Inverse First-order Dependence Research Articles

Mechanistic Study of Nickel-Catalyzed Reductive Coupling of Ynoates and Aldehydes.

In this work, (1,5-hexadiene)Ni(SIPr) (SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene) is used in place of Ni(COD)2/SIPr·HBF4/KOtBu (COD = 1,5-cyclooctadiene) as a more robust catalyst for regioselective reductive coupling of ynoates and aldehydes with triethylsilane. The catalytic reaction of ethyl 3-(trimethylsilyl)propiolate and methyl 4-formylbenzoate shows first-order dependence on aldehyde and catalyst concentrations, inverse first-order dependence on [ynoate], and no dependence on [silane]. The kinetics data, coupled with deuterium-labeling experiments, support a mechanism involving dissociation of the ynoate from a catalytically dormant nickelacyclopentadiene intermediate prior to turnover-limiting formation of a catalytically active nickeladihydrofuran.

A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid

First-row metals have been a target for the development of oxygen evolution reaction (OER) catalysts because they comprise noncritical elements. We now report a comprehensive electrochemical characterization of manganese oxide (MnOx) over a wide pH range, and establish MnOx as a functionally stable OER catalyst owing to self-healing, is derived from MnOx redeposition that offsets catalyst dissolution during turnover. To study this process in detail, the oxygen evolution mechanism of MnOx was investigated electrokinetically over a pH range spanning acidic, neutral, and alkaline conditions. In the alkaline pH regime, a ∼60 mV/decade Tafel slope and inverse first-order dependence on proton concentration were observed, whereas the OER acidic pH regime exhibited a quasi-infinite Tafel slope and zeroth-order dependence on proton concentration. The results reflect two competing mechanisms: a one-electron one-proton PCET pathway that is dominant under alkaline conditions and a Mn(3+) disproportionation process, which predominates under acidic conditions. Reconciling the rate laws of these two OER pathways with that of MnOx electrodeposition elucidates the self-healing characteristics of these catalyst films. The intersection of the kinetic profile of deposition and that of water oxidation as a function of pH defines the region of kinetic stability for MnOx and importantly establishes that a non-noble metal oxide OER catalyst may be operated in acid by exploiting a self-healing process.

Inhibitory Effect of Ethylene in Ene–Yne Metathesis: The Case for Ruthenacyclobutane Resting States

Reaction kinetics and mechanistic studies for ethylene-internal alkyne metathesis promoted by the phosphine-free initiator Ru1 (Piers's catalyst) is described. The kinetic order of reactants and catalyst was determined. The effect of ethylene was studied at different solution concentrations using ethylene gas mixtures applied at constant pressure. Unlike earlier studies with the second-generation Grubbs complex, ethylene was found to show an inverse first-order rate dependence. Under catalytic conditions, a ruthenacyclobutane intermediate was observed by proton NMR spectroscopy at low temperature. Combined with the kinetic study, these data suggest a catalytic cycle involving a reactive L(n)Ru═CH2 species in equilibrium with ethylene to form a ruthenacyclobutane, a catalyst resting state. Rates were determined for a variety of internal alkynes of varying substitution. Also, at low ethylene pressures, preparative syntheses of several 2,3-disubstituted 1,3-butadienes were achieved. Using the kinetic method, several phosphine-free inhibitors were examined for their ability to promote ethylene-alkyne metathesis and to guide selection of the optimal catalyst.

An electrochemical study of H 2O 2 decomposition on single-phase γ-FeOOH films

The electrochemical reduction and oxidation kinetics of hydrogen peroxide on γ-FeOOH films chemically deposited on indium tin oxide substrates were studied over the pH range of 9.2–12.6 and the H 2O 2 concentration range of 10 −4 to 10 −2 mol dm −3. The Tafel slopes for H 2O 2 reduction and oxidation obtained from polarization measurements are 106 ± 4 and 93 ± 15 mV dec −1, respectively, independent of pH and the concentration of H 2O 2. Both the reduction and oxidation of H 2O 2 on γ-FeOOH have a first-order dependence on the concentration of molecular H 2O 2. However, for the pH dependence, the reduction has an inverse first-order dependence, whereas the oxidation has a first-order dependence, on the concentration of OH −. For both cases the electroactive species is the molecular H 2O 2, not its base form, HO 2 −. Based on these observations, reaction kinetic mechanisms are proposed which involve adsorbed radical intermediates; HO OH − and HO for the reduction, and HO 2 H +, HO 2 , and O 2 − for the oxidation. These intermediates are assumed to be in linear adsorption equilibria with OH − and H + in the bulk aqueous phase, respectively, giving the observed pH dependences. The rate-determining step is the reduction or oxidation of the adsorbed H 2O 2 to the corresponding intermediates, a reaction step which involves the use of Fe III/Fe II sites in the γ-FeOOH surface as an electron donor–acceptor relay. The rate constant for the H 2O 2 decomposition on γ-FeOOH determined from the oxidation and reduction of Tafel lines is very low, indicating that the γ-FeOOH surface is a very poor catalyst for H 2O 2 decomposition.

Kinetics and Mechanism of Iridium-Catalyzed Dehydrogenation of Primary Amines to Nitriles

A family of primary amines has been catalytically dehydrogenated to nitriles by [C6H3-2,6-(OPtBu2)2]IrH2 using tert-butylethylene as a hydrogen acceptor. The catalytic mechanism has been investigated by a series of kinetic and isotopic labeling experiments, in addition to the isolation of intermediates along the reaction pathway. The turnover frequency exhibits a first-order dependence on the concentration of amine, an inverse first-order dependence on nitrile, and a zero-order dependence on tert-butylethylene. The mechanism of amine dehydrogenation is proposed to proceed from an iridium(I) nitrile complex, the catalyst resting state, via two preturnover limiting equilibria, followed by a slow β-hydride elimination event from a transient iridium(III) amido hydride species. Measuring rate constants for the stoichiometric dehydrogenation of isobutylamine over a 41 °C temperature range established activation parameters for β-hydride elimination of ΔH⧧ 24.8(9) kcal/mol and ΔS⧧ = −10(2) eu. Dehydrogenation of ...

Kinetics and mechanism of reduction of 2,8-dimethyl-1,9-diphenyl-3,7-diaza-2,7-nonadiene-1,9-dione dioximatocopper(III) ([CuIII A]+) and 4,6,9-trimethyl-5,8-diaza-4,8-dodecadiene-2,3,10,11-tetraone 3,10-dioximatocopper(III) ([CuIIIB]+) by p-methoxyphenol, 1,2-dihydroxybenzene, 1,4-dihydroxybenzene and some of its derivatives

The kinetics of reduction of two copper(III)-imine-oxime complexes, [CuIIIA]+ and [CuIIIB]+, (H2A and H2B=2,8-dimethyl-1,9-diphenyl-3,7-nonadiene-1,9-dione dioxime and 4,6,9-trimethyl-5,8-diaza-4,8-dodecadiene-2,3,10,11-tetraone 3,10-dioxime respectively) by hydroquinone (H2Q), 2-methylhydroquinone (MH2Q), 2-chlorohydroquinone (ClH2Q), catechol (H2Cat) and p-methoxyphenol (pMHP) have been examined in aqueous acidic solution. Under fixed reaction conditions, the kinetics display first-order dependence on each oxidant and reductant. The pH-dependence is complex for the reduction of [CuIIIA]+, since both the copper(III) complex and the reductants undergo protonation–deprotonation equilibria. In the lower pH range, the second-order rate constant, k2, decreases with increasing pH. In the higher pH range, k2 increases with increasing pH. In the lower pH range the most important oxidant is [CuIIIHA]2+, whereas, in the higher pH range the most important reactants are deprotonated reductants. However for H2Cat, as was observed before, two reaction pathways seem to operate in the high pH range. In one pathway, HCat− seems to be involved; whereas, in the other pathway Cat2− seems to be the reactive species. Doubly deprotonated catechol, Cat2−, is very unlikely to be formed at pH ≤ 5. It was therefore necessary to invoke a strong interaction between [CuIIIA]+ and HCat− followed by loss of the second proton. The pH dependence for the reduction of [CuIIIB]+ is less complex. Thus H2Q and MH2Q showed no pH dependence up to pH ∼ 4.60, whereas ClH2Q, pMHP and H2Cat displayed an inverse first-order dependence on [H+]. Observed rate constants showing first-order dependence and inverse first-order dependence on [H+] correlate reasonably well with those calculated using the Marcus equation. The reaction path involving Cat2− is believed to proceed by an inner-sphere mechanism. The agreement between the calculated and observed values for the [CuIIIA]+ complex is lower than was found for the [CuIIIA1]+(A1=3,9-diethyl-4,8-diaza-3,8-undeca-2,10-dionedioxime). It seems that the replacement of methyl groups in the latter complex by phenyl groups in the former complex causes both electronic and steric effects, and both effects seem to retard electron transfer. The electronic effect is readily seen in the decrease of the reduction potential of [CuIIIA]+ (E0=1.09 V) compared to the reduction potential of [CuIIIA1]+(E0=1.16 V) and thus making the former a weaker oxidant. The self-exchange rate constant (5 × 105 M−1 s−1) estimated for complexes with type H2A ligands seem to work well for complexes with type H2B ligands. This situation is supported by the findings of a fairly constant value for the self-exchange rate constant for CuIII/II–peptide complexes with varying substituents.

Oxidation of 3‐(3,4‐dihydroxy phenyl)‐l‐alanine (levodopa) and 3‐(3,4‐dihydroxy phenyl)‐2‐methyl‐l‐alanine (methyl dopa) by manganese(III) in pyrophosphate media: Kinetic and mechanistic study

AbstractManganese(III) (Mn(III)) has been stabilized in weakly acidic solution by means of pyrophosphate and the nature of the complex was elucidated spectrophotometrically. Stoichiometry of Mn(III)‐oxidation of levodopa and methyl dopa in pyrophosphate medium was established in the pH range 2.5–4.0 by iodometric and spectrophotometric methods. The reaction shows a distinct variation in kinetic order with respect to [Mn(III)], a first‐order dependence in the pH range 1.9–2.6, decreasing to fractional order above pH 3. Other common features include first‐order dependence on [dopa], positive fractional order dependence on [H+], and inverse first‐order dependence on [Mn(III)] in the pH range studied. The effects of varying ionic strength and solvent composition were studied. Added ions such as SO42− and ClO4− alter the reaction rate, probably due to the change in the formal redox potential of Mn(III)–Mn(II) couple because of the changes in coordination environment of the oxidizing species. Evidence for the transient existence of the free radical intermediate is given. Cyclic voltametric sensing of levodopa and methyl dopa has ruled out the formation of dopaquinones as oxidation products in the pH range studied. Activation parameters have been evaluated using the Arrhenius and Erying plots. Mechanisms consistent with the kinetic data have been proposed and discussed. These studies are expected to throw some light on dopa metabolism. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 449–457, 2001

Mechanistic Studies of the Palladium-Catalyzed Amination of Aryl Halides and the Oxidative Addition of Aryl Bromides to Pd(BINAP)2 and Pd(DPPF)2: An Unusual Case of Zero-Order Kinetic Behavior and Product Inhibition

Mechanistic studies of the amination of aryl bromides catalyzed by palladium complexes containing the chelating phosphines BINAP and DPPF are reported. The coupling of primary alkyl- and arylamines, secondary cyclic alkylamines, and secondary arylalkylamines with bromoarenes in the presence of stoichiometric base and Pd(BINAP)2 (1a) as catalyst, and the reaction of aniline with 4-Br-C6H4-t-Bu in the presence of base catalyzed by Pd(DPPF)2 (2), were studied. The stoichiometric oxidative additions of PhBr to 1a and to 2 were turnover limiting, and kinetic studies were also conducted on this individual step. The stoichiometric oxidative addition of PhBr to 1a showed an inverse first-order dependence on added ligand when the PhBr concentration was low but depended solely on the rate of chelating ligand dissociation at high [PhBr]. There was no measurable solvent effect. In addition, the rates were indistinguishable in the presence and in the absence of amines and salts that are present in the catalytic aminat...

Study of interactive free-energy relationships on ruthenium(III) catalyzed oxidation of phenyl styryl ketone and its substituted analogues by V(V) in acid medium

Kinetics of Ru(III) catalyzed oxidation of phenyl styryl ketone (PSK) and its substituted analogues by V(V) has been investigated in aqueous acetic acid-sulphuric acid medium in the temperature range 298–313 K. First-order dependence each on [V(V)], [PSK], [Ru(III)] was observed. Inverse first-order dependence was observed for [V(IV)]. The rate decreased with the increase in dielectric constant (D) of the medium. The rates were enhanced by electron-donating substituents in both the phenyl rings and decreased by electron-withdrawing substituents. Linear Hammett's plots were obtained for various substituents in benzaldehyde moiety of PSK for a given substituent in acetophenone moiety and vice versa. The mechanism proposed envisages formation of Ru(IV) from V(V) + Ru(III) reaction followed by the attack on by Ru(IV). Applicability of interactive free-energy relationship has been tested. The cross-interaction constants qx and qy have been determined at different temperatures and possible interpretations discussed. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 581–588, 2000

Ruthenium(III) Catalyzed Oxidation of Aliphatic Primary Amines by Bromamine-T in Hydrochloric Acid Medium: A Kinetic Study

The kinetics of the ruthenium(III) catalyzed oxidation of the aliphatic primary amines n-propylamine, n-butylamine and isoamylamine, by sodium N-bromo-p-toluenesulfonamide or bromamine-T (BAT) in HCl medium has been studied in the temperature range 298–313 K. The reaction rate shows a first-order dependence each on BAT, amine, and ruthenium(III). The reaction also shows an inverse fractional- and inverse first-order dependence on acid at low and high [HCl] ranges, respectively. Added halide ions and p-toluenesulfonamide (reduction product of BAT), and variation of ionic strength of the solvent medium have no effect on the rate. The activation parameters have been evaluated. Mechanisms consistent with the kinetic data have been proposed. The protonation constant of monobromamine-T has been evaluated to be 29 ± 2. A Taft LFE relationship is observed for the ruthenium(III) catalyzed reaction with ρ* = −4.6 indicating that the electron donating groups enhance the reaction rate. An isokinetic relationship is observed with β = 355 K indicating that enthalpy factors control the rate.

Kinetic role of tert-amines in the osmium tetroxide catalyzed trimethylamine N-oxide dihydroxylation of cyclohexene

Effect of some tert-amines on the catalytic osmium tetroxide dihydroxylation of cyclohexene in aqueous tert-butyl alcohol has been investigated All amines have been found to retard the catalysis greatly and beyond a definite concentration of amine, the rate reaches a minimal and remains constant. The oxidation of cyclohexene is inhibited by pyridine, 2.2′-bipyridyl and DABCO with an inverse first-order dependence whereas inhibition by triphenylamine NN-diethylaniline, picoline pyrazine hexamethylenetetraamine and TMEDA shows an inverse partial order dependence. The inolvement of dioxomonoglycolatoosmium(VI) esters and their monoamine adducts in the rate determining oxidation step was established by the linear plots of 1/Δk2 vs. 1/[L] where Δk2 is the decrease in the second-order rate constant in the presence of [L] concentration of tert-amine. The ligand-accelerated or ligand-decelerated catalysis of tert-amines in the catalytic osmium tetraoxide dihydroxylation of alkenes may vary depending on the secondary oxidant on the alkene and on the structure and concentration of the tert-amine. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 359–366, 1997

Kinetics of oxidation of certain pyrimidines by Ce(IV)

The kinetics of the oxidation of certain biologically important pyrimidine bases (Uracil, Thymine, and 6-Methyluracil) by Ce(IV) in aqueous H2SO4 has been investigated. A first-order dependence of rate each on [Ce(IV)], [pyrimidine], and an inverse first-order dependence on [H2SO4] has been observed. Rate and activation parameters for the oxidation of these pyrimidines have been computed. A suitable rate law and a mechanism consistent with the kinetic observations and product analysis have been proposed. © 1996 John Wiley & Sons, Inc.

Catalytic epoxidation of olefins by an effective monooxygenase system: Ru(III)—EDTA—ascorbate—O 2

The Ru(III)—EDTA—ascorbate—molecular oxygen system was found to be an efficient catalyst for the epoxidation of olefins in mixed aqueous acidic solvent, water:dioxane (1:2) (v/v). Detailed kinetic, catalytic and equilibrium studies of the epoxidation of cis-cyclooctene, styrene and trans-stilbene with molecular oxygen catalyzed by Ru(III)—EDTA—ascorbate complex are reported at 30 °C and μ=0.1 M KNO 3. The Kinetic parameters of the complicated epoxidation reactions were found to follow first-order kinetics with respect to substrate, ascorbic acid, catalyst and molecular oxygen concentrations. An inverse first-order dependence in hydrogen ion concentration was observed for all the epoxidation reactions studied. A monooxygenase model mechanism is proposed for the epoxidation reaction, in which the transfer of one of the oxygen atoms to olefin and the reduction of the other oxygen atom to water take place by heterolytic cleavage of the OO bond. The reactivity of the olefins was observed to decrease in the order: cis-cyclooctene > styrene > trans-stilbene.

Kinetics and mechanism of back-extraction: selected palladium extraction systems

The equilibrium and kinetics of back-extraction (stripping) of palladium originally extracted as PdCl 2− 4 from the chloroform extracts obtained with 1-(2-pyridylazo)-2-naphthol (PAN), 7-(4-ethyl-1-methyloctyl)quinolin-8-ol (Kelex 100) or dioctyl sulfide (R 2S) were investigated. Replacement of chloride in extracted species by thiocyanate occurs prior to back-extraction. The back-extraction equilibria have been described by Pd(SCN) 2(R 2S) 2(o) + 2SCN − K BX 1 Pd(SCN) 2− 4 + 2R 2S(o), with K BX 1=10 −(1.16±0.05), and Pd(SCN)PAN(o) + 3SCN − + H + K BX 2 Pd(SCN) 2− 4 + PAN(o), with K BX 2=10 4.89±0.06. The rate of stripping from PdPAN and PdKelex 100 displayed an inverse first-order dependence on the solution pH, a second-order dependence on the thiocyanate concentration and was zero order in both the chloride and the organic phase chelate concentration. More complicated kinetics were observed for palladium stripping in the dioctyl sulfide system. In all systems, the enhancement in stripping rate parallels the size of the “ trans effect”.

The study of kinetics and mechanism of ligand substitution reactions of [PdPar(OH)] − and [Cd(Par) 2] 2− (Par=4-(2-pyridylazo)resorcinol) with cyanide ions by stopped flow technique

The reaction of [PdPar(OH)] − with cyanide ion has been followed at 510 nm (λ max of [PdPar(OH)] −) while the reaction of [Cd(Par) 2] 2− with cyanide ions has been followed at 495 nm (λ max of [Cd(Par)] or [Cd(Par) 2]) by the stopped flow technique at pH= 11.0 ± 0.02, I=0.1 M (NaClO 4) and temperature= 25 ± 0.1 °C under pseudo-first-order conditions taking cyanide in large excess. The reaction between [PdPar(OH)] − and cyanide ion follows first-order kinetics in both [CN −] and [PdPar(OH)] −. The reactions of [Cd(Par)] and [Cd(Par) 2] with cyanide ions also follow first-order kinetics in each reactant at higher cyanide concentrations. The reactions of [PdPar(OH)] and [Cd(Par) 2] with cyanide ions exhibit zero-order dependences in [CN −] at extremely low cyanide concentrations. The reverse reaction between [Pd(CN) 4] 2− and Par, and [Cd(CN) 4] 2− and Par follow first-order kinetics in each reactant and an inverse first-order dependence in cyanide ion. It is inferred that the fourth step is the rate determining one in a proposed five step mechanistic scheme. Additional studies viz. effect of ionic strength, pH dependence and temperature dependence were also conducted and support the proposed mechanistic scheme.

Thermodynamics of homogeneous oxidations: Part I. Thermodynamics of oxidation of ascorbic acid and cyclohexane by the Ru(III) analogue of the model peroxidase system: Ru(III)-EDTA-ascorbate-H 2O 2

The homogeneous oxidation of ascorbic acid to dehydroascorbic acid and cyclohexane to cyclohexanol by hydrogen peroxide catalyzed by Ru(III)-EDTA and Ru(III)-EDTA-ascorbate complexes, respectively, has been investigated in the temperature range 288–313 K at μ = 0.1 M KNO 3. In Ru(III)-EDTA catalyzed oxidation of ascorbic acid, an overall secondorder kinetics, first order in both [Ru(III)-EDTA] and [ascorbic acid] and zero order in hydrogen peroxide concentration, was observed in the temperature range studied. The oxidation of cyclohexane to cyclohexanol exhibits first-order dependence in [Ru(III)-EDTA], [ascorbic acid] and [H 2O 2] concentration. The dependence in [cyclohexane] was found to be fractional order. An inverse first-order dependence in hydrogen ion concentration was observed for both reactions. The activation parameters for the oxidation steps corresponding to the rate constants and thermodynamic parameters for the formation of kinetic intermediate complexes corresponding to the equilibrium constants were computed, and the mechanistic implications of the oxidations catalyzed by Ru(III)-EDTA and its thermodynamics are discussed. The thermodynamics of the Ru(III)-EDTA-ascorbate system is compared with that of the Fe(III)-EDTA-ascorbate system.

Kinetic studies on ph dependent oxidase vs. oxygenase mechanism for Ru(III)-EDTA catalyzed oxidation of allyl alcohol by molecular oxygen in aqueous acidic medium

The kinetics of the oxidation of allyl alcohol by molecular oxygen catalyzed by Ru(III)-EDTA complex is investigated in the pH range 1.0–3.0 at 35 °C (μ = 0.1 M) in aqueous medium. In the pH range studied, the reaction is first order with respect to catalyst and substrate concentrations. An inverse first-order dependence on H + concentration is observed below pH 2.0, while the rate is independent of [H +] above pH 2.0. The rate of reaction is independent of O 2 concentration at pH 1.5 and first order with respect to O 2 concentration at pH 2.5. Mechanisms for the oxidation of allyl alcohol catalyzed by Ru(III)-EDTA have been proposed in the pH ranges 1.0–2.0 and 2.0–3.0, wherein the reactions are shown to proceed through oxidase and oxygenase routes, respectively. The rate laws for these routes are derived and the corresponding rate constants reported.

Kinetics and mechanism of oxidation of thiosemicarbazide in the pure state and in its metal complex by chloramine T, bromamine T, and dichloramine T in acid medium

The kinetics and mechanism of oxidation of thiosemicarbazide (TSC) in the pure state and in its zinc metal complex by chloramine T(CAT) and bromamine T(BAT) in aqueous perchloric acid medium and by dichloramine T(DCT) in 1 : 1 (v/v) water–methanol medium have been investigated. The rate followed first-order kinetics in [oxidant] and fractional order in [TSC] in both the pure state and in the metal complex with all the oxidants. With chloramine T, the rate had an inverse first-order and inverse fractional order dependence in [H+] for two ranges of [HClO4], 0.01–0.05 and 0.05–0.03 mol dm–3, respectively. The rate had inverse fractional and inverse first-order dependence in [H+] with BAT and DCT respectively over the entire [HClO4] range (0.01–0.2 mol dm–3). Addition of the reduced product of the oxidants (toluene-p-sulphonamide) had no significant effect on the rate with all the oxidants. But the increase of ionic strength of the medium decreased the rate. Decrease of dielectric constant of the reaction medium by adding methanol increased the rate indicating positive ion–dipole interactions in the rate-limiting steps. The coefficients of the rate-limiting steps have been computed at different temperatures with all the oxidants and these constants were used to calculate the activation parameters from the Arrhenius plots. The oxidation process under the conditions employed has been shown to proceed by two paths, one involving the direct interaction of monochloramine T(RNHCl) or dichloramine T(RNCl2) with TSC to give an N-chloro intermediate of the substrate which subsequently undergoes disproportionation and further reactions with the oxidant molecules in fast steps to give the final products, and the other involving the interaction of H2OCl+ or H2OBr+, produced from the disproportionation of oxidants, with the substrate to yield the products. The mechanisms proposed and the rate laws deduced are in conformity with the observed kinetics. The rate constants predicted by the derived rate laws as [TSC], [Complex], or [H+] vary are in good agreement with the experimental rate constants thus justifying the rate laws and hence the suggested mechanistic schemes.

Oxidation of thiourea and thioacetamide by octacyanomolybdate(v) anion in perchloric acid

The kinetics of the reduction of octacyanomolybdate(V) anion by thiourea and thioacetamide have been studied in aqueous HClO4 at constant ionic strengthI=0.10 mol dm−3 (NaClO4). The rate of oxidation of these substrates by the oxidant shows a first order dependence in both the oxidant and the substrates and while the thiourea system exhibits an inverse first-order dependence on [H+] that of thioacetamide is found to be first-order in [H+]. The variation observed in [H+] dependences in these reactions is attributed to the nature of the thiourea in the pH range used in this study and the inductive effect of the methyl group in thioacetamide. A mechanistic interpretation of these observations is advanced.

Ru(III)-EDTA catalyzed oxidation of ascorbic acid by hydrogen peroxide in aqueous solution

The kinetics of oxidation of ascorbic acid to dehydroascorbic acid by hydrogen peroxide catalyzed by ethylenediaminetetraacetatoruthenate(III) has been studied over the pH range 1.50 – 2.50, at 30°C and μ = 0.1 M KNO 3. The reaction has a first-order dependence on ascorbic acid and Ru(III)-EDTA concentrations, an inverse first-order dependence on hydrogen ion concentration, and is independent of hydrogen peroxide concentration in the pH range studied. A mechanism has been proposed in which ascorbate anion forms a kinetic intermediate with the catalyst in a pre-equilibrium step. Ruthenium(III) is reduced to ruthenium(II) in a rate-determining step and is reoxidized with hydrogen peroxide back to the Ru(III) complex in a fast step.