Kinetics and mechanism of the redox reaction between malachite green and iron(III) in aqueous and micellar media

The kinetics of the oxidation of malachite green (MG⁺) by Fe(III) were investigated spectrophotometrically by monitoring the absorbance change at 618 nm in aqueous and micellar media at a temperature range 20–40 °C; I = 0.10 mol dm^?3 for [H⁺] range (2.50–15.00) × 10^?4 mol dm^?3. The rate of reaction increases with increasing [H⁺]. The reaction was carried out under pseudo-first-order conditions by taking the [Fe(III)] (>10-fold) the [MG⁺]. A mechanism of the reaction between malachite green and Fe(III) is proposed, and the rate equation derived from the mechanism was consistent with the experimental rate law as follows: Rate = (k ₄ + K ₁ k ₅[H⁺]) [MG⁺][Fe(III)]. The effect of surfactants, such as cetyltrimethylammonium bromide (CTAB, a cationic surfactant) and sodium dodecylsulfate (SDS, an anionic surfactant), on the reaction rate has been studied. CTAB has no effect on the rate of reaction while SDS inhibits it. Also, the effect of ligands on the reaction rate has been investigated. It is proposed that electron transfer proceeds through an outer-sphere mechanism. The enthalpy and the entropy of the activation were calculated using the transition state theory equation.     

Kinetics and mechanism of the reaction of aquachromium(III) with l-ascorbic acid in aqueous media

The kinetics of the reaction of l-ascorbic acid (H2A) with aquachromium(III) has been studied over the range 0.04 [H2A]T 0.16 mol dm–3, 3.5 pH 6.0, 25.0 t 50.0C, 0.03 I 1.0 mol dm–3 (KNO3) and 0% [MeOH] 20% (v/v). The reaction takes place via an outer-sphere association between Cr3+/CrOH2+ and H2A, followed by transformation of the outer- into an inner-sphere complex by slow interchange. The anation rate constants for Cr3+ and CrOH2+ species (k1 and k2) at 25C and I=1.0moldm–3 (KNO3) are 1.68×10–4 and 2.14×10–4s–1, respectively. H and S for the k1 and k2 paths are 27.0±2kJmol–1, –227±5J K–1mol–1 and 57.2± 4kJmol–1, –123±14JK–1mol–1, respectively. Anation of Cr(H2O)3+6 and Cr(H2O)5OH2+ follow an Ia path.     

Kinetics and mechanism of electron transfer from phenolate anion to hexacyanoferrate (III) in aqueous alkaline media

A kinetic reinvestigation of the title redox system in aqueous alkaline media at 35°C and an ionic strength of 0.5 mol dm^?3 shows that the reaction follows a pseudosecond-order Fe(CN) disappearance. While varying [phenol]₀ and [OH^?] exhibit a linear influence on the pseudo-second-order rate constant, varying[Fe(CN)]₀ and [Fe(CN)]₀, initially taken, have a complicated inhibitory effect on the same. The major phenoloxidation products isolated under a chosen condition are 2,2′- and 4,4′- dihydroxydiphenyl. Results are interpreted in terms of a probable mechanism which envisages a reversible formation, by the first one-electron transfer, of a reactive phenoxy radical (PhO^˙) which on the second one-electron transfer forms a less reactive ion-pair intermediate (stabilized by the Fe(CN) produced) to decompose rate-determiningly to phenoxonium cation (PhO⁺) and Fe(CN), the product-formation steps being very rapid and kinetically indistinguishable.     

Kinetics and mechanism of the reaction between chromium(III) and picolinic acid in weak acidic aqueous solution

Abstract  

The interaction between chromium(III) and picolinic acid in weak acid aqueous solution was studied, resulting in the formation of a complex upon substitution of water molecules in the chromium(III) coordination sphere. Experimental results show that the reaction takes place in multiple steps. The first step is the formation of an ion pair, the second step (two consecutive steps) is the slow one corresponding to substitution of the first water molecule from the chromium aqueous complex coordination sphere by a picolinic acid molecule via oxygen atom of the carboxylic acid group and substitution of the second water molecule via nitrogen of the pyridine ring forming an 1:1 complex. Both consecutive steps were independent of chromium concentration. The rate constants of the 1st and 2nd consecutive steps were increased by increasing picolinic acid concentration. The corresponding activation parameters are ∆H _1obs ^* = 28.4 ± 4 kJ mol⁻¹, ∆S _1obs ^* = −202 ± 26 J K⁻¹ mol⁻¹, ∆H _2obs ^* = 39.6 ± 5 kJ mol⁻¹, and ∆S _2obs ^* = −175 ± 19 J K⁻¹ mol⁻¹. The third step is fast, corresponding to formation of the final complex [Cr(pic)₃]. The logarithms of the formation constants of 1:1 and 1:3 complexes were found to be 1.724 and 4.274, respectively.     

Kinetics and mechanism of the ligand substitution reaction of aquodiethylenetriaminepentaacetatoruthenate (III) in aqueous solution

The kinetics of ligand substitution reactions of [Ru(H₂dtpa) (H₂O)] (2) (H₂dtpa=diprotonated diethylenetriaminepentaacetic acid) were studied as a function of ligand (L) concentration, pH (2.5–8.0) and temperature (30–45 °C) at 0.2 M ionic strength. The equilibrium constants for the formation of mixed ligand complex [Ru^III(dtpa) (L)] (L=2-mercaptopyrimidine, cysteine) and the distribution of various species in solution in the pH range of 2.5–8.0 were computed from potentiometric results. [Ru(H₂dtpa) (H₂O)] ( H₂dtpa= ) , pH (2,5–8,0) (30–45°C) 0,2 M. [Ru^III(dtpa) (L)] ( L=2-, ) pH=2,5–8,0.     

Kinetics and mechanism of a trans-tetraazamacrocyclic chromium(III) complex oxidation by hexacyanoferrate(III) in strongly alkaline media

Oxidation of the trans-[Cr(cyca)(OH)₂]⁺ complex, where cyca = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, by [Fe(CN)₆ ]^3- ion in strongly alkaline media, leading to [Cr^V O(cyca_ox )]³⁺ ion, has been studied using electronic and e.p.r. spectroscopy. The kinetics of the Cr^III → Cr^IV transformation have been studied using a large excess of the reductant and OH^- ion over the oxidant. The reaction is a second order process: first order in [Cr^III] and [Fe^III] at constant [OH^-]. The second order rate constant is higher than linearly dependent on the OH^- concentration. The mechanism of the reaction has been discussed. A relatively inert intermediate chromium(V) species was detected based on characteristic bands in the visible region and the e.p.r. signal at g_iso = 1.987 for the systems where an excess of oxidant was used. The hyperfine structure of the main e.p.r. signal is consistent with the d¹ -electron interactions with four equivalent nitrogen nuclei and [Cr^V = O(cyca_ox)]³⁺ formula, where cyca_ox = oxidized cyca, can be postulated for the intermediate Cr^V complex.     

Kinetics and mechanisms of the oxidation of the octacyanoniobate(III)ion by oxyanions in alkaline aqueous media

Summary The kinetics and mechanisms of the oxidation of Nb(CN) _inf8 ^sup5– by the oxyanions S₂O _inf8 ^sup2– , BrO _inf3 ^sup– , and IO _inf4 ^sup– have been investigated in alkaline aqueous media (pH 12). The second-order rate constant for the electron transfer reaction between Nb(CN) _inf8 ^sup5– and S₂O _inf8 ^sup2– at 25.0 °C, I = 0.36m (K⁺), is 11.1± 0.3 m ^–1 s ^–1 with H = 30 ± 2kJmol^–1 and S = - 125 + 7JK^–1 mol^–1. The rate constant for the oxidation of Nb(CN) _inf8 ^sup5– by BrO _inf3 ^sup– at 25.0 °C, I = 0.20m (Na⁺), is 2.39 ± 0.08m ^–1 s ^–1 with H = 28 ± 2kJmol^–1 and S = -139 ± 7JK^–1mol^–1. The oxidation of Nb(CN) _inf8 ^sup5– by IO _inf4 ^sup– proceeds by two parallel pathways involving the monomeric IO _inf4 ^sup– ion and the hydrated dimer H₂I₂O _inf10 ^sup4– . The second-order rate constant for the oxidation of Nb(CN) _inf8 ^sup5– by monomeric IO _inf4 ^sup– at 5.0 °C, I = 0.050m (Na⁺), is (3.3 ± 0.6) × 10³ m ^–1 s ^–1 with H = 75 ± 6 kJ mol^–1 and S = 94 ± 15 J K^–1 mol^–1, while the rate constant for the oxidation by H₂I₂O _inf10 ^sup4– is (1.8 ± 0.1) × 10³ m ^–1 s ^–1 with H = 97 ± 5 kJ mol^–1 and S = 166 ± 16 J K^–1 mol^–1 under the same reaction conditions. The rate constants for each of the oxidants employed display specific cation catalysis with the order of increasing rate constants: Li⁺ < Na+ < NH _inf4 ^sup+ < K⁺ < Rb⁺ < Cs⁺, in the same direction as the electronic polarizability of the cations. The results are discussed in terms of the outer-sphere electron-transfer processes and compared with the corresponding data and mechanisms reported for other metal-cyano reductants.     

Kinetics and mechanism of the reaction between thiourea and iodate in unbuffered medium

The significances of thiourea and its derivativesnow spill into nonlinear dynamics in chemistry. A re-markable variety of complex dynamical phenomena,including oscillation, oligooscillation, variable stoichio-metries, autoinhibition, autocatalysis, bistability andbirhythmicity, traveling wave and chemical chaos,have been known in thiourea-based reaction systemsfor some time[1—7]. Also, a series of researches on thekinetics and mechanisms of the oxidations of thiourea,thiourea derivatives and …     

Kinetics and mechanism of the reaction of ozone with phenol in alkaline media

The kinetics of the reaction of O₃ with PhOH in alkaline medium has been studied. The rate of oxidation of phenol by ozone is directly proportional to the concentrations of reactants and increases in a complex manner with increase in alkali content in aqueous solution. The composition of the reaction products was investigated and it was found that dimers and oligomers of phenoxyl radicals predominate in alkaline medium. A mechanism is proposed for the process.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 320–324, February, 1991.     

Kinetics and mechanism of the oxidation of hydrogen peroxide by the octacyanotungstate(V) ion in alkaline aqueous media

Summary The oxidation of H₂O₂ by [W(CN)₈]^3– has been studied in aqueous media between pH 7.87 and 12.10 using both conventional and stopped-flow spectrophotometry. The reaction proceeds without generation of free radicals. The experimental overall rate law, , strongly suggests two types of mechanisms. The first pathway, characterized by the pH-dependent rate constant k _s, given by , involves the formation of [W(CN)₈· H₂O₂]^3–, [W(CN)₈· H₂O₂·W(CN)₈]^6– and [W(CN)₈· HO]^3– intermediates in rapid pre-equilibria steps, and is followed by a one-electron transfer step involving [W(CN)₈·HO]^3– (k _a) and its conjugate base [W(CN)₈·O]^4– (k _b). At 25 °C, I = 0.20 m (NaCl), the rate constant with H _a =40±6kJmol^–1 and S _a =–151±22JK^–1mol^–1; the rate constant with H _b =36±1kJmol^–1 and S _b =–136±2JK^–1mol^–1 at 25 °C, I = 0.20 m (NaCl); the acid dissociation constant of [W(CN)₈·HO]^3–, K ₅ =(5.9±1.7)×10^–10 m, with and is the first acid dissociation constant of H₂O₂. The second pathway, with rate constant, k _f, involves the formation of [W(CN)₈· HO₂]^4– and is followed by a formal two-electron redox process with [W(CN)₈]^3–. The pH-dependent rate constant, k _f, is given by . The rate constant k ₇ =23±6m ^–1 s ^–1 with and at 25°C, I = 0.20 m (NaCl).     

The reaction of p-benzenediols with Co(III) in aqueous perchlorate media. Kinetics of oxidation

The kinetics of redox reactions between Co(III) and a series of 1,4-benzenediol derivatives has been investigated in aqueous perchlorate media by means of a stopped-flow technique. Reaction rates dependences on oxidant, organic substrate (S), and acidity, were found to conform with: $\text{rate}\text{= (k + k′[}\text{H}{}^{\mathrm{+}}\text{]}{}^{\mathrm{-1}}\text{)[}\text{Co(III)}\text{][S]}$ for all the investigated compounds. Substituents effects on the rate of the single reaction pathways are discussed with reference to Co(III) reactive species and reaction mechanisms.     

Kinetics and mechanism of dissociation of oxalatobis(phenanthroline)cobalt(III) ion in aqueous hydrochloric acid media

The kinetics of dissociation of oxalatobis(phenanthroline)cobalt(III) ion into cis-diaquobis(phenanthroline)cobalt(III) ion in aqueous HClKCl media have been studied. The rate is first-order with respect to acid concentration (1–2 M) with a specific rate constant, k_H = 6.2 × 10⁻⁴M⁻¹min⁻¹ at 75°C (μ, 2 M); ΔH^≠ and ΔS^≠ values are 22.1 kcal mole⁻¹ and −18.7 e.u., respectively. The magnitude of the ΔS^≠ value appears consistent with a dissociation mechanism involving reaction of the conjugate acid form of the complex (S_N 1CA mechanism).     

Kinetics and mechanism of oxidation of xylitol and galactitol by hexacyanoferrate(III) ion in aqueous alkaline medium

Kinetics of oxidation of xylitol and galactitol by hexacyanoferrate(III) ion in aqueous alkaline medium is reported. The reaction rate is of first order with respect to hexacyanoferrate(III) in each substrate. The reaction is first order at lower concentrations of xylitol and galactitol and tends towards zero order as the concentration increases. Similarly first order kinetics was obtained with respect to hydroxide ion at lower concentrations and tends to lower order at higher concentration in the oxidation of xylitol; in the oxidation of galactitol the reaction is first order with respect to hydroxide ion even up to manyfold variation. The course of reaction has been considered to proceed through the formation of an activated complex between [K Fe(CN)₆]^2– and substrate anion which decomposes slowly into radical and [K Fe(CN)₆]^3–. A probable reaction mechanism is proposed.

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Kinetik und Mechanismus der Oxidation von Xylit und Galaktit mit Hexacyanoferrat(III) in wäßriger, alkalischer Lösung

Zusammenfassung Das Geschwindigkeitsgesetz der Titelreaktion ist in beiden Fällen erster Ordnung bezüglich Hexacyanoferrat(III). Die Oxidation ist erster Ordnung bei niedrigen Konzentrationen von Xylit und Galaktit und geht bei Erhöhung der Konzentration gegen null. In gleicher Weise wurde eine Kinetik erster Ordnung bezüglich Hydroxyl bei niedrigen Konzentrationen und eine erniedrigte Ordnung bei höheren Konzentrationen für die Oxidation von Xylit beobachtet; bei Galaktit bleibt die Oxidation auch bei höheren Hydroxyl-Konzentrationen erster Ordnung. Es wird angenommen, daß die Reaktion über einen aktivierten Komplex zwischen [KFe(CN)₆]^2– und dem Substrat-Anion verläuft; dieser Komplex zerfällt in [KFe(CN)₆]^3– und ein Substrat-Radikal. Ein möglicher Reaktionsmechanismus wird vorgeschlagen.

     

Kinetics of oxidation of pyruvic acid by [ethylenebis(biguanide)]silver(III) in aqueous acidic media

The oxidation of pyruvic acid by the title silver(III) complex in aqueous acidic (pH, 1.1–4.5) media is described. The reaction products are MeCO₂H and CO₂, together with a colourless solution of the Ag⁺ ion. The free ligand, ethylenebis(biguanide) is released in near-quantitative yield upon completion of the reduction. The parent complex, [Ag(H₂L)]³⁺ and one of its conjugate bases, [Ag(HL)]²⁺, participate in the reaction with both pyruvic acid (HPy) and the pyruvate anion (Py^–) as the reactive reducing species. Ag⁺ was found to be catalytically inactive. At 25.0°C, I=1.0moldm^–3, rate constants for the reactions [Ag(H₂L)]³⁺+HPy (k ₁), [Ag(H₂L)]³⁺+Py^– (k ₂), [Ag(HL)]²⁺+HPy (k ₃) and [Ag(HL)]²⁺+Py^– (k ₄) arek ₁=(94±6)×10^–5dm³mol^–1s^–1, (k ₂ K _a+k ₃ K _a1)= (1.3±0.1)×10^–5s^–1 and k ₄=(58±4)×10^–5dm³mol^–1s^–1, respectively, where K _a1is the first acid dissociation constant of the [Ag(H₂L)]³⁺ and K _a is for pyruvic acid. A comparison between the k ₁ and k ₄ values is indicative of the judgement that k ₂k ₃. A one-electron inner-sphere redox mechanism seems more justified than an outer-sphere electron-transfer between the redox partners.     

Kinetics and mechanism of a macrocyclic chromium(III) complex oxidation to chromium(IV) by hexacyanoferrate(III) in strongly alkaline media

Oxidation of the macrocyclic Cr(III) complex cis-[Cr(cycb)(OH)₂]⁺, where cycb=rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, by an excess of the hexacyanoferrate(III) in basic solution, slowly produces Cr(V) species. These species, detected using e.p.r. spectroscopy, are stable under ambient conditions for many hours, and the hyperfine structure of the e.p.r. spectrum is consistent with the interaction of the d-electron with four equivalent nitrogen nuclei. Electro-spray ionization mass spectrometry suggests a concomitant oxidation of the macrocyclic ligand, in which double bonds and double bonded oxygen atoms have been introduced. By comparison basic chromate(III) solutions are oxidized rapidly to chromate(VI) by hexacyanoferrate(III) without any detectable generation of stable Cr(V) intermediates.Kinetics of oxidation of the macrocyclic Cr(III) complex in alkaline solution has been studied under excess of the reductant. Rate determining formation of Cr(IV) by a second order process involving the Cr(III) and the Fe(III) reactants is seen. This reaction also involves a characteristic higher order than linear dependence on the hydroxide concentration. Reaction mechanisms for the processes, including oxidation of the coordinated macrocyclic ligand – under excess of the oxidant- are proposed.     

Kinetics and mechanism of the ligand substitution reaction of di-μ-hydroxobis(bipyridyl)dipalladium(II) ion with thiourea in aqueous solution

The kinetics of the interaction between thiourea and the title complex has been studied spectrophotometrically in aqueous medium as a function of nucleophile concentration, temperature and pH at constant ionic strength. The reaction is a two-step process in which the first step is ligand dependent, but the second step is ligand independent and is assigned to ring closure. The rate and activation parameters, conductivity studies and IR data were used to deduce a plausible mechanism.