Effect of Charged Substituents on Rates of the Thiol-Disulfide Interchange Reaction

Article abstract of DOI:10.1021/jo01303a033

Rate constants in aqueous solution at 25 deg C are reported for the production of p-nitrothiophenol from the mixed disulfides RSSC6H4NO2 upon reaction with the thiol anions R'S^-.The R' groups used varied in charge and include ^-O2CCH2CH2, ⁺H3NCH2CH2, and HOCH2CH2.The R groups used were these same three in addition to ^-O2CCH2CH2CH2.The solutions of RSSC6H4NO2 were prepared by allowing O2NC6H4S^- to react with an excess of RSSR, followed by rapidly mixing R'S^- and measuring the return of the absorption due to O2NC6H4S^-.The rates for reactions involvingcharged R and R' groups differ from those with uncharged groups of identical pK_a by factors of up to 2.5.Negatively charged groups, especially on the central thiol, slow the reaction and positively charged groups speed the reaction to a greater extent than that predicted by the calibrating ionization reaction.Ramifications of these observations are considered for the prediction of rate constants of thiol-disulfide interchange reactions and for the interpretation of β values when reactants of various charge types are used.