Three Rate-Constant Kinetic Model for Permanganate Reactions Autocatalyzed by Colloidal Manganese Dioxide: The Oxidation ofl-Phenylalanine

Abstract

The reduction of permanganate ion to MnO(2)-Mn(2)O(3) soluble colloidal mixed oxide by l-phenylalanine in aqueous phosphate-buffered neutral solutions has been followed by a spectrophotometric method, monitoring the decay of permanganate ion at 525 nm and the formation of the colloidal oxide at 420 nm. The reaction is autocatalyzed by the manganese product, and three rate constants have been required to fit the experimental absorbance-time kinetic data. The reaction shows base catalysis, and the values of the activation parameters at different pHs have been determined. A mechanism including both the nonautocatalytic and the autocatalytic reaction pathways, and in agreement with the available experimental data, has been proposed. Some key features of this mechanism are the following: (i) of the two predominant forms of the amino acid, the anionic form exhibits a stronger reducing power than the zwitterionic form; (ii) the nonautocatalytic reaction pathway starts with the transfer of the hydrogen atom in the α position of the amino acid to permanganate ion; and (iii) the autocatalytic reaction pathway involves the reduction of Mn(IV) to Mn(II) by the amino acid and the posterior reoxidation of Mn(II) to Mn(IV) by permanganate ion.

Kinetics of the chromium(III)/l‐glutamic acid complexation reaction: Formation, decay, and UV‐vis spectrum of a long‐lived intermediate

“…This variation of the rate constants can be ascribed to the decrease of the pH as the reaction advanced caused by the release of protons from the amino acid to coordinate with Cr(III), suggesting the involvement of base catalysis that was confirmed later by independent experiments. Actually, the occurrence of systematic experimental errors leading to rate coefficients showing a dependence on the value of the limiting reactant initial concentration is somehow ubiquitous in kinetic studies, especially when two or three rate constants are required to account for the observed absorbance versus time curves. Therefore, the rate constants obtained in this work must be considered as average values for the pH range covered in each kinetic experiment.…”

Pathways in permanganate oxidation of mandelic acid: reactivity and selectivity of intermediate manganese species

“…19,20 Permanganate oxidations have been reviewed extensively. [21][22][23][24][25][26][27][28][29] Definitive works of Simandi, [30][31][32][33][34] Wiberg, [35][36][37] Freeman, [38][39][40] Perez-Benito, [41][42][43][44][45] Lee, [46][47][48][49] and Stewart [50][51][52] have greatly contributed to our understanding of the mechanisms of oxidations of a diverse group of organic compounds. These include aromatic hydrocarbons (C-H oxidation), 35,53,54 alcohols, 50,51 aldehydes, 37 alkenes, 30,38,39 oxalic acid, [55][56][57][58][59] amines, 52,…”

“…These include aromatic hydrocarbons (C-H oxidation), 35,53,54 alcohols, 50,51 aldehydes, 37 alkenes, 30,38,39 oxalic acid, [55][56][57][58][59] amines, 52,60 sulfoxides, 61 and amino acids. [41][42][43][44][45] Permanganate oxidations are inherently complex due to a number of intermediates resulting from initial two and one electron reductions of Mn(VII). Both Mn(V) and Mn(VI) have no known aqueous chemistry except in a highly alkaline medium.…”

“…Adding to the complexity, there are a number of pH dependent disproportionation/comproportionation reactions. 62,63 A soluble, tobacco colored manganese(IV) species has been observed in a large number of reactions in neutral pH ( phosphate buffer) including oxidation of sulfite, 31 amino acids, [41][42][43][44] thiosulfate, 64 and alkenes. 34,36,39,47 A colloidal Mn(IV) has also been identified in permanganate oscillating reactions.…”

“…These include aromatic hydrocarbons (C–H oxidation), 35,53,54 alcohols, 50,51 aldehydes, 37 alkenes, 30,38,39 oxalic acid, 55–59 amines, 52,60 sulfoxides, 61 and amino acids. 41–45…”