions are larger in DMSO and DMF than in water b1 a iactor of approximately 2300 at 24 oC. The log of the rate constant is directly proportional to the mole fraction ol DrO in mixtures of DMSO and D2O, even at small mole fractions of D2O. This linear proportionality suggests that thiolate anion is not specifically solvated by water and that hydrogen bonding is relatively unimportant in stabilizing this species. The valucs of -\Sr for thiolate-disulfide interchange are approximately -10 cal/(deg mol), presumptively because of loss in the entropy of the reactants in going from ground to transition state, partially compensated by a gain in entropy from solvent release. Introduction of a hydroxyl group p to the C-S bond slows the reaction by a factor of 2-15; the introduction of methyl groups p to the C-S bond slows the rate by factors of 3-20. A number of substances have been screened as potential catalysts ior thiolate-disulfide interchange in water: none showed useful levels of catalytic activity, although phenylselenol did accelerate the interchange significantly.