Donald Hupe1,2, Eric Pohl2
1Merck Sharp & Dohme Research Laboratory, P.O. Box 2000, Rahway, New Jersey, USA
2University of Michigan, Department of Chemistry, Ann Arbor, MI 48109, USA
Tóm tắt
AbstractEquilibrium constants for the reaction of aryl thiol anions with hydroxyethyl disulfide have been measured which, along with literature data, demonstrate a slope of 1.21 for a plot of log KS− (R'S‐ + RSSR ⇌ R'SSR + ‐SR) vs pKa. Rate constants were measured also for these endothermic reactions of aryl thiol anions with hydroxyethyl disulfide and also for the exothermic reactions of alkyl thiol anions with the mixed disulfide of mercaptoethanol and 4‐nitro‐2,3,5,6‐tetrafluorothiophenol. These kinetic data, obtained over a range of KS− of 1021, show the gradual curvature expected for Hammond postulate type behavior. A quantitative measure of this curvature in terms of the Marcus formalism was carried out for these two data sets along with four others having more moderate values of ΔG. The data were fit with a value for the intrinsic barrier, λ/4 = 11.6 kcal, and a value for the work term Wr = 4.0 kcal. A comparison is made of these values with the similar values found for alkyl, proton and acyl transfer reactions. The importance of using a variety of substrates with a series of bases or nucleophiles, rather than a single substrate, is discussed, as are the cause for curvature other than Hammond postulate behavior.
