On Mechanistic Implications in L-Ascorbic Acid Reduction Reactions: Differential Reactivities of Ascorbic Acid and Acid Ascorbate Monoanion Species Compared vis-a-vis Mechanistic Types

Kinetic Study of the Hexacyanoferrate (III) Oxidation of Dihydroxyfumaric Acid in Acid Media

“…The reaction stoichiometry was determined by measuring the oxidant concentration remaining after completion of a kinetic run with the oxidant in excess. The 2:1 ratio found with other endiols such as 2,3-dihydroxy-2-propenal, 3 or ascorbic acid, – was not found here, but rather the [Ferricyanide] completed /[HADF] initial ratio determined at three acidity levels, [H + ] = 0.1, 1.0, and 2.0 M, was always 1.4:1. This feature should not be set down to interference of any HADF decarboxylation over the time needed to complete the redox reaction, because decarboxylation is 2 orders of magnitude slower.…”

“…The mechanism put forward resembles that of endiols such as 2,3-dihydroxy-2-propenal or ascorbic acid; however, in the latter cases, the initial step is irreversible. The hydroxyfumarate radical species bears similarity with the ascorbate radical suggested in the oxidation of ascorbic acid. – The product P (eq ) would be the corresponding diketone, that is, diketosuccinic acid, as occurs in electrochemical oxidations , or by 2,6-dichlorophenolindophenol . The rate equation applicable to this scheme leads to …”

“…Given the observed nondependence of the rate constant with ionic strength at 2.0 M HCl, it can be concluded that the rate-determining step involves neutral HADF, the steps involving the radical cation being fast. The rate-determining step of the hexacyanoferrate (III) oxidation of many organic substrates that follow first order in both the oxidant and substrate is the transfer of the first electron from the substrate to the oxidant, ,, with formation, in this case, of the dihydroxyfumarate free radical; this radical should generate hexacyanoferrate (II) and diketosuccinic acid in the follow-up fast step. In this acidity range the hexacyanoferrate (III)/(II) formal reduction potential rises from 0.688 V (1.0 M HCl) to 0.764 V (3.0 M HCl); therefore, the process described will be even faster when HFe(CN) 6 2− is the reactive species.…”

“…The hydroxyfumarate radical species bears similarity with the ascorbate radical suggested in the oxidation of ascorbic acid. [32][33][34][35][36][37][38] The product P (eq 12) would be the corresponding diketone, that is, diketosuccinic acid, as occurs in electrochemical oxidations 10,39 or by 2,6-dichlorophenolindophenol. 13 The rate equation applicable to this scheme leads to…”