Mahua Das

Micellar effect on the reaction of picolinic acid catalyzed chromium(VI) oxidation of dimethyl sulfoxide in aqueous acidic media: A kinetic study

The kinetics and mechanism of picolinic acid (PA) catalyzed oxidation of dimethyl sulfoxide (DMSO) to dimethyl sulfone by chromium(VI) in both aqueous H2SO4 and HClO4 media have been studied in the absence and presence of surfactants at different temperatures. Cr(VI)–PA complex formed in preequilibrium steps is the active oxidant that experiences the nucleophilic attack by DMSO to form a positively charged intermediate ternary complex. Within the proposed ternary complex, an oxygen transfer or a ligand coupling or both occurs to generate the product, dimethyl sulfone. Cr(VI) is ultimately converted to Cr(III)–PA complex. Under the experimental conditions, the process shows a first-order dependence on each of the reactants (i.e., [Cr(VI)]T, [PA]T, [DMSO]T, and [H+]). HCrO4− has been found kinetically active. The reaction is catalyzed by sodium dodecyl sulfate (SDS, a representative anionic surfactant) monotonically, while cetylpyridinium chloride (CPC, a representative cationic surfactant) retards the reaction continuously. The observed micellar effects have been explained by considering the hydrophobic and electrostatic interaction between the surfactants and reactants. A pseudo-phase ion exchange (PIE) model has been applied to explain the micellar effect. The Piszkiewicz cooperative model has been applied to determine the kinetic parameters, and it indicates the existence of catalytically productive submicellar aggregates. Because of this reactant-promoted micellization of the surfactant before or below the cmc value, the present systems do not show any discontinuity at the respective reported cmc values of the surfactants.

Micellar Effect on Chromium(VI) Oxidation of D-Glucose in the Presence and Absence of Picolinic Acid in Aqueous Media: A Kinetic Study

Abstract The kinetics and mechanism of Cr(VI) oxidation of D-glucose in the presence and absence of picolinic acid (PA) in aqueous acid media have been carried out under the conditions, [D-glucose]T ⋟ [Cr(VI)]T at different temperatures. Under the kinetic conditions, HCrO4- has been found kinetically active in the absence of PA while in the ΡΑ-catalysed path Cr(VI)-PA complex has been established as the active oxidant. In the ΡΑ-catalysed path, Cr(VI)-PA complex receives a nucleophilic attack by the substrate to form a ternary complex which subsequently experiences a redox decomposition (through 2e transfer) leading to lactone (oxidised product) and Cr(IV)-PA complex. Then Cr(IV)-PA complex participates further in the oxidation of D-glucose and ultimately is converted into Cr(III)-PA complex. In the uncatalysed path, Cr(VI)-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+], Both the paths show first-order dependence on [D-glucose]T and [Cr(VI)]T. The ΡΑ-catalysed path is first-order in [PA]T. These observations remain unaltered in the presence of externally added surfactants. Effect of cationic surfactant (i.e. cetylpyridinium chloride, CPC) and anionic surfactant (i.e. sodium dodecyl sulfate, SDS) on both the uncatalysed and ΡΑ-catalysed path has been studied. CPC inhibits both the uncatalysed and ΡΑ-catalysed path while SDS catalyses the reactions. The observed micellar effects have been explained by considering the hydrophobic and electrostatic interaction between the surfactants and reactants. Applicability of different kinetic models, e.g. pseudo-phase ion exchange (PIE) model, Menger-Portnoy model, Piszkiewicz cooperative model, has been tested to explain the observed micellar effects. Effect of [surfactant]T on the activation parameters has been explored to rationalise the micellar effect.

Kinetics and mechanism of osmium(VIII) mediated cerium(IV) oxidation of dimethylsulfoxide in aqueous sulfuric acid

Under the experimental conditions [DMSO]T ≫[CeIV]T ≫[Os]T the kinetics of oxidation of dimethylsulfoxide (DMSO) to dimethylsulfone (DMSO2) have been followed at different temperatures (40–55°C) in 1.0 mol dm−3 sulfuric acid media. The rate of disappearance of [CeIV] shows a first-order dependence on both [Os]T and [DMSO]T and zeroth-order kinetics with respect to [CeIV]. The suggested mechanism involves oxidation of DMSO by OsVIII in a rate-determining step through an outer-sphere mechanism, followed by rapid regeneration of OsVIII by CeIV from OsVI. The rate law conforms to: −d[CeIV]/dt=k0=k[Os]T[DMSO]T. The values of k and the activation parameters are: 102k=(4.9 ± 0.10) mol−1 dm3 s−1 at 40°C, [H2SO4] =1.0 mol dm−3;ΔH‡=58±3kJmol−1, ΔS‡= −88 ±5JK−1mol−1.

Micellar effects on the reaction of Cr(VI) oxidation of hexitols in the presence and absence of picolinic acid in aqueous acid media

In aqueous acidic media, picolinic acid (PA) promoted CrVI-oxidation of the hexitols to the respective aldohexoses goes on, along with a slower uncatalysed path; the anionic surfactant (SDS) accelerates the process while the cationic surfactant (CPC) retards the reaction.

A comparative kinetic study of iridium(III) catalysis in cerium(IV) oxidation of dioxane in aqueous sulfuric acid and perchloric acid media

The kinetics and mechanism of IrIII-catalysed oxidation of dioxane by CeIV in both aqueous H2SO4 and HClO4 media have been studied at different temperatures under the conditions: [dioxane]T ≫ [CeIV]T ≫ [Ir]T (ca. 10−6–10−8moldm−3). In aqueous HClO4 media a slow uncatalysed path exists alongside the catalysed path, while in aqueous H2SO4 media the catalysed path is the only kinetically detectable path. In both media, the overall process shows a first-order dependence on [CeIV]T; the catalytic path is first order in [Ir]T and exhibits a non-linear dependence on [dioxane]T. The catalysed path probably involves a pre-equilibrium interaction between the catalyst and substrate leading to an outer-sphere complex followed by the electron transfer in the rate-determining step involving CeIV and an outer-sphere complex formed in pre-equilibrium steps. The catalytic path presumably involves the IrIII/IrIV cycle. In HClO4 media the catalytic efficiency is greater than in H2SO4 media. Activation parameters for different paths have been determined in order to rationalise the mechanistic steps.

KINETICS AND MECHANISM OF PICOLINIC ACID PROMOTED CHROMIUM(VI) OXIDATION OF DIMETHYL SULFOXIDE IN THE PRESENCE AND ABSENCE OF SURFACTANTS

In the picolinic acid (PA) promoted CrVI oxidation of dimethyl sulfoxide (DMSO), the CrVI–PA complex formed at the pre-equilibrium step undergoes nucleophilic attack by the S of DMSO to form a positively charged reactive intermediate which 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.

Kinetics and mechanism of ruthenium(III) catalysed oxidation of formic acid by cerium(IV) in aqueous sulfuric acid media

The kinetics of oxidation of formic acid by cerium(IV) in the presence of ruthenium(III)(ca. 10–6 mol dm–3) in aqueous sulfuric acid media has been followed at different temperatures (30–50 °C). The rate of disappearance of cerium(IV) in the reaction has been found to be zero order with respect to cerium(IV) concentration. At a fixed [H+], under the conditions, [HCO2H]T[CeIV]T[Ru]T the observed zero-order rate constant (k0) conforms to: –d[CeIV]t/dt=k0=[Ru]T[HCO2H]T{kb+kc[HCO2H]T} where [Ru]T and [HCO2H]T represent the total concentration of ruthenium(III) and formic acid respectively. At 40 °C, [H2SO4]= 1.0 mol dm–3 and I= 2.75 mol dm–3 the values of 102kb and 102kc are 6.0 ± 0.1 dm3 mol–1 s–1 and 5.4 ± 0.1 dm6 mol–2 s–1 respectively. Both kb and kc are found to have an inverse hydrogen-ion dependence. Out of the different possible mono- and bis-complexes, RuIII(HCO2–) and RuIII(HCO2–)(HCO2H) have been found to be kinetically active in the slow oxidative steps (through an inner-sphere mechanism) leading to RuIIIH (through hydride transfer from the C–H bond of metal-bound formate) and CO2 followed by the rapid oxidation of RuIIIH to RuIII by cerium(IV). The activation parameters are ΔH‡= 46 ± 3 kJ mol–1, ΔS‡=–125 ± 5 J K–1 mol–1(for the kb path) and ΔH‡= 47 ± 3 kJ mol–1, ΔS‡=–120 ± 5 J K–1 mol–1(for the kc path).

Cooxidation of formic acid and oxalic acid by chromium(VI) in aqueous acid media: a kinetic study

The Cr(VI) oxidation of a mixture of formic acid and oxalic acid in aqueous acid media occurs much faster than that of either of the two substrates alone; in the mixture, both substrates undergo oxidation simultaneously in a ternary complex of Cr(VI) through a three-electron transfer step (i.e. Cr(VI) → Cr(III)).

Kinetics and Mechanism of Chromium(III) Catalysed Oxidation of Propan-1-ol and Butan-1-ol by Cerium(IV) in Aqueous Sulfuric acid Media

The kinetics and mechanism of chromium(III) catalysed oxidation of propan-1-ol and butan-1-ol by cerium(IV) in aqueous sulfuric acid media have been studied at different temperatures (30-40° C) under the conditions, [substrate]T(0.025-0.70 mol dm-3) × [CeIV]T (2.5-4.5 × 10-3 mol dm-3) in the presence of [Cr]T = 0.0025-0.07 mol dm-3. At fixed [H+] and [HSO-4 ] the observed rate law conforms to: -d ln[CeIV]/df =kobs =A[S]T[Cr]T/{B + C[Cr]T(D + E[S]T)}, where, A, B, C, D and Ε are constants expressed in terms of different rate constants and equilibrium constants; [S]Tand [Cr]T denote the total substrate (i.e. propan-l-ol or butan-l-ol) and total chromium concentration respectively. The proposed mechanism involves the association of catalyst, substrate and oxidant at some pre-equilibrium steps before the electron transfer step. The overall process is found catalysed by H+ . From the [HSO-4] dependence, Ce(SO4)2 has been found kinetically active.

Kinetics and Mechanism of Iridium(III) Catalysed Oxidation of Formaldehyde by Cerium(IV) in Aqueous Sulfuric Acid Media

In the IrIII (ca. 10–6moldm–3) catalysed oxidation of formaldehyde by CeIV in aqueous H2SO4 media an intermediate involving an association of the catalyst, substrate and oxidant is formed before the electron transfer step and IrIII/IrIV catalytic cycle operates.