The Catalytic Influence of Polymers and Surfactants on the Rate Constants of Reaction of Maltose with Cerium (IV) in Acidic Aqueous Medium

Kinetics of the reaction of maltose with cerium ammonium sulfate were analyzed spectrophotometrically by observing the decrease of the absorbance of cerium (IV) at 385 nm in the presence and absence of polyethylene glycols (600, 1500, and 4000) and polyvinylpyrrolidone (PVP), in addition to anionic micelles of sodium dodecyl sulfate (SDS), cationic micelles of cetyltrimethylammonium bromide (CTAB) and non-ionic micelles of Tween 20 surfactants. Generally, there is little literature about using the polymers (PEGs and PVP) as catalysts in the oxidation-reduction reactions. Therefore, the major target of this work was to investigate the influence of the nature of polymers and surfactants on the oxidation rates of maltose by cerium (IV) in acidic aqueous media, as well as employing the Piszkiewicz model to explain the catalytic effect. The kinetic runs were derived by adaptation of the pseudo first-order reaction conditions with respect to the cerium (IV). The reaction was found to be first-order with respect to the oxidant and fractional-order to maltose and H2SO4. The reaction rates were enhanced in the presence of polymer and micellar catalysis. Indeed, the surfactants were found to work perfectly close to their critical micelle concentrations (CMC). Electrostatic interaction and H-bonding appear to play an influential role in binding maltose molecules to polymer/surfactant micelles, while oxidant ions remain at the periphery of the Stern layer within the micelle.

A Review on Micellar Catalyzed Oxidation Reactions of Organic Functional Groups in Aqueous Medium Using Various Transition Metals

The current requirement for science and research concerns the absolute sustainable development of a chemistry that is inherently safer, smarter and more environmentally friendly. The oxidation reaction is a very fundamental transformation reaction in organic synthesis and likely plays a significant role in the production of various value-added chemicals from biomass and others precursors. In the focus of making kinetic experiments greener several modified methodologies and safe chemicals have been employed. Surfactants are such suitable alternate that go with the requirments. Surfactant aggregates i. e. micelles are nano-sized supra molecules, able to act as catalysts. They can be used to catalyze the organic functional group transformation reactions mediated with transition metals and promoted with various aromatic bases. This allowed water to be used as a solvent, where the reactions became more sustainable. The recyclability of used surfactants, enhancement of reaction kinetics and speed of reaction with no consumption of energy has added more value to this type of catalytic oxidation. This article aims to contribute to the discussion of the mechanistic aspects of various types of surfactant-catalyzed oxidation of organic functional groups.

Kinetics and Mechanism of 2,2′-bipyridine Catalysed Chromium(VI) Oxidation of Dimethyl Sulfoxide in the Presence and Absence of Surfactants†

In the 2,2′-bipyridine (bipy) catalysed CrVI oxidation of dimethyl sulfoxide (DMSO) to dimethyl sulfone, the CrVI–bipy complex formed at the pre - equilibrium step undergoes a nucleophilic attack by the S or O of DMSO to form a positively charged reactive intermediate. This intermediate experiences an oxygen transfer or a ligand coupling to give the products. The anionic surfactant (SDS) accelerates the process while the cationic surfactant (CPC) retards the reaction.

Kinetic and Mechanistic Aspects of the Chromic Acid Oxidation of D-Galactose in the Presence and Absence of Picolinic Acid Catalyst in Aqueous Micellar Acid Media

The kinetics and mechanism of the chromic acid oxidation of D-galactose in the presence and absence of picolinic acid (PA) in aqueous acid media have been determined under the conditions, [D — galactose]T≥ [CrVI]Tat different temperatures. Under the kinetic conditions, HCrO4-has been found kinetically active in the absence of PA while in the PA catalysed path, a CrVI- PA complex has been established as the active oxidant. In the PA-catalysed path, the CrVI- PA complex receives a nucleophilic attack by the substrate to form a ternary complex which subsequently undergoes a redox decomposition through a two-electron transfer leading to a lactone (oxidised product) and a CrIV- PA complex. Then the CrIV-PA complex participates further in the oxidation of D-galactose and ultimately is converted into a CrIII- PA complex. In the uncatalysed path, a CrVI- substrate ester experiences an acid catalysed redox decomposition (2e-transfer) at the rate-determining step. The uncatalysed path shows a second-order dependence on [H+]. Under the experimental conditions, both paths show first-order dependences on [D-galactose]Tand [CrVI]T. The PA-catalysed path is first-order in [PA]T. These observations remain unaltered in the presence of externally added surfactants. The effect of surfactants like N-cetylpyridinium chloride (CPC, a cationic surfactant) and sodium dodecyl sulfate (SDS, an anionic surfactant), on both the uncatalysed and PA-catalysed paths have been studied. CPC retards both the uncatalysed and PA-catalysed path, while SDS accelerates the reactions. The observed micellar effects have been explained by considering the hydrophobic and electrostatic interactions between the reactants and surfactants.

Influence of various organic molecules on the reduction of hexavalent chromium mediated by zero-valent iron

Hexavalent chromium is a priority pollutant in many countries. Reduction of Cr(VI) to Cr(III) is desirable as the latter specie is an essential nutrient for maintaining normal physiological function and also has a low mobility and bioavailability. A variety of naturally-occurring organic molecules (containing alpha-hydroxyl carbonyl, alpha-hydroxyl carboxylate, alpha-carbonyl carboxylate, phenolate, carboxylates and/or thiol groups, siderophore, ascorbic acid); chelating agents (ethylenediaminetetraacetic acid derivates, acetyacetone) and others were examined their reducing activity towards a surfactant preparation (Tween 20) containing Cr(VI) and Fe(0) under a variety of reaction conditions. An appreciable enhancement (up to 50-fold) of the pseudo-first-order rate constant was achieved at acidic and circum neutral pH values for those compounds capable of reducing Cr(VI) (alpha-hydroxyl carboxylate, ascorbic acid, cysteine). Comparable enhancements were obtained for certain chelating agents (ethylenediaminetetraacetic acid derivates and siderophores) which is attributed to the formation of complexes with reaction products, such as Cr(III) and Fe(III), which impede the precipitation of Cr(III) and Fe(III) hydroxides and Cr(x)Fe(1-)(x)(OH)(3) and thus reduce passivation of the Fe(0) surface. The results suggest that these molecules might be used in effective remediation mediated by Fe(0) of Cr(VI)-contaminated soils or groundwater in a wide range of pH, thus increasing reaction rates and long-term performance of permeable reductive barriers.

Picolinic Acid Assisted Three-Electron Transfer Chromic Acid Oxidation ofdl-Mandelic Acid in Aqueous Micellar Media: A Kinetic Study

The three-electron transfer chromic acid oxidations of dl-mandelic acid (MA) in the presence and absence of picolinic acid (PA) have been studied in aqueous micellar media under the kinetic conditions, [ dl-mandelic acid] ≫ [PA]T≫ [CrVI]Tat different temperatures. Under these kinetic conditions, HCrO4-has been found to be kinetically active in the absence of picolinic acid while in the PA-catalysed path, a CrVI—PA complex has been established as the active oxidant. In the PA-catalysed path, the CrVI—PA complex experiences nucleophilic attack by the substrate to form a ternary complex which subsequently undergoes a redox decomposition involving 3e-transfer, leading to oxidative decarboxylation through C—C bond cleavage. This rate-determining step produces CO2, benzoyl radical and the corresponding CrIII—PA complex. Benzoyl radical is subsequently oxidized to benzoic acid in later, faster steps. In the uncatalysed path, the CrVI—substrate ester also undergoes an acid-catalysed redox decomposition through 3e-transfer as the rate-determining step. Both the catalysed and uncatalysed paths show a first order dependence on [H+] and both paths also show a first order dependence on [ dl-mandelic acid]Tand [CrVI]T. The PA-catalysed path is first order in [PA]T. These observations remain unaltered in the presence of externally added surfactants. The effects of a cationic surfactant such as N-cetylpyridinium chloride (CPC) and an anionic surfactant such as sodium dodecyl sulfate (SDS) on both the uncatalysed and PA-catalysed paths have been studied. CPC has been found to accelerate the uncatalysed path and to retard the PA-catalysed path. SDS shows a rate-accelerating effect for both paths in different ways.