title: Kinetics and mechanism of the oxidation of ascorbic acid in aqueous solutions by a trans-dioxoruthenium(VI) complex.
abstract: The oxidation of ascorbic acid (H(2)A) by a trans-dioxoruthenium(VI) species, trans-[Ru(VI)(tmc)(O)(2)](2+) (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), has been studied in aqueous solutions under argon. The reaction occurs in two phases: trans-[Ru(VI)(tmc)(O)(2)](2+) + H(2)A --> trans-[Ru(IV)(tmc)(O)(OH(2))](2+) + A, trans-[Ru(IV)(tmc)(O)(OH(2))](2+) + H(2)A --> trans-[Ru(II)(tmc)(OH(2))(2)](2+) + A. Further reaction involving anation by H(2)A occurs, and the species [Ru(III)(tmc)(A(2-))(MeOH)](+) can be isolated upon aerial oxidation of the solution at the end of phase two. The rate laws for both phases are first-order in both Ru(VI) and H(2)A, with the second-order rate constants k(2) = (2.58 +/- 0.04) x 10(3) M(-1) s(-1) at pH = 1.19 and k(2)' = (1.90 +/- 0.03) M(-1) s(-1) at pH = 1.24, T = 298 K and I = 0.1 M for the first and second phase, respectively. Studies on the effects of acidity on k(2) and k(2)(') suggest that HA(-) is the kinetically active species. Kinetic studies have also been carried out in D(2)O, and the deuterium isotope effects for oxidation of HA(-) by Ru(VI) and Ru(IV) are 5.0 +/- 0.3 and 19.3 +/- 2.9, respectively, consistent with a hydrogen atom transfer (HAT) mechanism for both phases. A linear correlation between log(rate constants) for oxidation by Ru(VI) and the O-H bond dissociation energies of HA(-) and hydroquinones is obtained, which also supports a HAT mechanism.
authors.affiliation: Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, P. R. China.
authors.forename: Yi-Ning
authors.initials: YN
authors.lastname: Wang
authors.forename: Kai-Chung
authors.initials: KC
authors.lastname: Lau
authors.forename: William W Y
authors.initials: WW
authors.lastname: Lam
authors.forename: Wai-Lun
authors.initials: WL
authors.lastname: Man
authors.forename: Chi-Fai
authors.initials: CF
authors.lastname: Leung
authors.forename: Tai-Chu
authors.initials: TC
authors.lastname: Lau
mesh_terms: D001205:Ascorbic Acid / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D012428:Ruthenium / Q000737:chemistry*; D012996:Solutions; D013053:Spectrophotometry; D013237:Stereoisomerism; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D009942:Organometallic Compounds; D012996:Solutions; D014867:Water; D012428:Ruthenium; D001205:Ascorbic Acid