Abstract
The kinetics and mechanism of picolinic acid promoted reaction of phenylsulfinylacetic acid (PSAA) with Cr(VI) was carried out in aqueous acetonitrile medium under pseudo first order conditions. The reaction follows Michaelis-Menten type of kinetics with respect to PSAA. The catalytic activity by picolinic acid can be interpreted on the basis of the formation of a highly active oxidizing species, Cr(VI)-PA complex. The mechanism involves the formation of a termolecular complex, Cr(VI)-PA-PSAA by the nucleophilic attack of the sulfur atom of PSAA on chromium of Cr(VI)-PA complex in an equilibrium step followed by ligand coupling in a slow step. Electron releasing substituents in the phenyl ring of PSAA accelerate while electron withdrawing groups retard the reaction rate. The overall rate constants for the para- and meta-substituted PSAAs are found to correlate excellently with Hammett σ constants with a very low reaction constant, ρ. . KEY WORDS : Phenylsulfinylacetic acid, Cr(VI), Picolinic acid, Oxidative decarboxylation, Substituent effect, Catalysis Bull. Chem. Soc. Ethiop. 2016 , 30(1), 137-146. DOI: http://dx.doi.org/10.4314/bcse.v30i1.13
Highlights
Cr(VI) is an excellent oxidizing agent for both preparative and analytical purposes
The pyridine bases and other ligands containing hetero nitrogen atoms were found to facilitate the Cr(VI) oxidation reactions to a greater extent and the various chelating agents explored for this purpose are picolinic acid, pyridine-2,6-dicarboxylic acid, 2,2’-bipyridyl, 1,10-phenanthroline, EDTA, etc
Picolinic acid (PA), an endogenous metabolite of L-tryptophan that has been detected in a variety of biological media including cell culture supernatants, blood serum [3], cerebrospinal fluid [4], pancreatic juice and intestinal homogenates [5], possesses a wide range of neuroprotective, immunological and anti-proliferative effects [6]
Summary
Cr(VI) is an excellent oxidizing agent for both preparative and analytical purposes. During oxidation reactions, the main co-ordination site of Cr binding involves alcoholato, carboxylato and thiolato groups [1, 2]. The pyridine bases and other ligands containing hetero nitrogen atoms were found to facilitate the Cr(VI) oxidation reactions to a greater extent and the various chelating agents explored for this purpose are picolinic acid, pyridine-2,6-dicarboxylic acid, 2,2’-bipyridyl, 1,10-phenanthroline, EDTA, etc. The complexing agents influence the nature of Cr(VI) species in the reaction mixture and even affect the mechanism of Cr(VI) oxidation in some cases. Picolinic acid (PA), an endogenous metabolite of L-tryptophan that has been detected in a variety of biological media including cell culture supernatants, blood serum [3], cerebrospinal fluid [4], pancreatic juice and intestinal homogenates [5], possesses a wide range of neuroprotective, immunological and anti-proliferative effects [6]. PA is an efficient chelating agent which forms mono and bis complexes through pyridine nitrogen and carboxylate oxygen [7] with a range of metals including Tl, Mo and W [8, 9]. The catalytic effect was attributed to the activation of oxidizing species of chromium through a precursor complex formation
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