Kinetics and mechanism of the oxidation of ascorbic acid in aqueous solutions by a trans-dioxoruthenium(VI) complex

Yi-Ning WANG, Kai-Chung LAU, William W. Y. LAM, Wai-Lun MAN, Chi Fai LEUNG, Tai-Chu LAU

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Abstract

The oxidation of ascorbic acid (H₂A) by a trans-dioxoruthenium(VI) species, trans-[RuVI(tmc)(O)₂]²⁺ (tmc =1,4,8,11-tetramethyl-1,4,8,11 -tetraazacyclotetradecane), has been studied inaqueous solutions under argon. The reaction occurs in two phases: trans-[RuVI(tmc)(O)₂]²⁺ + H₂A→trans-[RuIV(tmc)(O)(OH₂)]²⁺ + A, trans-[RuIV(tmc)(O)(OH₂)]²⁺ + H₂A → trans-[Ruˡˡ(tmc)(OH₂)₂]²⁺+ A. Further reaction involving anation by H₂A occurs, and the species [Ruˡˡˡ(tmc)(A²⁻)(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 RuVI and H²A, with the second-order rate constants k₂ = (2.58 ± 0.04) × 10³ M⁻¹ s⁻¹ at pH = 1.19 and k₂´ = (1.90 ± 0.03) M⁻¹ s⁻¹ at pH = 1.24, = 298 K and / = 0.1 M for the first and second phase, respectively. Studies on the effects of acidity on k₂ and k₂´suggest that HA⁻ is the kinetically active species. Kinetic studies have also been carried out in D₂O, and the deuterium isotope effects for oxidation of HA⁻ by RuVI and RuIV are 5.0 ± 0.3 and 19.3 ± 2.9, respectively, consistent with ahydrogen atom transfer (HAT) mechanism for both phases. A linear correlation between log(rate constants) for oxidation by RuVI and the O-H bond dissociation energies of HA⁻ and hydroquinones is obtained, which also supports a HAT mechanism. Copyright © 2008 American Chemical Society.

Original languageEnglish
Pages (from-to)400-406
JournalInorganic Chemistry
Volume48
Issue number1
DOIs
Publication statusPublished - 05 Jan 2009

Citation

Wang, Y.-N., Lau, K.-C., Lam, W. W. Y., Man, W.-L., Leung, C.-F., & Lau, T.-C. (2009). Kinetics and mechanism of the oxidation of ascorbic acid in aqueous solutions by a trans-dioxoruthenium(VI) complex. Inorganic Chemistry, 48(1), 400-406. doi: 10.1021/ic8015904

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