Kinetics and mechanism of base hydrolysis of Reinecke's salt revisited – specific salt effects

The kinetics of base hydrolysis of the trans-[Cr(NH₃)₂(NCS)₄]^– anion follows the rate law: -d[complex]/dt = k ₀ + k ₁[OH^–] (50–70 °C, [OH^–] = 0.1–1.9 M and = 2.0 M). The specific salt effect has been investigated for eight aqueous media: NaCl, NaBr, NaI, NaClO₄, KCl, KBr, CsCl and CsBr. The alkali-independent path (k ₀) does not show any specific effect of inert electrolyte ions, the activation parameters: H = 113.5 ± 0.4 kJ mol^–1 and S = 24.1 ± 1.3 J mol^–1 K^–1 are interpreted in the frame of a dissociative interchange mechanism (I _d). For the alkali-dependent path (k ₁) the specific salt effect is observed for cations of the inert electrolyte, showing an important role for ion-pair formation between the cations and reagent complex anion in the activation process. A linear correlation between lnk ₁ and lnK ₀ (K ₀ – ion-pair formation constant) has been found for the cations studied. The dissociative, via conjugate base, mechanism (D _CB) has been proposed for the alkali-dependent path.     

Kinetics and mechanism of the anation of (αβ)-hydroxo(tetraethylenepentamine)cobalt(III) by thiosulfate, the base hydrolysis of the (sulfur-bonded) thiosulfato(tetraethylenepentamine)-cobalt(III) and photochemistry of oxygen and sulfur-bonded thiosulfato complexes

Summary The reaction of ()-(tetren)CoOH²⁺ with S₂O ₃ ^2- in the 7.25–8.28 pH range at 20–40 °C yielded S- (yellow) and O- (purple) bonded thiosulfato(tetren)cobalt(III) complexes, the former in larger quantities. The rate determining step is preceded by diffusion-controlled ion-pair [(tetren)CoOH²⁺,S₂O ₃ ^2- ] formation. Replacement of coordinated OH^- by S₂O ₃ ^2- is interpreted in terms of an internal conjugate base mechanism: (tetren)CoOH²⁺ (tetren-H)CoOH ₂ ²⁺ , the reactive amido conjugate base being generated by intramolecular proton transfer from the coordinated NH site.In acid medium the S-bonded (tetren)Co(S₂O₃)⁺ is highly stable to redox decomposition, in contrast to its pentaammine analogue. The complex however, undergoes base hydrolysis yielding the corresponding hydroxo complex. The rate and activation parameters for the base hydrolysis have been reported. Photolysis of O- and S-bonded isomers of [(tetren)CoS₂O₃]⁺ in acidic medium at 254 and 313 nm, respectively, yielded aquation products accompanied by some decomposition of S₂O ₃ ^2- .     

Kinetics of oxidation of phenylhydrazine by a μ-oxo diiron(III,III) complex in acidic aqueous media

Phenylhydrazine (R) quantitatively reduces [Fe₂(μ-O)(phen)₄(H₂O)₂]⁴⁺ (1) (phen?=?1,10-phenanthroline) and its conjugate base [Fe₂(μ-O)(phen)₄(H₂O)(OH)]³⁺ (2) to [Fe(phen)₃]²⁺ in presence of excess 1,10-phenanthroline in the pH range 4.12–5.55. Oxidation products of phenylhydrazine are dinitrogen and phenol. The reaction proceeds through two parallel paths: 1?+?R?→?products (k ₁), 2?+?R?→?products (k ₂); neither RH⁺ nor the doubly deprotonated conjugate base of the oxidant, [Fe₂(μ-O)(phen)₄(OH)₂]²⁺ (3) is kinetically reactive though both are present in the reaction media. At 25.0°C, I?=?1.0?M (NaNO₃), the rate constants are k ₁?=?425?±?10?M^?1?s^?1 and k ₂?=?103?±?5?M^?1?s^?1. An inner-sphere, one-electron, rate-limiting step is proposed.     

Kinetics and mechanism of ruthenium(III) catalyzed oxidation of chloromphenicol—An antibiotic drug by diperiodatocuprate(III) in aqueous alkaline medium

The kinetics of ruthenium(III) catalyzed oxidation of chloramphenicol (CHP) by diperiodatocuprate(III) (DPC) in aqueous alkaline medium at a constant ionic strength of 0.1 mol l⁻¹ was studied spectrophotometrically. The reaction between DPC and CHP in alkaline medium exhibits 1: 2 stoichiometry (CHP: DPC). The main oxidation products were identified by spot test, IR, NMR, and GC-MS spectral studies. The reaction is first order with respect to ruthenium(III) and DPC concentrations. The order with respect to chloramphenicol concentration varies from first order to zero order as the chloramphenicol concentration increases. As the alkali concentration increases the reaction rate increases with fractional order dependence on alkali concentration. Increase in periodate concentration decreases the rate. A mechanism adequately describing the observed regularities is proposed. The reaction constants involved in the different steps of the mechanism were calculated. The activation parameters with respect to limiting step of the mechanism are computed and discussed. Thermodynamic quantities are determined.     

Kinetics and mechanism of the oxidation of neutralized α-hydroxy acids by tris(pyridine-2-carboxylato)manganese(III)

The kinetics of oxidation of the neutralized -hydroxy acids: lactic, -hydroxyisobutyric, mandelic, benzilic and atrolactic acids by tris(pyridine-2-carboxylato)manganese(III) have been studied. The reactions were carried out in a Na(pic)-picH [Na(pic) = sodium salt of pyridine-2-carboxylic acid and picH = pyridine-2-carboxylic acid] buffer medium in the 4.89–6.10pH range. The oxidation rate was found to be independent of pH, and rate follows the order: benzilate > mandelate >atrolactate>lactate > -hydroxy isobutyrate. The oxidation products are MeCHO, Me2CO, PhCHO, Ph2CO and PhCOMe for the respective reactions. A mechanism is proposed involving intermediate formation of hepta-coordinated MnIII complexes in a fast step. The complexes then decompose to give free radicals and MnII in the rate determining step. The free radicals subsequently react with another molecule of the MnIII species to give the respective carbonyl compounds in a fast step.     

Kinetics of Cyclopolymerization of N,N′-Methylenebisacrylamide Initiated by Redox Couples with Mn(III). Part III

Redox-initiated free-radical cyclopolymerization of the nonconjugated divinyl monomer N,N′-methylenebisacrylamide was studied at 25–40°C, involving trisacetatomanganese(III) dihydrate as oxidant with four different reductants, methyl ethyl ketone (2-butanone), cyanoacetic acid, malic acid, and thiomalic acid. While the general mechanistic sequence is the same, the modes of termination are different in these cases. Because the reactivities of the different free radicals from the four redox pairs are different, the kinetic order with respect to the monomer, oxidant, and reductant differ considerably in magnitude. The kinetic and thermodynamic parameters were calculated.     

Kinetics and mechanism of the reduction of diaquatetrakis (2,2′-bipyridine)-μ-oxo diruthenium(III) by ascorbic acid

Summary The kinetics of the oxidation of ascorbic acid by diaquatetrakis (2,2-bipyridine)--oxo diruthenium(III) in aqueous HClO₄ were investigated. The dependence of the second order rate constantk ₂ on [H⁺] is given by k ₂=a+b[H⁺]^–, indicating that both the undissociated form and the monoanion of ascorbic acid are reactive. Marcus theory was used to estimate the redox potential for the Ru^III-O-Ru^III/Ru^III-O-Ru^II couple and a feasible mechanism has been proposed to explain the results.     

Electrochemical behavior of uranium(III) in NaCl–KCl molten salt

The electro-redox behavior of uranium(III) on Mo electrode in NaCl–KCl molten salt in the temperature range 973–1073 K has been investigated using cyclic voltammetry electrochemical method and so on, such research will help to understand uranium behavior in pyro-reprocessing. The results showed that UCl₃ could be reduced into uranium metal in a quasi-reversible one-step process exchanging three electrons. The diffusion coefficients of U(III) ions were determined and the activation energy for diffusion was found to be 55.794 kJ mol⁻¹. The apparent standard potentials of U(III)/U(0) at several temperatures were calculated. The thermodynamic properties of UCl₃ have also been investigated.

     

Kinetics of oxidation of glutathione by an octahedral cobalt(III) complex with phenolate–amide–amine coordination

The reduction of the octahedral cobalt(III) complex Co^III(HL)·9H₂O, H₄L = 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane by glutathione (GSH) has been studied by conventional spectrophotometry at 25.0 ≤ t/°C ≤ 45.0, 0.02 ≤ [H⁺]/mol dm^?3 ≤ 0.20 and I = 0.3 mol dm^?3 (NaClO₄). The reaction is biphasic. The fast initial phase is attributed to the H⁺-induced formation of the mixed ligand complex, [Co^III(H₂L)GSH]⁺, for which the rate-limiting step is the chelate ring opening via Co^III–NH (amide–N) bond cleavage of the protonated species, [Co^III(H₂L)]⁺. Outer-sphere association equilibria between GSH/GSH₂ ⁺ and [Co^III(H₂L)]⁺ substantially retard the ring opening process and consequently the mixed ligand complex formation. This is then followed by a slow phase involving reduction of [Co^III(H₂L)GSH]⁺ by both GSH and GSH₂ ⁺. The final products are the corresponding Co(II) complex and the oxidized form of GSH, GS–SG. The kinetic data and activation parameters for the redox process are interpreted in terms of an outer-sphere electron transfer mechanism.     

Kinetics of the reduction of tetrakis (2,2′-bipyridine)-μ-oxo-di-iron(III) by 1,3-benzenediol in aqueous acidic medium

The stoichiometry and kinetics of the reduction of Fe₂(III)(bpy)₄OCl₄ by 1,3-benzenediol have been investigated in aqueous hydrochloric acid medium. The reaction is first order in [oxidant] but zero order in [reductant]. There is no evidence for the formation of an intermediate complex of significant stability. The presence of NO₃ ⁻ or ClO₄ ⁻ had no effect on the rate of the reaction. Also, the reaction rate is not affected by the changes in the ionic strength and dielectric constant of the reaction media. A plausible mechanism involving an outer-sphere complex formed via an ion pair is proposed for the reaction.     

Electroreduction of Dy(III) assisted by Zn and its co-deposition with Zn(II) in LiCl–KCl molten salt

To recover dysprosium (Dy) from LiCl–KCl molten salt, the electrochemical mechanism of Dy(III) on liquid Zn electrode and co-deposition of Dy(III) and Zn(II) on W electrode were studied using electrochemical methods. Cyclic voltammetry results demonstrated that the redox process of Dy on liquid Zn electrode is reversible and controlled by diffusion. Reverse chronopotentiograms showed that the transition time ratio of reduction and oxidation is ~3:1, revealing the redox of Dy on liquid Zn electrode is a kind of soluble–soluble system: Dy(III) + 3e⁻ = (Dy–Zn)_solution. The half-wave potential of Dy(III) was almost constant with the increase in scanning rate. The electrochemical separation of metallic Dy from the molten salt was performed using constant potential electrolysis, and the product characterized using X-ray diffraction and scanning electron microscopy–energy-dispersive X-ray spectroscopy was the thermodynamic unstable compound DyZn₅. Also, the co-deposition mechanism of Dy(III) and Zn(II) was explored, indicating that Dy(III) could deposit on pre-deposited Zn and form Dy–Zn compounds: Zn(II) + 2e⁻ = Zn and xDy(III) + yZn + 3xe⁻ = Dy_xZn_y. Moreover, the effect of Dy(III) concentration on the formation of Dy–Zn compounds was investigated. The redox peak currents corresponding to different Dy–Zn compounds changed with the increase in Dy(III) concentration. The co-deposition of Dy(III) and Zn(II) was performed using constant current electrolysis at diverse Dy(III) concentrations. The different Dy–Zn compounds were produced by controlling Dy(III) concentration.     

The Kinetics of Crystallization of Poly(tetrafluoroethylene) by the Action of γ-Radiation

The kinetics of crystallization of PTFE over the temperature range 287–310 K upon its -irradiation to a dose of 220 kGy was studied using calorimetric procedures. Parameters for the models of radiation-induced growth in the degree of crystallinity (expressed as the mass fraction) of bulk and film polymer specimens were calculated. It was shown that the degree of perfection of crystals produced upon irradiation can be determined from the heat of transition at T 293 K characteristic of PTFE.     

Kinetics of Protonolysis of o-Substituted Phenylmercuric Chlorides by HCl(NaI) in Ethanol

Yadavetalhaveexaminedthereactionsbetweeniodineando-XC6H4HgCl(X=H,0H,CH3,N0z)inethanol,foundthatthetotalorderofthereacti0nwastwo,andthattheorderoftherateconstantsofthesedifferentderivativesofphenylmercuricchlorideis0H>H>CH3>NO2.Theyprop0sedthattheproximityofCH3grouptoHgCIgroup(orthoposition)isresponsibleforitsreverseeffect1.ButinourearlierresearhesonprotonolysisofarylmercuricchloridesbyHCl(NaI)inaqueousdio....2',n0obvioussterichindranceofo-substituentswasobserved,instead,abnormalelec…     

Kinetics and mechanism of the reduction of diaquotetrakis (2,2′-bipyridine)-µ-oxodiruthenium(III) ion by hypophosphorous acid in acidic medium

The kinetics and mechanism of the reduction of diaquotetrakis(2,2′-bipyridine)-µ-oxodiruthenium(III), [(H₂O)₂(bipy)₄Ru₂O]⁴⁺, by H₃PO₂ has been studied in aqueous acid at ionic strength = 0.5 mol dm^?3 (NaClO₄), [H⁺] = 5.0 × 10^?2 mol dm^?3 and temperature = 31 ± 1 °C. Measurement of the stoichiometry showed that 1 mole of [(H₂O)₂(bipy)₄Ru₂O]⁴⁺ was reduced by 1 mole of H₃PO₂. The reaction was found to be first order with respect to both [(H₂O)₂(bipy)₄Ru₂O⁴⁺] and [H₃PO₂], hence second order overall. Variations in the ionic strength and dielectric constant of the reaction medium had no effect on the rate. Also, addition of various ions to the reaction medium did not significantly alter the rate. Free radicals were identified during the course of the reaction by a polymerisation test. Spectroscopic information and Michaelis–Menten plots suggested the absence of an intermediate complex prior to electron transfer. [(H₂O)₂(bipy)₂Ru]²⁺, the reduction product of [(H₂O)₂(bipy)₄Ru₂O]⁴⁺, plus H₃PO₃, the oxidation product of H₃PO₂, were identified in the product solutions. It is suggested that the reaction proceeds through the outer sphere pathway. A mechanism for the reaction is proposed.     

Kinetics and mechanism of base hydrolysis of cisα-oxalato(amine)(trien)cobalt(III) and salicylato(amine)(trien)cobalt(III)cations

Oxalato(amine)(trien)cobalt(III) and salicylato(amine)(trien)cobalt(III) perchlorates have been synthesized and tentatively assigned a cis α configuration. The dissociation constants of the complexes have been determined. The base hydrolysis of the complexes have been investigated at 30, 35, 38.6°C and I = 1.0 mol dm⁻³. The rate laws for the oxalato and salicylato complexes are −dln[Complex]_T/dt = k₂[OH⁻] and −dln[Complex]_T = k₁ + k₂[OH⁻] respectively. For the oxalato complex, k₂(30°C) = (2.95 ± 0.05) × 10⁻² dm³ mol⁻¹ s⁻¹, ΔH¹ = (109 ± 4) KJ mol⁻¹, ΔS¹ = (85 ± 13) JK⁻¹ mol⁻¹ and for the salicylato complex, k₁(30°C) = (1.58 ± 0.28) × 10⁻⁴ s⁻¹, ΔH¹ = (166 ± 7) KJ mol⁻¹, ΔS¹ = (229 ± 21) JK⁻¹ mol⁻¹ and k₂(30°C) = (3.56 ± 0.10) × 10⁻³ dm³ mol⁻¹ s⁻¹, ΔH¹ = (101 ± 6) KJ mol⁻¹, ΔS¹ = (42 ± 20) JK⁻¹ mol⁻¹. The base hydrolysis reactions of the complexes were followed in presence of imidazole and ethanolamine, in the presence of added anions and also in D₂O medium. The results are discussed in terms of δ SN₁CB mechanism involving rate limiting CoO bond fission for both k₁ and k₂ paths. However, the possibility of CO bond cleavage in the base hydrolysis of the oxalato complex is not ruled out.     

Kinetics and mechanism of oxidation of α,β-unsaturated alcohols by manganese(III) acetate in aqueous sulphuric acid medium

Summary The kinetics of oxidation of ,-unsaturated alcohols (UA's), such as prop-2-ene-1-ol, but-2-ene-1-ol and 3-phenyl-prop-2-ene-1-ol, by manganese(III) acetate in aqueous H₂SO₄ at constant ionic strength and different acidities has been studied. The reaction was found to proceed through an outer sphere mechanism. The reactions were first order with respect to [Mn^III] and fractional order in [UA]. The reaction showed first order dependence in [H⁺], and the rate decreased on addition of [Mn^II]. Added salts, such as Na₂SO₄, had a negligible effect on the rate. The data suggested that disproportionation of the Mn^III-UA complex into free radicals was the rate determining step in the presence of [Mn^II]. A mechanism consistent with the experimental data is proposed. The activation parameters have been evaluated for the temperature range 298–313 K.