Isotope effects on the ionization of the thiol groups of pentafluorothiophenol, thioacetic acid, 4-nitrothiophenol, thiophenol, methyl mercaptoacetate, mercaptoethanol, and mercaptoacetate in water and deuterium oxide range from íTrsh/^rsd = 2.0-2.5 and approximately follow the equation ApK = 0.26 + 0.012pA% Bending as well as stretching frequencies contribute significantly to the isotope effects for reactions of thiols. The increase in the isotope effect with decreasing acidity of the thiol and the magnitude of the difference in the isotope effects for the ionization of RSH and ROH provide evidence that isotope effects on the interaction of RSand ROwith the solvent contribute significantly to the observed effects.Human Development of the National Institutes of Health (HD 01247).
The reactions of acetylimidazole with weakly acidic thiols undergo a change in rate-determining step with increasing imidazole buffer concentration. The imidazole-catalyzed step is assigned to the breakdown of a tetrahedral addition intermediate and the other step to attack of thiol anion on free acetylimidazole. However, the concurrent, uncatalyzed reaction of acetylimidazolium ion with thiol anion does not undergo this change in ratedetermining step. The phosphate-catalyzed reaction also does not undergo a change in rate-determining step and phosphate can bring about a rate increase under conditions in which imidazole brings about no further rate increase. It is suggested that these results may be explained in terms of bimolecular reactions in aqueous solution that proceed by independent, concurrent pathways; i.e., the intermediates or transition states need not be at equilibrium with respect to transport processes.
The aminolysis of acetylimidazole by ethylenediamine shows a rate enhancement of more than lo3 compared to glycine, which is attributed to intramolecular general base catalysis ; the reaction with acetylimidazolium ion shows no significant rate enhancement.
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