Ruthenium(III), osmium(VIII) and ruthenium(III) + osmium(VIII) catalysed oxidations ofi-propanol by N-bromosuccinimide in basic solution

Summary The oxidation ofi-propanol (IPA) by N-bromosuccinimide (NBS) in basic solution was investigated separately in the presence of Ru^III, Os^VIII and Ru^III + Os^VIII ions. The order in [IPA] was found to be 0.7, 0.5 and 0.3 respectively in the above three cases in the concentration range studied. The order in [NBS] was unity in the presence of Ru^III chloride but was found to be zero in the case of Os^VIII and Ru^III+Os^VIII catalysis. The order in [metalion] was found to be nearly unity in all the three catalysed reactions. Increase in [^–OH] increased the rate of reaction while addition of succinimide retarded the rate of reaction. Decrease in dielectric constantsof the medium decreased the rate of oxidation. The pseudo first order rate constants (k), zero order rate constants (k₀) and the formation constants (k_f) of the substrate-catalyst complexes and the thermodynamic parameters have been evaluated. Suitable mechanisms in conformity with the experimental observations have been proposed for the three catalysed reactions.     

Ruthenium(III)- and osmium(VIII)-catalyzed oxidation of 2-thiouracil by bromamine-B in acid and alkaline media: a kinetic and mechanistic study

The kinetics of oxidation of 2-thiouracil (TU) by sodium N-bromobenzenesulphonamide or bromamine-B (BAB) have been studied in an HCl medium, catalyzed by RuCl₃, and in a NaOH media with OsO₄ as catalyst, at 313 K. The stoichiometry and oxidation products are the same in both cases, but their kinetic patterns were found to be different. In acid medium the rate shows a first order dependence in each of [BAB] and [TU], and is dependent on [Ru^III]. The reaction rate is inversely dependent on [H⁺]. In alkaline medium, the rate is first order in [BAB] and in [Os^VIII] and zero order in [TU]. The reaction rate is dependent on [NaOH]. Activation parameters have been evaluated, solvent isotope effects have been studied in D₂O medium, and equilibrium constants were calculated. The activation parameters and rate constants indicate that the catalytic efficiency is: Os^VIII > Ru^III. The proposed mechanisms and the derived rate laws are consistent with the observed kinetics.     

Oxidations by iron(VI). Kinetic study of uncatalysed and osmium(VIII) catalysed oxidation of dimethyl sulphoxide in alkaline medium

Summary Uncatalysed and Os^VIII-catalysed oxidation of dimethyl sulphoxide (DMSO) by potassium ferrate [Fe^VI] has been studied in alkaline medium in the 9.8–11.9 pH range, in the presence of IO ₄ ^– as a stabilizing agent. The order in [Fe^VI] and [DMSO] was found to be unity for the uncatalysed reaction and zero and fractional respectively for the catalysed reaction. The [Os^VIII] order was unity in the catalysed reaction. The rate of oxidation decreased with increase in pH and the order in [H⁺] was fractional for the uncatalysed oxidation. However in the catalysed oxidation, the rate at first decreased and then increased with increase in pH. The effect of changing [IO ₄ ^– ] and [buffer] on the oxidation rate was negligible in both cases. Suitable mechanisms consistent with the observed kinetics have been proposed.     

Kinetics and mechanisms of oxidation by metal ions. Part XIII. Osmium(VIII)-catalysed oxidation of cycloalkanols by alkaline hexacyanoferrate(III)

Summary The kinetics of OsO₄-catalysed oxidation of cyclopentanol, cyclohexanol and cyclooctanol by alkaline hexacyanoferrate(III) have been studied at low [OH^–] so that the equilibrium between alcohol and alkoxide ion is not unduly shifted towards the latter. The reaction shows a first-order dependence in [OH^–]. The order of the reaction with respect to cycloalcohol is fractional, indicating the formation of an intermediate complex with Os^VIII since the order with respect to hexacyanoferrate(III) ion is zero. The order with respect to Os^VIII may be expressed by the equation k_obs=a+b[Os^VIII]. The analysis of the rate data indicates a significant degree of complex formation between [OsO₃(OH)₃]^– and ROH. It was found that the bimolecular rate constant k for the redox reaction between complex and OH^–k₁, the forward rate constant for the formation of alkoxide ion. The activation parameters of these rate constants are reported.     

Kinetics and mechanism of osmium(VIII)-catalyzed oxidation of 1,4-thioxane by alkaline hexacyanoferrate(III)

The kinetics of oxidation of 1,4-thioxane (1,4-oxathiane) by alkaline K₃Fe(CN)₆ have been studied in the presence of Os^VIII as catalyst. The reaction is first order in hexacyanoferrate(III) and Os^VIII. The order in thioxane and OH^– is zero. While added salts and ethanol have a negligible effect on the oxidation rate, K₄Fe(CN)₆ retards it. On the basis of kinetic evidence, a mechanism has been proposed.     

Kinetics and mechanism of metal ion catalysis: Part II. Oxidation of PhCHO by osmium(VIII) continuously regenerated by hexacyanoferrate(III) ions

The kinetics of the oxidation of PhCHO by Os^VIII has been studied in 0.01–0.05 M [OH^–] range. Fe(CN)^3– ₆ was used to regenerate Os^VIII. The very low solubility of PhCHO in H₂O restricted the study to the 0.0024–0.0036 M [PhCHO] range. A mechanism involving the PhCHO hydrate has been proposed.     

Osmium(VIII)/palladium(II) catalysis of cerium(IV) oxidation of allyl alcohol in aqueous acid

Summary Catalysis of the Ce^IV-allyl alcohol (AA) reaction in acid solution depends both on the of rate enhancement and product distribution on the catalyst used: Os^VIII results mainly in acrolein, whereas Pd^II gives acrylic acid. The rate laws in the two cases also differ:viz., Equations 1 and 2K₁ is the equilibrium constant of formation of the Os^VIII-allyl alcohol complex and k₁ is the rate constant of its oxidation by Ce^IV; K₂ is the equilibrium constant for the formation of the Ce^IV-Pd^II-allyl alcohol complex and k₂ is its rate constant of decomposition. Rate = K₁k₁[Ce^IV][AA][Os^VIII]/(1+K₁[AA]) (1) Rate = K₁k₁[Ce^IV][Pd^II]/(1+K₂[Ce^IV]) (2)While Os^VIII is effective in H₂SO₄ solution, aqueous HClO₄ is needed for Pd^II. Both reactions proceed through formation of catalyst-allyl alcohol complexes with participation of free radicals. The details of these observations are discussed.     

Reactivities of osmium(VIII), iridium(IV) and platinum(IV) towards glycolaldehyde

Oxidations of glycolaldehyde (GA) to glyoxal by osmium(VIII), iridium(IV) and platinum(IV) have been investigated in aqueous solution, and the orders with respect to each [reactant] determined. The reaction involving iridium(IV) takes place through intermediate formation of free radicals in a MeCO₂Na-MeCO₂H buffer medium, whereas a one-step two-electron transfer process occurs in the oxidations by Os^VIII and Pt^IV in an alkaline medium. Mechanisms consistent with the experimental findings are proposed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Kinetics and mechanisms of oxidation by metalions. Part XI(1). Kinetics of the osmium(VIII)-catalysed oxidation of glycols by alkaline hexacyanoferrate(III)

Summary The kinetics of the Os^VIII-catalysed oxidation of glycols by alkaline hexacyanoferrate(III) ion exhibits zerothorder dependence in [Fe(CN) ₆ ^3– ] and first-order dependence in [OsO₄]. The order with respect to glycols is less than unity, whereas the rate dependence on [OH^–] is a combination of two rate constants; one independent of and the other first-order in [OH^–]. These observations are commensurate with a mechanism in which two complexes, [OsO₄(H₂O)G]^– and [OsO₄(OH)G]^2–, are formed either from [OsO₄(H₂O)(OH)]^– or [OsO₄(OH)₂]^2– and the glycol GH, or by [OsO₄(H₂O)₂] and [OsO₄(H₂O)(OH)]^– and the glycolate ion, G^–, which is in equilibrium with the glycol GH through the reaction between GH and OH^–. Hence there is an ambiguity about the true path for the formation of the two Os^VIII-glycol complexes. A reversal in the reactivity order of glycols in the two rate-determining steps, despite the common attack of OH^– ion on the two species of Os^VIII-complexes, indicates that the two complexes are structurally different because S changes from the negative (corresponding to k₁₁) to positive (related to k₂).     

Osmium(VIII)-catalyzed oxidation of pentamethylene sulfide

Although pentamethylene sulfide (tetrahydrothiopyran) lacks acidic hydrogen, Os^VIII has been found to catalyze its oxidation by alkaline K₃Fe(CN)₆ to produce 3-hydroxypentamethylene sulfide as the only product. The kinetics reveal first-order dependence on ferricyanide and Os^VIII, and zero order on pentamethylene sulfide and OH^–. The effects of introduced electrolytes, K₄Fe(CN)₆, relative permittivity and temperature have also been studied. On the basis of kinetic evidence, a mechanism that involves anation of the osmium catalyst (sulfide/water interchange) followed by intramolecular proton abstraction, followed by an electron transfer step has been proposed and discussed.     

Diperiodatoargentate(III) Oxidation of D-Galactose in Absence and Presence of Anionic and Cationic Surfactants

Diperiodatoargentate(III) (DPA) was used as an oxidizing titrant in the spectrophotometric degradation of D-galactose for the first time. The kinetics is based on the reduction of silver(III) to silver(I) by D-galactose at specified experimental conditions. Effects of added [H⁺] and [periodate] have also been investigated. The premicellar environment of cetyltrimethylammonium bromide (CTAB) and sodiumdodecyl sulphate (SDS) strongly inhibits the reaction rate. The observed rate constant is strongly affected by [CTAB] and [SDS] changes for [surfactant] <cmc. Surfactant concentration range above the cmc does not influence the reaction rate. The monoperiodatoargentate(III) ions act as an active oxidant in comparison to that of DPA. A suitable mechanism involving a two-electron transfer from D-galactose to the silver(III) species has been proposed and hence a corresponding rate equation has been derived.     

Osmium(VIII) oxidation of arsenic(III) in aqueous sulphuric acid

The oxidation of As^III by Os^VIII or Os^VI in aqueous H₂SO₄ follows the rate law:

Kinetic study of ruthenium(III)-catalyzed oxidation of <Emphasis Type="SmallCaps">l</Emphasis>-alanine by diperiodatoargentate(III) in aqueous alkaline medium

The kinetics of Ru(III)-catalyzed oxidation of l-alanine (Ala) by diperiodatoargentate(III) (DPA) in alkaline medium at 25 °C and a constant ionic strength of 0.90 mol dm⁻³ was studied spectrophotometrically. The products are acetaldehyde, Ag(I), ammonia and bicarbonate. The [Ala] to [DPA] stoichiometry is 1:1. The reaction is first order in both [Ru(III)] and [DPA] and has less than unit order in both [Ala] and [alkali]. Addition of periodate has a retarding effect on the reaction. The effects of added products, ionic strength and dielectric constant of the reaction medium have been investigated. The reaction proceeds via a Ru(III)–Ala complex, which further reacts with one molecule of monoperiodatoargentate(III) in the rate-determining step. The reaction constants were calculated at different temperatures and the activation parameters have been evaluated.     

Kinetics of Hydrolysis of Diperiodatoargentate(III) Ion and its Reduction by Paracetamol

The kinetics of hydrolysis and reduction of the diperiodatoargentate(III) ion (DPA) has been studied in aqueous acidic medium spectrophotometrically. Upon dilution the silver (III) complex was found to be unstable in the presence of H₂O. Addition of [H⁺], largely increased the hydrolysis rate, whereas [OH⁻] does not have any effect. Under pseudo-first-order conditions ([paracetamol] > [DPA]), the reduction rate was very fast. Second-order conditions were used to determine the reaction rate. The reaction was acid-catalyzed and the rate decreased by the addition of periodate. The Arrhenius equation was valid for the reaction. The changes observed in the direction of the rate constant-[H⁺] profile correspond to aquation of the diperiodatoargentatate(III) complex. The proposed mechanism and the derived rate law are consistent with the observed kinetics.     

Effects of anionic and cationic micelles on the oxidation of d-dextrose by diperiodatoargentate(III)

The oxidative behavior of d-dextrose toward diperiodatoargentate(III) (DPA) has been studied in the absence and presence of anionic and cationic micelles of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), respectively. The kinetics is based on the reduction of silver(III) to silver(I) by d-dextrose under pseudo-first-order conditions. The monoperiodatoargentate(III) ions act as active oxidants in comparison to that of DPA. The reactions are first- and fractional-order dependence with respect to [DPA] and [d-dextrose], respectively. The reaction rates decrease with [H⁺] and [periodate]. The premicellar environment of SDS and CTAB strongly inhibits the reaction rate. Inhibition is due to favorable thermodynamic/electrostatic binding between the Ag(III) complex and CTAB monomer aggregates. A suitable mechanism involving a one-electron transfer (rate-determining step) from d-dextrose to the silver(III) species has been proposed. Activation parameters have been evaluated and discussed.     

Ruthenium(III) catalysed oxidation of l-leucine by a new oxidant,diperiodatoargentate(III) in aqueous alkaline medium

The kinetics of Ru(III) catalysed oxidation of l-leucine by diperiodatoargentate(III) (DPA) in alkaline medium at 298 K and a constant ionic strength of 0.60 mol dm⁻³ was studied spectrophotometrically. The oxidation products are pentanoic acid and Ag(I). The stoichiometry is [l-leucine]:[DPA] = 1:2. The reaction is of first order in Ru(III) and [DPA] and has less than unit order in both [l-leu] and [alkali]. The oxidation reaction in alkaline medium has been shown to proceed via a Ru(III)–l-leucine complex, which further reacts with one molecule of monoperiodatoargentate(III) (MPA) in a rate determining step followed by other fast steps to give the products. The main products were identified by spot test and spectral studies. The reaction constants involved in the different steps of the mechanism are calculated. The catalytic constant (K_c) was also calculated for the Ru(III) catalysed reaction at different temperatures. From the plots of log K_c versus 1/T, values of activation parameters with respect to the catalyst have been evaluated. The activation parameters with respect to the slow step of the mechanism are computed and discussed, and thermodynamic quantities are also determined. The active species of catalyst and oxidant have been identified.     

Kinetics and mechanism of <Emphasis Type="Italic">L</Emphasis>-isoleucine oxidation by alkaline diperiodatoargentate(III)

The oxidation of L-isoleucine by alkaline diperiodatoargentate(III) (DPA) at 298 K and a constant ionic strength of 0.80 mol dm⁻³ was studied spectrophotometrically. The stoichiometry is [L-isoleucine]: [DPA] = 1:2. The reaction is first order in [DPA] and has less than unit order in both [L-isoleucine] and [alkali] and retarding effect in The oxidation reaction in alkaline medium has been shown to proceed via a L-isoleucine–DPA complex, which further reacts with one molecule of DPA in a rate determining step followed by other fast steps to give the products. Spot test and IR were used to identify the main products. The reaction constants involved in the different steps of the mechanism are calculated. The activation parameters with respect to the slow step of the mechanism are computed and discussed, and thermodynamic quantities are also determined. The probable active species of oxidant have been identified. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Osmium(VIII) mediated oxidation of mercury(I) by cerium(IV) in aqueous sulphuric acid – a kinetic and mechanistic study

The oxidation of Hg^I by Ce^IV has been studied in aqueous H₂SO₄. A minute amount (10^–6 mol dm^–3) of Os^VIII is sufficient to catalyse the reaction. The active catalyst, substrate and oxidant species are H₂OsO₅, [Hg₂(SO₄)HSO₄]^– and H₃Ce(SO₄)^– ₄, respectively. Possible mechanisms are proposed and the reaction constants involved have been determined.     

Oxidation of L-phenylalanine by diperiodatoargentate(III) in aqueous alkaline medium. A Mechanistic approach

The kinetics of oxidation of L-phenylalanine (L-Phe) by diperiodatoargentate(III) (DPA) in alkaline medium at a constant ionic strength of 0.25 mol/dm⁻³ has been studied spectrophotometrically. The reaction between DPA and L-phenylalanine in alkaline medium exhibits 1: 1 stoichiometry (L-phenylalanine: DPA). The reaction shows first order in [DPA] and has less than unit order dependence each in both [L-Phe] and [Alkali] and retarding effect of [IO₄⁻] under the reaction conditions. The active species of DPA is understood to be as monoperiodatoargentate(III) (MPA). The reaction is shown to proceed via a MPA-L-Phe complex, which decomposes in a rate-determining step to give intermediates followed by a fast step to give the products. The products were identified by spot and spectroscopic studies. The reaction constants involved in the different steps of the mechanisms were calculated. The activation parameters with respect to slow step of the mechanism were computed and discussed. The thermodynamic quantities were also determined for the reaction.     

Kinetics and mechanism of metal ion catalysis: Part I. Kinetics of electron transfer from cycloheptanone in aqueous alkali

The kinetics of the electron transfer from cycloheptanone to OsO₄ in alkaline medium has been studied spectrophotometrically. The oxidation of cycloheptanone by Os^VIII, continuously regenerated by Fe(CN)^3– ₆ in alkaline medium in the 0.00123–0.01 M range, is zeroth order with respect to Fe(CN)^3– ₆ and first order with respect to Os^VIII. A suitable mechanism, based on rate data analysis, is proposed.