Mechanism of oxidation of L-ascorbic acid by the pentaamminechromatocobalt(III) complex ion in aqueous solution

Abstract

Oxidation of -ascorbic acid by pentaamminechromatocobalt() nitrate has been investigated over the ranges 6.1 р pH р 8.7, 21.0 р θ р 30.0 ЊC, in excess of -ascorbic acid at a constant ionic strength of 0.50 mol dm Ϫ3 (NaClO 4 ). Overall the reaction occurs in two distinct stages. The first, involving saturation kinetics, proceeds with an increase in absorbance at wavelengths 350-390 nm. During this stage the pentaamminechromato complex and the ascorbate anion react via an outer-sphere mechanism to form an ion pair. The rate equation for this stage is (i)with [A] T being total ascorbate, K 4 and k 5 are the equilibrium constant for adduct formation and first-order rate constant for its decomposition, respectively. At 25.0 ЊC, k 5 and K 4 have respective values of (3.5 ± 0.4) × 10 Ϫ2 s Ϫ1 and (8.5 ± 1.7) × 10 2 dm 3 mol Ϫ1 . The second stage involving a decrease in absorbance at similar wavelengths is the reduction of the free chromium() ion which is postulated to have been formed during the first stage of the reaction. The pseudo-first-order rate constant for this stage can be expressed as in equation (ii) where K 1 and K 6

Micellar effect on the kinetics and mechanism of chromium(VI) oxidation of organic substrates

“…A similar reaction mechanism is known for the reaction with the 1e-reductants of transition metal ions [41] and [Fe(phen) 3 ] 2+ [107]. In some other cases, this mechanism has also been suggested [82,97,[108][109][110][111]. In the oxidation of alkyl/aryl and diphenyl sulfides by Cr(VI), to explain the excellent correlations obtained between the log k values and the oxidation potentials/ionization energies of the sulfides and the absence of any rate retardation in the presence of Mn(II), the one-electron transfer mechanism (i.e.…”

“…Other groups of workers [108,109,112] have suggested the formation of the ester-like intermediate at the rate determining step. Dasgupta and co-workers [108,109] have argued that the three successive 1e-transfer steps [i.e Cr(VI) → Cr(V) → Cr(IV) → Cr(III)] occur extremely fast to produce the Cr(III)-product and consequently no appreciable buildup of the Cr(VI)-ester like intermediate occurs.…”

“…In the interaction of Cr(VI) with ascorbic acid in aqueous acid media, different authors have suggested different mechanisms [17(a), 77,85,[108][109][110]112,113] depending on the conditions. The Cr(V)-complex and ascorbate anion radical were confirmed through an EPR study [17(a), 77,85,110].…”

The different kinetic and mechanistic behaviors of molybdenum and tungsten in the reduction of tris(benzene‐1,2‐dithiolato)Mo(VI) and W(VI) complexes by ascorbic acid in aqueous media

“…Usually, these oxidations have an inner-sphere nature due to the fact that the cations coordinate one of the hydroxyl groups of ascorbic acid, inducing a mono-electronic rate-limiting electron transfer toward the metallic center. In contrast, the oxidation of ascorbic acid by chelates is thought to proceed by uncomplicated outersphere mechanisms [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. Most of the ascorbic oxidations produced by metal-transition complexes are also mono-electronic, even for oxidants capable of exchanging two electrons, such as some binuclear Ru(III) amine complexes [27], or dioxo Ru(VI) species [37].…”

Kinetics and Mechanisms of the Reduction of Chromium(VI) by 2-Mercaptoethanesulfonic Acid in Aqueous Solution: Difference in the Mechanistic Process of Reduction with Noncarboxylate Thiols

“…Thiols and ascorbic acid have been found to be the most reactive biological substrates in the reduction of Cr(VI) at physiological pH [14]. Our research group has investigated the kinetics of the reduction of Cr(VI) by many reductants under different reaction conditions [15][16][17][18][19], The two most recent publications involving 2-mercaptosuccinic acid [20] and thioglycolic acid [21], have provided evidence for the direct participation of neighbouring carboxylate functional groups during the reduction of Cr(VI) by thiols under acidic aqueous conditions. In order to confirm that carboxylate groups do participate, we need to show that the reduction of Cr(VI) by a thiol-containing compound is significantly different in the absence of a neighbouring carboxylate functional group.…”