Hydroxylamine derivatives in the Purex Process Part VII. The redox reactive kinetics and mechanism of dimethylhydroxylamine and vanadium(V) in nitric medium

The kinetics of the redox reaction between dimethylhydroxylamine (DMH) and vanadium(V) in nitric acid has been studied by spectrophotometry at 23.1 °C. The rate equation of the reaction is determined as -d[V(V)]/dt=k[V(V)][DMH] by investigating the influence of the concentrations of V(V) and DMH, acidity, ionic strength and the ratio of the initial concentrations of reactants on the redox reaction. The rate constant of the reaction k = 9.95±0.52 (mol/l)^-1.s^-1 when the ionic strength is 1.00 mol/l. The activation energy of the reaction is 22.1 kJ/mol. A possible mechanism of the redox reaction has been suggested on the basis of an electron spin resonance(ESR) spectrum of dimethyl nitroxyl radical, (CH₃)₂O.     

Radical scavenging properties of a flavouring agent–Vanillin

Using pulse radiolysis technique, the one-electron oxidation of vanillin (V-OH) with azide radicals, at pH 6 and 9 resulted in the formation of vanillin phenoxyl radical with k = 6.7 × 10⁷ and 2.5 × 10⁹ dm³ mol^-1 s^-1, respectively. The transient absorption spectra of the vanillin phenoxyl radical (V-O) formed either at pH 6 or 9, showed a _max at 410 nm. At pH 5, the OH radicals seem to form an adduct with vanillin, _max at 430 nm and k(OH + V-OH) = 3.3 × 10⁹ dm³ mol^-1 s^-1, while at pH 9, the OH radical reaction resulted in the formation of vanillin phenoxyl radical with _max at 410 nm and k(OH + V-O-) = 6.6 × 10⁹ dm³ mol^-1 s^-1. The reactivity of NO₂radicals with vanillin is lower by orders of magnitude signifying an incomplete reaction. In general, the rate constants for the reaction of OH, N, NO radicals with vanillin were higher at pH 9 than at the lower pH. Its reactivity with other one-electron oxidants like CCl₃OO, CHCl₂OO and CH radicals and the ability to chemically repair tryptophanyl and guanosyl radicals with k = 1.5 - 4 × 10⁷ dm³ mol^-1 s^-1 indicate its antioxidative behaviour.     

Square-wave voltammetry of 4,4′-diisopropyl-2,2′-bithiazoline and its complex with copper(II)

(4S)-4′-diisopropyl-2,2′-bithiazoline (DPT) is an electroactive organic chiral compound giving two reduction responses in square-wave voltammograms at potentials about −0.2 and −0.4 V by forming a complex with mercury which deposits at the electrode surface. By the addition of copper(II) ion to the solution of DPT a third peak appears between them at about −0.3 V, which corresponds to the reduction of adsorbed Cu-DPT complex. Optimal pH for the investigation of those redox processes was found to be 2.8. By square-wave voltammetric measurements it was interpreted that these redox reactions were quasireversible with immobilized reactants. By plotting i_p/f vs. frequency a quasireversible maximum was obtained, and the apparent standard reaction rate constants were calculated: log (k_s)DPT=(0.91 ± 0.9) and 1 < k_s < 65_S⁻¹, log (k_s)_CuDPT= (0.35 ± 0.9) and 0.3 < k_s < 18 S⁻¹ in 0.55 M NaCl.     

Ac polarography and faradaic impedance of strongly adsorbed electroactive species: Part IV. Experimental study of the faradaic impedance of the redox systems benzo(c)cinnoline-dihydrobenzo(c) cinnoline

The faradaic impedance of the surface redox system benzo(c)cinnoline-dihydrobenzo(c)-cinnoline is studied experimentally in aqueous medium between pH 5 and 13. The variations of the impedance components are in good accord with the theoretical predictions. A V-shaped curve is found for log k_s=f(pH) (k_s=rate constant of the surface electrochemical reaction). It is estimated that the determination of rate constant values up to 2×10⁴ s^?1 on a mercury electrode is possible by this method.     

Kinetics and electrochemical studies on superoxide

Rate constants for the reaction of superoxide O^- ₂ with various substrates were obtained through stationary electrode polarography theory and technique. In solvent acetonitrile, the substrate and the rate constants of the reaction O^- ₂ + AH^{- k2}Product, are, AH = isopropanol (k₂ < 0.01 M^-1 s^-1); ethanol (k₂ = 1.42 × 10² M^-1 s^-1); methanol (k₂ = 1.1 × 10⁷ M^-1 s^-1), H₂O (k₂ = 1.0 × 10⁵ M^-1 s^-1). In MeCN, O^-2 was found to be rather unreactive towards glucose and acetone but it reacts with fructose and sucrose catalytically. However, in DMF2, O^- ₂reacts with glucose and fructose with k2 order of 105 M-1 s-1. The mechanism of the reaction of O^- ₂ with the substrates (AH) is proposed as O^- ₃ + AH k₂O, AH^k2 _k-1 k [O₂H + AH]^-, _k-2O₂H + A^- with k₁ = 10⁹ M^-1 s^-1 and k^-1 = 10⁸ -10⁹ s^-1. With these values of k_-1 and k₁, k k₂(obs). The reversible E_1/2 for O₂ + e O^- ₂ in various solvents: MeCN, acetone, isopropanol, methanol, H₂O were obtained either directly from the reversible voltammogram or from experimental voltammograms and the rate constants obtained (as above) using stationary electrode polargraphy theory; E_1/2 being -0.82 (MeCN),-0.85 (acetone),-0.72 (isopropanol);-0.66 (MeOH),-0.56 (H2O) vs SCE.     

Hydroxylamine derivative in Purex process Part VIII. The kinetics and mechanism of the redox reaction of N-methylhydroxylamine and vanadium(V)

The kinetic properties of the oxidation-reduction reaction between N-methylhydroxylamine (NMHAN) and vanadium(V) in nitric acid medium has been studied by spectrophotometry at 23.1 °C. The rate equation of the redox reaction was determined as -d[V(V)]/dt=k[V(V)] [NMHAN] by investigating the influence of concentration of NMHAN, acidity, ionic strength and the ratio of initial concentration of V(V) to NMHAN on the reaction. The rate constant of the reaction k> = 0.818±0.051 (mol/l)^–1·s^–1 at the ionic strength of 1.00 mol/l. The activation energy of the redox reaction was calculated to be 39.6 kJ/mol. A possibly radical mechanism of the redox reaction between NMHAN and V(V) has been suggested on the basis of electron spin resonance (ESR) spectra of nitroxyl radical, i.e., CH₃

Electrode kinetic studies of single electron charge transfer redox couples using the faradaic rectification method

Three single electron charge transfer redox reactions have been studied using the faradaic rectification method. The kinetic parameters obtained for the ferricyanide-ferrocyanide redox couple are α=0.49, k_a⁰=12×10^?2 cm s^?1; for the chromic-chromous system α=0.47, k_a⁰=2×10^?3 cm s^?1 and for the titanic-titanous reaction α=0.49 and k_a^o=6×10^?4 cm s^?1 at 27°C.     

Kinetics and mechanism of the copper(II) promoted hydrolysis of the methyl ester of ethylenediaminemonoacetate

Summary Base hydrolysis of methyl ethylenediaminemonoacetate has been studied at I=0.1 mol dm^–3 (NaClO₄) over the pH range 7.4–8.8 at 25 °C. The proton equilibria of the ligand can be represented by the equations, where E is the free unprotonated ester species. Values of pK₁ and pK₂ are 4.69 andca. 7.5 at 25° (I=0.1 mol dm^–3). For base hydrolysis of EH⁺, k_OH=1.1×10³ dm³ mol^–1 s^–1 at 25 °C. The species E is shown to undergo lactamisation to give 2-oxopiperazine (k_lact ca. 1×10^–3 s^–1) at 25 °C. Formation of the lactam is indicated both by u.v. measurements and by isolation and characterisation of the compound.Base hydrolysis of the ester ligand in the complex [CuE]²⁺ has been studied over a range of pH and temperature, k _OH ²⁵ =9.3×10⁴ dm³ mol^–1 s^–1 with H=107 kJ mol^–1 and S ₂₉₈ =209 JK^–1 mol^–1. Base hydrolysis of [CuE]²⁺ is estimated to be some 10⁵5 fold faster than that of the free ester ligand. The results suggest that base hydrolysis occursvia a chelate ester species in which the methoxycarbonyl group of the ligand is bonded to copper(II).     

Kinetic Studies on the Reaction of Sodium Hexacyanoferrate（ll） Complex with Peroxydisulfate Anion

The oxidation of Na₄Fe(CN)₆ complex by S₂O anion was found to follow an outer‐sphere electron transfer mechanism. We firstly carried out the reaction at pH=1. The specific rate constants of the reaction, k_ox, are (8.1±0.07)×10^?2 and (4.3±0.1)×10^?2 mol^?1·L·s^?1 at μ=1.0 mol·L^?1 NaClO₄, T=298 K for pH=1 (0.1 mol·L^?1 HCl0₄) and 8, respectively. The activation parameters, obtained by measuring the rate constants of oxidation 283–303 K, were ΔH^≠=(69.0±5.6) kJ·mol^?1, ΔS^≠=(?0.34±0.041)×10² J·mol^?1·K^?1 at pH=l and ΔH^≠=(41.3±5.5) kJ·mol^?1, ΔS^≠=(?1.27±0.33)×10² J·mol^?1·K^?1 at pH=8, respectively. The cyclic voltammetry of Fe(CN) shows that the oxidation is a one‐electron reversible redox process with E_1/2 values of 0.55 and 0.46 V vs. normal hydrogen electrode at μ=1.0 mol·L^?1 LiClO₄, for pH=1 and pH=8 (Tris). respectively. The kinetic results were discussed on the basis of Marcus theory.     

Metal Complexes with Macrocyclic Ligands. Part XLIV Kinetics of the Cu2+ incorporation into a macrocyclic ligand conjugated to proteins: Model studies for the d‘post-labeling’ technique

The kinetics of the Cu²⁺ complexation by macrocycles 1 (4-[(l,4,8,11-tetraazacyclotetradec-1-yl)methyl]-benzoic acid) and 2 (N-propyl-4-[(1,4,8,11-tetraazacyclotetradec-1-yl)methyl]-benzamide) as well as by macrocycle 1 conjugated to bovine serum albumin (bsa) and to ribonuclease A (rnase) were studied by stopped flow techniques. For 1 and 2 , the kinetics were followed in the mM range monitoring the d-d* absorption band of the Cu²⁺ complex. From the pH dependence of k_obs, the rate law is v = [Cu²⁺] (k_LH[LH] + k[LH₂]), where k_LH and k are the bimolecular rate constants for Cu²⁺ with the diprotonated (LH₂) and monoprotonated (LH₁) form of the ligand, respectively. The values are k = 1.7( 1 ) M^?1s^?1 and k_LH = 2.3(1) 10⁵ M^?1s^?1 for 1 , and k, = 0.28(9) M^?1s^?1 and k_LH = 2.0(1) 10⁵ M^?1s^?1 for 2. The kinetics of the Cu²⁺ incorporation into 1,2 and 1 conjugated to bsa and rnase, i.e., 3 and 4 , respectively, were also followed using nitroso-R salt as a metal indicator in the μM range, i.e., under conditions typical for the ‘post-labeling’ technique to give radiolabeled monoclonal antibodies. In these cases, the reaction takes place between the 1:1 complex of Cu²⁺ with nitroso-R-salt and the macrocycle. At pH 6.5, the rates are very similar to each other indicating that the complexation properties of the macrocycle attached to a protein are not very different from those of the free ligand under comparable conditions.     

Pulse radiolysis study on one-electron oxidation of 1-naphthylamine-4-sulphonic acid in aqueous solutions

Reactions of 1-naphthylamine-1-sulphonic acid (NASA) with hydroxyl radicals and oneelectron oxidants such as N₃, Br₂ ^- and Cl₂ ^- radicals have been studied at various pHs using pulse radiolysis technique. Rate constants for the reaction of N₃ and Br₂ ^-. radicals with NASA at neutral pH were found to be 5 × 10⁹ and 4 × 10⁸ dm³ mol^-1 s^-1 respectively. These reactions led to the formation of a cation radical (semi-oxidized species). OH radical reaction with NASA (k = 7.2 × 10⁹ dm³ mol^-1 s^-1) at neutral pH gave a mixture of species, namely, a semi-oxidized species as well as an adduct species. Cl₂ ^-. radicals reacted with NASA rather slowly (k = 7 × 10⁷ dm³ mol^-1 s^-1) at pH 1 to give the semioxidised species. However, even at pH 1, OH radical reaction with NASA gave a mixture containing semi-oxidized as well as an adduct species. The OH-adduct species having _max at 340 nm decays at acidic pHs to give semi-oxidized species having _max at 370 nm. Electron adduct of NASA was found to be a strong reducing radical.     

Kinetics and mechanism of oxidation of hydrazinium ion with peroxomonophosphoric acid in acid perchlorate solutions and role of trace iodide ions

The reaction of peroxomonophosphoric acid and hydrazinium ion in acid perchlorate solutions occurs as per stoichiometry (i), and the rate law (ii) at large [N₂H₅ ⁺], where K′_d is the first acid dissociation constant of H₃PO₅ and k ₁ and k ₂ are rate constants found to be 2.6 × 10^?4 s^?1 and 5.0 × 10^?2 M^?1 s^?1, respectively, at 35°. The reaction is greatly catalyzed by iodide ions. The mechanism involves a redox cycle I^?/I₂ and the rate is independent of [N₂H₅ ⁺] in the presence of iodide ions. K′_d was found to be 0.55 M^?1 and independent of temperature.     

ArticleMicrocalorimetric Investigation on the Kinetics of the Oxidation of Ascorbic Acid with Hydrogen Peroxide

Introduction Ascorbic acid is still attractive due to its wide-rang-ing role in biological processes and chemical fields. Consequently the knowledge of the kinetics of ascorbate reduction is highly desirable. In previous papers,1-11 spectroscopy and electronics methods were generally used to study the oxidation of ascorbic acid, and only a first-order reaction rate constant was obtained by these methods. Since the reaction is a complicated process involving several reactive species, the previo…     

Reactions of [NiIII(cyclam)] with aqueous sulfur dioxide: Exhibition of a deuterium isotope effect and catalysis by chloride and bromide

One unit of S(IV) (SO₂ or SHO₃^?) is oxidized per 2 units of [Ni^III(cyclam)] species to obtain sulfate. Kinetic analyses have been done by varying the acidities (0.013 ? [H⁺] ? 1.0 M) and halide concentrations (0.000 ? [X^?] ? 0.012 M; X=Cl and Br) at constant ionic strength (μ = 1.0 M). The rate law that incorporates the [X^?] and [H⁺] dependence is ?d[Ni^III]_T/dt=2k[Ni^III]_T[S(IV)]_T where 2k={k_a[H⁺] + k_bK + kK′_X[H⁺] [X^?] + kK′_XK[X^?]} {[H⁺] + K}^?1 {1 + K′_X[X^?]}^?1, here k_a=87 ± 7 M^?1 s^?1, k_b=(2.5 ± 0.5)×10³ M^?1 s^?1 and pK = 1.8 ± 0.2. Rate constants k_a and k_b are attributed to the reactions of [Ni^III(cyclam) (H₂O)₂]³⁺ with SO₂ and SHO₃^?, respectively. Monohalo species apparent equilibrium constants K′_Cl=(1600 ± 400) M^?1 and K′_Br=(190 ± 20) M^?1 and rate constants k=80 ± 8 M^?1 s^?1 and k = 140 ± 15 M^?1 s^?1 are ascribed to the protonated pathway, involving the [Ni^III(cyclam) (H₂O)X]²⁺ and SO_2(aq) reaction pairs. The other two rate constants of k=(5 ± 1)×10³ M^?1 s^?1 and k=(3.1 ± 0.5)×10⁴ M^?1 s^?1, refer to the deprotonated pathway and are assigned to the [Ni^III(cyclam) (H₂O)X]²⁺ /SHO₃^? redox couple. A deuterium H₂O / D₂O isotope effect of 2.1–2.8 can be attributed partially to an equilibrium isotope effect at low acidity though a small kinetic isotope (2.5 ± 0.5) effect is evident for the dihydrogen sulfito pathway, k_a. The kinetic isotope effect and the absence of sulfite radical scavenging effects are explained by a mechanism entailing migration of a hydride from sulfur to the Ni^III center to produce a Ni^III—H species, which rapidly comproportionates, and S(VI). © 1993 John Wiley & Sons, Inc.     

Kinetics and mechanism of the reaction between bis 4-(2-pyridylazo)resorcinol ferrate(III) complex and cyanide ion

Summary Kinetics and mechanism of formation of a 113 mixed cyano-complex from [Fe^III (Par)₂]^– (where Par represents 4-(2-pyridylazo)resorcional) and cyanide ion has been studied spectrophotometrically at 720 nm [_max=Fe^(III)(Par) ₂ ^– ], pH=10.0±0.02, temp=25±0.1°C and 1=0.1 M (NaClO₄). The order with respect to cyanidevaries from one to two at high and low cyanide concentrations respectively. The rate constants for respective reactions are k₁=10.8±0.6×10^–2 M^–1 s^–1, k₂=7.7±0.5 M^–2 s^–1. The reverse reaction does not occur at a measurable rate even in presence of large excess of par. These observations suggest that Fe^III (Par) ₂ ^– forms a mixed complex, [FePar(CN)₃]^2-, in presence of an excess of cyanide ions. A three-step mechanism consistent with these results is proposed. The activation parameters for the reaction have been derived and used to support the proposed mechanism. The effect of ionic strength lends further support to the mechanism.     

A new route to the asymmetric Schiff base ligands. The ligand exchange in [Mo(CN)<Subscript>3</Subscript>O(salhy)]<Superscript>2−</Superscript> ion. Kinetics and mechanism

The kinetics of the ligand exchange in (PPh₄)₂[Mo(CN)₃O(salhy)]. 6H₂O (Hsalhy = salicylaldehyde hydrazone) by a solvent molecule and by 2,2-bipyridine (bpy) have been studied in EtOH. For the ligand exchange by a solvent molecule the pseudo-first order rate constant equals k _obs = 3.2 (±0.2) × 10^–3 s^–1 (t=25 °C), H =67 (± 7) kJ mol^–1, S =–75 (±23) J mol^–1 K^–1, while for the exchange by a bpy molecule k _obs=3.5 (±0.2) × 10^–3 s^–1 (t=25 °C), H =56 (±7) KJ mol^–1, S = –104 (±8) J mol^–1 K^–1. It was found, that all reactions proceed via the same mechanism which involves the chelate ring opening cis to the Mo=O bond. The mechanism of the reaction was proposed and was proved by the synthesis of (PPh₄)₂[Mo(CN)₃O(N-pic)]. 2.5H₂O (N-pic denotes that the nitrogen of picolinic acid is trans to Mo=O) by ligand exchange in EtOH, while in aqueous solution the O-pic analogue is formed exclusively.     

Arrhenius parameters for the reactions CH3. + C4H10 → CH4 + C4H9. and C2H5. + C4H10 → C2H6 + C4H9.

Study of n-butane pyrolysis at high temperature in a flow system allows measurement of the sum of the rate constants of the initiation reactions and of the Arrhenius parameters of the reactions Established data for k₁/k₂ allow estimation of k₁ for 951°K and this, with recent thermochemical data, yields the result log k_?1 (l.mole s^?1) = 8.5, in remarkable agreement with a recent measurement [20] but over si×ty times smaller than conventional assumption. The product k₃k₄ (l.²mole^?2s^?2) is found to be associated with the Arrhenius parameters log (A₃A₄) = 21.90 ± 0.6 and (E₃ + E₄) = 38.3 ± 2.7 kcal/mole. These values are much higher than would be e×pected on the basis of low temperature estimates. Independent evaluation gives log A₄ = 10.5 ± 0.4 (l.mole^?1s^?1) and E₄ = 20.1 ± 1.7 kcal/mole, hence log A₃ = 11.4 ± 0.8 (l.mole^?1s^?1) and E₃ = 18.2 ± 3.2 kcal/mole. These values are shown to be entirely consistent with a wide range of results from pyrolytic studies, and it is argued that they further confirm the view that Arrhenius plots for alkyl radical–alkane metathetical reactions are strongly curved, in part due to tunneling and, appreciably, to other as yet unidentified effects. Since there is published evidence that metathetical reactions involving hydrogen atoms show even greater curvature, it is suggested that this may be a characteristic of many metathetical reactions.     

The gas phase pyrolysis of alkyl nitrites. I. t-butyl nitrite

The rate of decomposition of t-butyl nitrite (TBN) has been studied in a static system over the temperature range of 120–160°C. For low concentrations of TBN (10^?5- 10^?4M), but with a high total pressure of CF₄ (～0.9 atm) and small extents of reaction (～1%), the first-order homogeneous rates of acetone (M₂K) formation are a direct measure of reaction (1), since k₃» k₂ (NO): TBN . Addition of large amounts of NO in place of CF₄ almost completely suppresses M₂K formation. This shows that reaction (1) is the only route for this product. The rate of reaction (1) is given by k₁ = 10^16.3–40.3/θ s^?1. Since (E₁ + RT) and ΔH are identical, both may be equated with D(RO-NO) = 40.9 ± 0.8 kcal/mole and E₂ = O ± 1 kcal/mole. From ΔS and A₁, k₂ is calculated to be 10^10.4M^?1 ·s^?1, implying that combination of t? BuO and NO occurs once every ten collisions. From an independent observation that k₂/k₂′ = 1.7 ± 0.25 independent of temperature, it is concluded that k₂′ = 10^10.2M^?1 · s^?1 and k₁′ = 10^15.9?40.2/θ s^?1; . This study shows that MeNO arises solely as a result of the combination of Me and NO. Since NO is such an excellent radical trap for t-Bu\documentclass{article}\pagestyle{empty}\begin{document}${\rm Me\dot O}$\end{document}, reaction (2) may be used in a competitive study of the decomposition of t? Bu\documentclass{article}\pagestyle{empty}\begin{document}${\rm Me\dot O}$\end{document} in order to obtain the first absolute value for k₃. Preliminary results show that k₃ (∞) = 10^15.7–17.0/θ s^?1. The pressure dependence of k₃ is demonstrated over the range of 10^?2?1 atm (160°C). The thermochemistry for reaction (3) implies that the Hg 6(³P₁) sensitised decomposition of t-BuOH occurs via reaction (m): In addition to the products accounted for by the TBN radical split, isobutene is formed as a result of the 6-centre elimination of HONO: TBN \documentclass{article}\pagestyle{empty}\begin{document}$\mathop \to \limits^7 $\end{document} isobutene + HONO. The rate of formation of isobutene is given by k₇ = 10^12.9–33.6/θ s^?1. t-BuOH, formed at a rate comparable to that of isobutene–at least in the initial stages–is thought to arise as a result of secondary reactions between TBN and HONO. The apparent discrepancy between this and previous studies is reconciled in terms of the above parallel reactions (1) and (7), such that k + 2k₇ = 10^14.7–36.2/θ s^?1.