Kinetics of the oxidation of ascorbic acid and substituted 1,2- and 1,4-dihydroxybenzenes by the hexacyanoruthenate(III) ion in acidic perchlorate media

The kinetics of oxidation of ascorbic acid and a series of substituted 1,2- and 1,4-dihydroxybenzene compounds (H2Q) by [Ru(CN)6]3– have been investigated in acidic perchlorate media. The inverse dependences of the rate constants on acid concentrations for 2,3-dicyano-1,4-dihydroxybenzene, 4,5-dihydroxybenzene-1,3-disulphonate, and ascorbic acid have been attributed to concurrent rate-determining pathways involving the one-electron oxidations of H2Q or HQ– by [Ru(CN)6]3– to the corresponding semiquinone or ascorbate radical intermediate. The cross-reaction rate constants have been correlated with the semiquinone or ascorbate reduction potentials in terms of the Marcus relationship to yield a [Ru(CN)6]3––[Ru(CN)6]4– electron self-exchange rate constant of (1.0 ± 0.8)× 105 dm3 mol–1 s–1. This is compared with those for other low-spin d5–d6 transition-metal complex couples and discussed in terms of the inner-sphere and solvent reorganization energies.