Free Energy Hydride Affinities of Quinones in Dimethyl Sulfoxide Solution

Article abstract of DOI:10.1021/jo00071a011

Free energy hydride affinities in solution defined by reaction i <(i) Q_s + H(1-)_s -->/<-- (QH(1-))_s, ΔG_hydride(Q)_s> were evaluated for a number of quinones (Q) in dimethyl sulfoxide solution using reaction ii <(ii) -ΔG_hydride(Q)_s = 2.303RTpK_a(QH(1-))_s + FΔE⁰_NHE<(Q^.-/Q(2-))_s + (Q/Q^.-)_s> + C> derived from a thermochemical cycle.The constant C corresponds to -FE⁰_NHE<(H(1+))/H^.)_s + (H^./H(1-))_s> which is equal to 69.9 kcal/mol in dimethyl sulfoxide.The pK_a of hydroquinone monoanions (QH(1-)) were determined by the overlapping indicator method.Reversible electrode potentials for both the first and second charge transfers to Q were determined by cyclic voltammetry.Values of ΔG_hydride(Q)_DMSO ranging from -58 to -101 kcal/mol were observed.Hydride affinities of substituted benzoquinones (BQ) were found to be linearly correlated with the corresponding electron affinities in solution.Equilibrium constants for the reactions of BQ with the NADH model compound 10-methylacridan were estimated, and the thermochemical results are discussed in terms of the one- and the two-step mechanisms of hydride-transfer reactions.