Effect of manganese(II) ions on the oxidation of malic and oxaloethanoic acids by aqueous HCrO−4

The catalytic effect of Mn^II ions on the pseudo-first-order rate constants (k _obs) for chromic acid oxidations of malic and oxaloethanoic acids (an oxidation product of malic acid) has been studied spectrophotometrically at 25 °C. The rates show a first-order dependence on the Cr^VI concentration for each reductant. The order with respect to [malic acid] was found to lie between 1 and 2, and 1 for [oxaloethanoic acid]. The rate increased markedly with increasing [Mn^II] in both the cases. The catalytic effects of Mn^II have been ascribed to a one-step three-electron process in which a termolecular complex is formed between the reductant, Mn^II and HCrO^– ₄. The intermediate Cr^IV is ruled out; details of such a process are discussed. Mechanisms in accordance with the experimental data are proposed for the reactions.     

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.     

Role of cetyltrimethylammonium bromide (cationic surfactant) on the tryptophan–MnO4 reaction

Upon addition of permanganate to a solution of tryptophan (Trp), yellow-brown color species appears within the time of mixing of tryptophan in absence and presence of cetyltrimethylammonium bromide (CTAB), which was stable for some days. Spectroscopic and kinetic evidences suggest the formation of water-soluble colloidal MnO₂ as the most stable reduction product of MnO₄⁻. Carbon dioxide and ammonia are not formed as the oxidation products. Carbon–carbon double bond of indole moiety of Trp is responsible for the fast reduction of permanganate. Cetyltrimethylammonium bromide catalyses the permanganate oxidation of Trp with a rate enhancement of ca. 200-fold. Sub- and postmicellar catalytic effect of CTAB ascribed to the association/incorporation/solubilization of both reactants (MnO₄⁻ and Trp) with the CTAB aggregates and into the Stern layer of cationic micelles. Quantitative kinetic analysis of the rate constant–[CTAB] data has been performed on the basis of modified pseudo-phase model of the micelles. A comparison was made of the oxidation rates of different amino acids by permanganate. The order of the effectiveness was as follows: tryptophan  tyrosine  phenylalanine.     

Kinetics and reactivities of ruthenium(III)- and osmium(VIII)-catalyzed oxidation of ornidazole with chloramine-T in acid and alkaline media: A mechanistic approach

Ornidazole is an antiparasitic drug having a wide spectrum of activity. Literature survey has revealed that no attention has been paid towards the oxidation of ornidazole with any oxidant from the kinetic and mechanistic view point. Also no one has examined the role of platinum group metal ions as catalysts in the oxidation of this drug. Such studies are of much use in understanding the mechanistic profile of ornidazole in redox reactions and provide an insight into the interaction of metal ions with the substrate in biological systems. For these reasons, the Ru(III)- and Os(VIII)-catalyzed kinetics of oxidation of ornidazole with chloramine-T have been studied in HCl and NaOH media, respectively at 313 K. The oxidation products and kinetic patterns were found to be different in acid and alkaline media. Under comparable experimental conditions, in Ru(III)-catalyzed oxidation the rate law is −d[CAT]/dt = k [CAT]_o[ornidazole]_o^x[H⁺]^−y[Ru(III)]^z and it takes the form −d[CAT]/dt = k [CAT]_o[ornidazole]_o^x[OH⁻]^y[Os(VIII)][ArSO₂NH₂]^−z for Os(VIII)-catalyzed reaction, where x, y and z are less than unity. In acid medium, 1-chloro-3-(2-methyl-5-nitroimidazole-1-yl)propan-2-one and in alkaline medium, 1-hydroxy-3-(2-methyl-5-nitroimidazole-1-yl)propan-2-one were characterized as the oxidation products of ornidazole by GC–MS analysis. The reactions were studied at different temperatures and the overall activation parameters have been computed. The solvent isotope effect was studied using D₂O. Under identical set of experimental conditions, the kinetics of Ru(III) catalyzed oxidation of ornidazole by CAT in acid medium have been compared with uncatalyzed reactions. The relative rates revealed that the catalyzed reactions are about 5-fold faster whereas in Os(VIII) catalyzed reactions, it is around 9 times. The catalytic constant (K_C) has been calculated for both the catalysts at different temperatures and activation parameters with respect to each catalyst have been evaluated. The observed experimental results have been explained by plausible mechanisms. Related rate laws have been worked out.     

Kinetics of diamminesilver(I)-catalysed oxidation of sulphur(IV) by dioxygen in ammonia buffers

The kinetics of the silver(I)-catalysed autoxidation of SO₃ ^2– into SO₄ ^2– in ammonia–ammonium nitrate buffer obeyed the rate law:R _obs=k₁ k₂ K[Ag^I]_T[SO₃ ^2-}][O₂] / ([NH₃]+K[SO₃ ^2-])(k₁+k₂[O₂])The values of k ₁, k ₂/k _–1 and K were found to be 1.2l mol^–1 s^–1, 5.3 × 10² l mol^–1 and 0.6 respectively at 30 °C. Two alternative free radical mechanisms have been proposed.     

Relative reactivities of 1,2-dicarbonyl compounds towards permanganate in ethanoic acid medium. Mechanism of the oxidation processes

The oxidative behaviour of the 1,2-dicarbonyl compounds, viz., glyoxal, biacetyl and benzil, towards permanganate in ethanoic acid medium in the presence of HClO₄ has been investigated. The reaction is first order with respect to MnO^– ₄, substrate, as well as H⁺. The rate decreases with an increase in ionic strength. Different thermodynamic parameters have been evaluated. The protonated dialdehyde or diketone reacts with permanganate ion to form an intermediate ester which decomposes in a slow step to produce the corresponding carboxylic acid via C—C bond cleavage.     

Kinetic,solvation and reactivity studies of iron(II) complexes of monoxime and dioxime ligands

Complexes of Fe^II with monoxime and dioxime ligands have been isolated and characterised. Kinetic results and rate laws are reported for acid aquation and base hydrolysis of these complexes in H₂O and in MeOH–H₂O mixtures. Kinetics of acid catalysed aquation of Fe^II–monoxime complexes follow a rate law with k_obs = k₂[H⁺] + k₃[H⁺]², while kinetics of acid dissociation and base hydrolysis of the Fe^II–dioxime complex follow rate laws with k_obs = k₂[H⁺] and k_obs = k₂[OH⁻]. Acid aquation and base hydrolysis mechanisms are proposed. The solubilities of Fe^II–monoxime and –dioxime complex salts are reported and transfer chemical potentials of their complex cations are calculated. Solvent effects on reactivity trends have been analysed into initial and transition state components. These are determined from transfer chemical potentials of reactant and kinetic data. Rate constant trends from these complexes are compared and discussed in terms of ligand structure and solvation properties. Our kinetic results give information relevant to the application of these ligands as analytical reagents for trace Fe^II in acidic and neutral media, in water and in aqueous alcohols.     

Kinetics of Chlorohydrination of Allyl Chloride

The chlorohydrination of allyl chloride with chlorine in water was studied at 20–80°C. The effect of the concentration of chloride ions within the range 0–3.6 mol/l on the selectivity of formation of glycerol dichlorohydrins was studied. An equation that relates the selectivity and the concentration of Cl^–was derived, which adequately describes experimental data. The schemes of parallel and consecutive reactions occurring in the system were suggested. The ratios between the rate constants of the following reactions were found: the reactions of chlorine with water and allyl chloride dissolved in water (k ₁/k ₄= 4.1 × 10^–4), the reaction of allyl chloride with hypochlorous acid and the decomposition of hypochlorous acid (k ₂/k ₃= 1.7 × 10³), and the reactions of the allyl chloride–chlorine complex with a water molecule and Cl^–(k ₅/k ₆= 2.9 × 10^–2).     

A kinetic study of the permanganate—oxalate reaction and kinetic determination of manganese(II) and oxalic acid by the stopped-flow technique

The reduction of permanganate by oxalate in the presence of manganese(II) ion in acidic media is described. All reactions were run at 525 nm and constant ionic strength 1.0 M. The reaction was found to obey the rate expression —$\text{d[MnO}{}_4{}^{\mathrm{-}}\text{]}\text{d}\text{t}$ = k [Mn²⁺] [C₂O₄^2-]² [MnO₄^-] [H⁺]^-2 = k' [MnC₂O₄] [MnO₄^-]. The values of k and k' were shown to be 5.4 × 10⁴ M^-1 s^-1 and 8.2 × 10⁴ M^-1 s^-1, respectively. Reaction rate methods for the determination of manganese(II) and oxalic acid are reported. The rate of disappearance of permanganate was monitored automatically and related directly to manganese-(II) and oxalic acid concentrations. Manganese(II) in the ranges 1–10 × 10^-4 M and 1–10 × 10^-3 M and oxalic acid in the range 0–20 μg ml^-1 can be determined very rapidly with a precision of 1–2%.     

Kinetics and mechanism of the chromium(III)–picolinato chelate ring opening in some chromium(III)–oxalato–picolinato complexes in acidic aqueous solutions

Two Cr^III–picolinato complexes were obtained and characterized in solution. The [Cr(C₂O₄)(pyac)₂]^– and [Cr(C₂O₄)₂(pyac)]^2– ions (pyac = picolinic acid anion) in acidic solutions undergo a reversible one-end Cr^III–picolinato chelate ring opening via Cr^III—N bond breaking. The reaction rate was determined spectrophotometrically in the 0.1–1.0 M HClO₄ range at I = 1.0 M. The observed pseudo-first order rate constant depends on [H⁺] according to the equation: k _obs = a + b[H⁺] + c/[H⁺]. A reaction mechanism, which assumes participation of the protonated and unprotonated forms of the reactants, has been proposed. The kinetic parameters a, b, c have been defined as a = k ₁, b = k ₂ Q ₁, c = k _–1/Q ₂, where k ₁, k _–1,k ₂ are rate constants for the forward and reverse processes and Q ₁, Q ₂ are the protolytic equilibrium constants in the term of the proposed mechanism. The activation parameters have been determined and discussed.     

Dissociative chemistry of ionic transition metal cluster fragments

The ionic fragments formed by collision-induced dissociation of Mn₂(CO) _y ⁺ ions (y=1–10) are reported. The ratio of product ions formed by metal-metal vs. metal-ligand bond cleavage are discussed in terms of the dependence of the metal-metal bond energy on the metal-to-ligand ratio. The collision-induced dissociation data indicate that the metal-metal bond energy of Mn₂(CO) ₅ ⁺ and Mn₂(CO) ₁₀ ⁺ is less than that for Mn₂(CO) _y ⁺ (y=1–4 and 6–9). The product ions arising by metal-metal and metal-ligand bond cleavage reactions for collision-induced dissociation and photodissociation are compared. On the basis of this data and the known photochemistry/photophysics of Mn₂(CO)₁₀, it is proposed that the difference in collision-induced dissociation and photodissociation product ion branching ratios is attributable to spin-orbit transitions of the activated ions.     

Kinetics and mechanism of formation of square-planar copper(II) and nickel(II) complexes of ethylenebisbiguanide in aqueous acetate buffer media

Summary The kinetics of formation of square-planar Cu^II and Ni^II complexes of the quadridentate ligand, ethylenebisbiguanide, have been studied spectrophotometrically in aqueous HOAc–NaOAc buffer, at ionic strength 0.2 mol dm^–3, in the 25–35°C temperature range. The observed rate constants for the formation reactions are independent of pH (and of OAc^– concentration) in the pH range used (3.6–4.8 for Cu^II and 5.0–5.8 for Ni^II) where the product complexes form stoichiometrically, but show first-order dependence on the ligand concentration;i.e. k_obs=k_f[L]_total. At 25°C k_f values (dm³ mol^–1s^–1) are 35.2±0.4 for Cu^II and (8.4±0.1)×10^–3 for Ni^II. The mechanism of the reactions is discussed.     

Kinetic study of Cr(VI) sorption on MnO2

The kinetics of adsorption of chromate ions has been investigated radiometrically over a wide range of concentration of chromate ions (10^–6–10^–2M) and temperature (303–323 K). The kinetics of the process follows essentially a first order rate law with respect to adsorptive concentration and obeys the Freundlich adsorption isotherm in the concentration range studied. In addition, the kinetics of desorption of the preadsorbed species also follows a first order rate law and the activation energy for desorption is greater than that of the adsorption process. On the basis of an adsorption kinetic study, the thermodynamic parameters have been calculated. Infrared spectroscopy has shown the chemical interaction of chromate ions on the surface of MnO₂.     

Kinetics and mechanisms of oxidation by metal ions. Part XIII. Osmium(VIII)-catalysed oxidation of cycloalkanols by alkaline hexacyanoferrate(III)

Summary The kinetics of OsO₄-catalysed oxidation of cyclopentanol, cyclohexanol and cyclooctanol by alkaline hexacyanoferrate(III) have been studied at low [OH^–] so that the equilibrium between alcohol and alkoxide ion is not unduly shifted towards the latter. The reaction shows a first-order dependence in [OH^–]. The order of the reaction with respect to cycloalcohol is fractional, indicating the formation of an intermediate complex with Os^VIII since the order with respect to hexacyanoferrate(III) ion is zero. The order with respect to Os^VIII may be expressed by the equation k_obs=a+b[Os^VIII]. The analysis of the rate data indicates a significant degree of complex formation between [OsO₃(OH)₃]^– and ROH. It was found that the bimolecular rate constant k for the redox reaction between complex and OH^–k₁, the forward rate constant for the formation of alkoxide ion. The activation parameters of these rate constants are reported.     

Nitrogen isotope exchange between nitric oxide and nitric acid

The rate of nitrogen isotope exchange between NO and HNO₃ has been measured as a function of nitric acid concentration of 1.5–4M·1^–1. The exchange rate law is shown to beR=k[HNO₃]²[N₂O₃] and the measured activation energy isE=67.78kJ ·M^–1 (16.2 kcal·M^–1). It is concluded that N₂O₃ participates in¹⁵N/¹⁴N exchange between NO and HNO₃ at nitric acid concentrations higher than 1.5M·1^–1.     

Studies on the kinetics and mechanism of dissociation of copper(II) and nickel(II) complexes of the macrocyclic ligand 1, 5, 9, 13-tetraaza-2, 4, 4, 10, 12, 12-hexamethyl-cyclohexadecane-1, 9-diene in perchloric acid media

Summary Acid catalysed dissociation of the copper(II) and nickel(II) complexes (ML²⁺ of the quadridentate macrocyclic ligand 1, 5, 9, 13-tetraaza-2, 4, 4, 10, 12, 12-hexamethyl-cyclohexadecane-1, 9-diene (L) has been studied spectrophotometrically. Both complexes dissociate quite slowly with the observed pseudo-first order rate constants (k_obs) showing acid dependence; for the nickel(II) complex (k_obs)=k_O+k_H[H⁺], the k_o path is however absent with the copper(II) complex. At 60°C (I=0.1M) the k_H values areca 10^–4 M^–1 s^–1 for both complexes; k _H ^Cu /k _H ^Ni =ca. 3.9, comparable to some other square-planar complexes of these metal ions. The rate difference is primarily due to H values [copper(II) complex, 29.4±0.5 kJ mol^–1; nickel(II) complex, 35.6±1.5 kJ mol^–1] with highly negative S values [for copper(II), –215.5 ±6.1 JK^–1 mol^–1 and for nickel(II), –208.1 ±5.6 JK^–1 mol^–1] which are much higher than the entropy of solvation of Ni²⁺ (ca. –160 JK^–1 mol^–1) and Cu²⁺ (ca. –99 JK^–1 mol^–1) ions; significant solvation of the released metal ions and the ligand is indicated.     

Synthesis,thermodynamic properties and kinetics of the acid-catalyzed dissociation of nickel(II) diamido polyamino complexes

The ligands 1,10-N,N-bis(2-hydroxymethylbenzoyl)-1,4,7,10-tetraazadecane (L₁) and 1,11-N,N-bis(2-hydroxymethylbenzoyl)-1,4,8,11-tetraazaundecane (L₂) have been synthesized. The stability constants of Ni^II complexes of ligands L₁ and L₂ have been studied at 25 °C using pH titrations. The kinetics of general acid (HCl, 0.04–2.34 mol dm^–3) or buffer (DEPP or DESPEN, 0.05 mol dm^–3, pH 4.83–5.72)-catalyzed dissociation of these Ni^II complexes have been investigated at 25 °C using a stopped-flow spectrophotometer. The ionic strength of solution was controlled at I = 2.34 mol dm^–3 (KCl + HCl) and I = 0.1 mol dm^–3 (KNO₃, buffer), respectively. The kinetic dissociation of Ni^II complexes catalyzed by HCl obeys the equilibrium k _obs = k _1d + k _2H[H⁺], whereas in buffer solution the observed rate constant k _obs = k _d + k _1H[H⁺]. At pH < 1.5, both the proton-assisted and direct protonation pathways contribute to the rates, whereas solvation is the dominant pathway at pH > 6. In the 4.8–5.7 pH range, the complexes dissociate mainly through a proton-assisted pathway.     

Kinetic investigation of sulfur(IV) oxidation by peroxo compounds R-OOH in aqueous solution

Summary Normal and rapid-scan stopped-flow spectrophotometry in the range of 260–300 nm was used to study the kinetics of sulfur(IV) oxidation by peroxo compounds R-OOH (such as hydrogen peroxide, R=H; peroxonitrous acid, R=NO; peroxoacetic acid, R=Ac; peroxomonosulfuric acid, R=SO ₃ ^– ) in the pH range 2–6 in buffered aqueous solution at an ionic strength of 0.5 M (NaClO₄) or 1.0 M (R=NO; Na₂SO₄). The kinetics follow a three-term rate law, rate=(k_H[H]+k_HX[HX]+k_p)[HSO ₃ ^– ][ROOH] ([H] = proton activity; HX = buffer acid = chloroacetic acid, formic acid, acetic acid, H₂PO ₄ ^– ). Ionic strength effects (I=0.05–0.5 M) and anion effects (Cl^–, ClO ₄ ^– , SO ₄ ^2– ) were not observed. In addition to proton-catalysis (k_H[H]) and general acid catalysis (k_HX[HX]), the rate constant k_p characterizes, most probably, a water induced reaction channel with k_p=k_HOH[H₂O]. It is found that k_Hf(R) with k_H(mean)=2.1·10⁷ M^–2 s^–1 at 298 K. The rate constant k_HX ranges from 0.85·10⁶ M^–2 s^–1 (HX=ClCH₂–COOH; R=NO; 293 K) to 0.47·10⁴ M^–2 s^–1 (HX=H₂PO ₄ ^– ; R=H; 298 K) and the rate constant k_p covers the range 0.2·M^–1 s^–1 (R=H) to 4.0·10⁴ M^–1 s^–1 (R=NO). LFE relationships can be established for both k_HX, correlating with the pK_a of HX, and k_p, correlating with the pK_a of the peroxo compounds R-OOH. These relationships imply interesting aspects concerning the mechanism of sulfur(IV) oxidation and the possible role of peroxonitrous acid in atmospheric chemistry. A UV-spectrum of the unstable peroxo acid ON-OOH is presented.     

Kinetics and mechanism of sulfite oxidation by permanganate in basic medium

The kinetics of the permanganate-sulfite redox reaction has been studied in the alkaline medium using a stopped-flow technique. In the pH range 10 to 12 the reaction was found to obey the rate expression: ?½d[MnO₄^?]/dt = (k₁ + k₂[OH^?]) [MnO₄^?] [SO₃^2?]. Mechanisms for the two paths involving the formation of a (O₃MnOSO₃)^3? complex prior to the rate-determining step are consistent with the data. © John Wiley & Sons, Inc.     

Unusual stabilization of manganese(III) during the cooxidation of dimethylformamide and manganese(II) by chromium(VI)

The kinetics of the formation and decomposition of Mn^III have been investigated spectrophotometrically in acidic media at 25 °C. The complete rate law for Mn^III formation isCr^VI + DMF + Mn^II {H⁺} Mn^III + CO₂ + Me₂NH + Cr^III ... (1)Mn^III + DMF {H⁺} Mn^II + CO₂ + Me₂NH ... (2)expressed by k _obs1 = k ₁ k₁ K _a1[H⁺][DMFH⁺][Mn^II]/{1 + K _a1[H⁺]}. Mn^III reduction by DMF follows the rate law k _obs2 = k ₂ K _h[DMF][H⁺]²/{[H⁺] + K _h}. The above results are accounted for by a mechanism involving the intermediacy of Cr^IV.