Kinetics and mechanism of the reduction of tetrachloroaurate(III) by formate in acidic aqueous solution

Rate data for the reduction of AuCl₄⁻ by formate, were measured at different temperatures, formate, hydrogen and chloride ion concentrations. AuCl₄⁻ and AuCl₃(OH)⁻ react with HCOOH and HCOO⁻ in the rate-determining steps of the suggested reaction mechanism to produce AuCl₃(HCOO)⁻. The latter species undergoes a rapid redox reaction in which AuCl₂⁻, Au(o) and CO₂ are produced.     

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 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 the reaction between silver(III) and thiourea in aqueous alkaline media

The tetrahydroxoargentate(III) ion, Ag(OH)₄⁻, is rapidly reduced by thiourea (tu) in accordance with the three term rate law RATE = {k₁+(k₂+k₃[OH⁻])[tu]}[Ag^III] where k₁ = 1.08 s⁻¹, k₂ = 1.46 x 10³ M⁻¹ s⁻¹, and k₃ = 2.02 x 10³ M⁻² s⁻¹. The k₁ path occurs via the rate-determining aquation of Ag(OH)₄⁻ while the other two paths involve axial attack of thiourea on silver. The higher values of k₂ and k₃ compared to the ethylenediamine reaction, which obeys the same rate law, is a reflection of the greater nucleophilicity of tu.

Following the redox reaction, solutions become brown in a reaction that obeys pseudo-first-order kinetics. Similar behaviour is observed when tu is replaced by Na₂S or thio-acetamide and when Ag^I reacts with any of these sulphur containing compounds. We attribute this process to the Ag^I promoted formation of sulphide species which eventually precipitate as Ag₂S.     

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. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 173–181, 2001     

Kinetics and mechanism of the reaction between dimethylformamide and chromium(VI)

The kinetics of oxidation of N,N‐dimethylformamide by chromium(VI) has been studied spectrophotometrically in aqueous perchloric acid media at 20°C. The rate showed a first‐order dependence on both [Cr(VI)] and [DMF], and increased markedly with increasing [H⁺]. The order with respect to [HClO₄] was found to lie between 1 and 2. The rate was found to be independent of ionic strength as well as of any inhibition effect of Mn(II). The formation of superoxochromium(III) ion was detected in an aerated solution of chromium(VI), DMF and HClO₄. The proposed mechanism, involving two reaction pathways, leads to the rate law, rate = K_a1 [HCrO₄⁻] [DMF] (k_I K_a2 [H⁺]²+k_II[H⁺]). The first pathway, with rate constant k_I, involves the formation of chromium(V) and a free radical. The second pathway, with rate constant k_II, involves the formation of Cr(IV), CO₂ and dimethylamine. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 409–415, 1999     

Early stages of the hydrolysis of chromium(III) in aqueous solution-XII. Kinetics of cleavage of the trimer and tetramer in acidic solution

The kinetics of cleavage of the hydrolytic trimer and tetramer of chromium(III) have been studied in acidic solution and at elevated temperatures using spectrophotometric methods. For both reactions, the variation in absorbance with time was consistent with the presence of two consecutive rate-determining processes. The two reactions were concluded to proceed according to: Product analyses and consideration of the stability constants for these oligomers indicated that their overall cleavage reactions were irreversible under the conditions studied. However, it was unclear whether the initial step in tetramer cleavage was irreversible or not. The dimer was identified as an intermediate in the cleavage of the trimer. Each of the bridge-cleavage processes (k₁ and k₂) occurred via acid-dependent pathways, involving protonation processes in addition to bridge-cleavage processes, while only some of them displayed acid independent pathways. The acid dependence of the rate constants and associated parameters at 298 K are: (i) for trimer cleavage k₁ = k_b [H⁺], where the dimer to monomer was found to be the rate-determining step at higher temperatures (≈350 K), while cleavage of the trimer to dimer and monomer becomes rate determining at lower temperature (≈ 300 K). This change in the rate-determining step is attributable to the much higher value for k₁ compared with k₂. Possible mechanisms for trimer and tetramer cleavage are discussed and their rates compared to previously studied cleavage reactions of Cr^III oligomers.     

Kinetics and mechanism of an unique hydrolysis reaction of (hexafluoroacetylacetonato)bis(ethylenediamine)cobalt(III) in aqueous solution

Summary Addition of base to the title complex results in the rapid reversible formation of the hydrolysed species Co(en)₂-(hfac · OH)⁺ in which the coordinated hexafluoroacetylacetonato ligand contains a hydroxyl group on the carbonyl carbon atom. The kinetics of both the forward hydrolysis and reverse acidolysis reactions were followed spectrophotometrically using stopped-flow and T-jump techniques. The corresponding rate constants areca. 3×106 and 1×10⁸ M^–1s^–1, respectively, for various buffer systems at 25 °C and ionic strength 1.0 M. A combination of the kinetic and equilibrium data enables the estimation of the uncatalyzed (spontaneous) forward and reverse reaction components. The results are discussed with reference to similar data reported for the hydrolysis and reverse acidolysis reactions of the uncoordinated acetylacetonato ligand.     

Kinetics of oxidation of manganese(II) by peroxomonosulfuric acid in aqueous acidic solution

Summary Oxidation of Mn _aq ²⁺ by HSO ₅ ^– in acetate buffer to manganese(IV) is autocatalytic, and obeys a rate expression of the general form -d[Mn^II]/dt = k₀[Mn^II] + k₁[Mn^II][MnO_x]. The first-order (k₀) and heterogenetic (k₁) rate constants show first-order dependences on [HSO ₅ ^– ] and on 1/[H⁺]. The reaction is catalyzed by the addition of the chelating ligand glycine; k₁ shows a first-order dependence on [glycine] at a fixed pH. This catalysis is ascribed to complexation, whereby the redox potential for Mn(gly) _n ^(2–n)+ is lower than that for Mn _aq ²⁺ , facilitating oxidation. The stoichiometry of the reaction is Mn²⁺: HSO ₅ ^– = 11, and the manganese(IV) oxide formed is of battery-active grade. Purity of the recovered product is not affected by the presence of high concentrations of natural sugars in the initial solution.     

Kinetics and mechanism of the reaction between biacetyl and hydroxylamine in acid solution

The kinetics and mechanism of formation of dimethylglyoxime from biacetyl and hydroxylamine in acid solution have been studied. Unprotonated hydroxylamine was found to react with the hydrogen-bonded carbonyl group to give the conjugate acid of the oxime. Detectable quantities of carbinolamine intermediates were not found in the acid concentration range studied.     

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 ketone-bisulfite addition reaction in aqueous solution

The kinetics of the addition of bisulfite to acetone has been studied by the polarographic technique. The specific rate increases with pH and hence the reaction is probably base catalyzed. Mechanisms have been suggested for the reaction in the presence and absence of added base.

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The kinetics of the anation reaction of aquopentaamminecobalt(III) by malonate in acidic aqueous solution

The Co(NH₃)₅OH₂³⁺ ion reacts with malonate to form Co(NH₃)₅O₂CCH₂CO₂H²⁺ or Co(NH₃)₅O₂CCH₂CO₂⁺, depending on the pH of the reaction solution. The kinetics of this anation reaction have been studied as a function of [H⁺] for the acidity range 1.5 ≤ pH ≤ 6.0 in the temperature range of 60 to 80°C, the [total malonate] ≤ 0.5 M, and the ionic strength 1.0M. The anation by malonic acid follows second-order kinetics, the rate constant being 8.0 × 10^?5 M^?1·sec^?1 at 70°C, and the anations by bimalonate (Q₁, k₁) and malonate ion (Q₂, k₂) are consistent with an I_d mechanism. Typical values at 70°C for the ion pair formation constants are Q₁ = 1.3, Q₂ = 5.4M^?1; and for the interchange rate constants k₁ = 5.3 × 10^?4; k₂ = 7.3 × 10^?4 sec^?1. The activation parameters for the various rate constants are reported and the results discussed with reference to previously reported data for similar systems.