Role of trace metal ions. Kinetics and mechanism of the copper(II)-catalysed oxidation of ascorbic acid with peroxodiphosphate in acetate buffers

Trace metal-ion catalysis: kinetics and mechanism of oxidation of l-ascorbic acid by chromium(VI) in phosphate buffers

“…The title reaction is catal---~ ysed only by copper(II); the reported (6) catalytic activities ~ 4o of iron(II) and iron(III) are significantly less than that exhibited by copper(II) both in autoxidations (9) and also oxidations of the ascorbic acid by metal and non-metal 30 oxidants in aqueous acidic medium <13). The catalytic role of copper(II) is normally reported (9'26'z7) to occur either via complexation with H2A and HA-or the copper(II)-zo copper(I) redox cycle (6). However, the latter does not appear to be operative here in view of the following facts: 10 (1) the reaction is second order overall, i.e.…”

“…Recently catalysis by traces of metal ions in the oxidation of ascorbic acid by peroxodiphosphate (6) and peroxobound chromium(IV) (7'8) in acetate buffers and other media (9) has been reported. These studies prompted us to undertake the title study for several reasons.…”

Kinetics and mechanism of oxidation of l‐ascorbic acid by peroxomonophosphate in aqueous acid medium

“…It is well established 40 that copper(II) forms complexes with various species of ascorbic acid. A qualitative portrait of the reaction events in copper(II)‐catalyzed oxidation of ascorbic acid by peroxomonophosphoric acid can be envisaged as follows: The peroxo‐coordinated adducts 41 are readily formed, and the reduction of such adducts proceeds through a transient species with properties similar to that of a peroxobridged precursor complex.…”

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

“…At a short timescale, the rate of ascorbate radical formation from HA − and A is expected to be of little significance (k −6 = 2.8 × 10 −10 s −1 M −1 ) as the equilibrium is strongly displaced toward the products (K 6 = 5 × 10 14 , k 6 = 1.4 × 10 5 s −1 M −1 at pH 7.5) 1 . As these highly reactive species remain at a low concentration, the amount of ascorbate radical eliminated through step 14 is expected to be much smaller compared with that of the competitive step 13 (k 6 …”

“…The oxidation of ascorbic acid by labile substitution and aquo transition-metal complexes has been widely studied during the past few decades [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. 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.…”