Paths and Mechanism for the Oxidation of Alkanes,Alkenes, and Arenes by Peroxynitrous Acid

The bell-shaped relations between the rate constants for the oxidation of hydrocarbons by peroxynitrous acid (HOONO) and the ratio of the volumes of the aqueous and gas phases in the reactor are explained quantitatively on the assumption that the HOONO and the substrate are distributed between the gas and the solution and that OH radicals are formed in the two phases and interact with the hydrocarbon. The distribution coefficient of the HOONO between the gas and aqueous phases is determined [ = (0.4-2)·10^–6].     

Kinetics,Substrate Selectivity and Mechanism of the Oxidation of Alkenes by Peroxynitrous Acid

The kinetic features of the oxidation of alkenes by peroxynitrous acid (HOONO) generated in the H₂O₂–HNO₂/acetate buffer (pH 4.27) system, are quantitatively explained assuming simultaneous reactions in the gas and liquid phases. A remarkable similarity is found for the substrate selectivities of the gas-phase reactions of alkenes as well as of alkanes and arenes with HOONO and with ^·OH radicals. The reaction mechanism is discussed.     

Kinetics, substrate selectivity, and mechanisms of alkane and alkylbenzene reactions with peroxynitrous acid in the gas phase and solution

Specific features of the kinetics of alkane and alkylbenzene oxidation with HOONO formed in the H₂O₂-NaNO₂ system (pH 4.27) are quantitatively explained assuming the simultaneous occurrence of reactions in the gas and liquid phases. A model of the kinetic distribution method is developed and verified that accounts for the equilibrium distribution of a substrate and a reagent between phases and their interaction in both phases. Relative rate constants for the oxidation ofn-alkanes (C₃-C₈), isobutane, cyclopentane, cyclohexane, benzene, and alkylbenzenes are measured over a wide range of the volume ratios of the gas and liquid phases (λ = V_g/V₁). Relative rate constants for the oxidation of alkanes in the gas phase and alkylbenzenes in gas and solution were determined. Similarity in substrate selectivities and kinetic isotope effects of the gasphase reactions of alkanes and arenes with peroxynitrous acid and^•OH radicals suggest that hydroxyl radical or the ˙OH...NO₂ radical pair is an active species in the gas phase. In solution, alkylbenzenes react nonselectively with HOONO, as well as with ˙OH radicals. In contrast to the liquid-phase oxidation of arenes, the liquidphase oxidation of all alkanes under study insignificantly contribute (5–15%) to the overall rate of the substrate consumption.     

Substrate selectivity and mechanism of oxidation of alkanes by peroxynitrous acid in aqueous solution

The relative rate constants (k_rel) were measured for the oxidation of alkanes (RH) by peroxynitrous acid (HOONO) in aqueous solutions at 25 °C. The krel values for the reactions of RH with HOONO and with OH radicals in the series of C₂-C₇ alkanes and the isotopic effects (c-C₆H₁₂/c-C₆D₁₂) agree. However, the k_rel value for methane was lower than for its reaction with OH. Possible reaction mechanisms are discussed. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 5, pp. 295–299, September–October, 2006.     

Kinetics and mechanism of the reactions of C6H12, C5H10 and C6D12 cycloalkanes with peroxynitrous acid in aqueous solution and the gas phase

The c-C₆H₁₂/c-C₆D₁₂ kinetic isotope effect (KIE), the k₆/k₅ rate constant ratio (c-C₆H₁₂/c-C₅H₁₀), and the temperature dependence of these ratios in the gas-phase reactions of cycloalkanes with peroxynitrous acid and OH radicals are identical. The same result was obtained for the reactions in aqueous solution. These data are in accord with the conclusion that OH^· radicals formed in the homolysis of the HO-ONO bond are the active species in the reactions of HOONO with hydrocarbons in aqueous solution and in the gas phase. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 2, pp. 105–110, March–April, 2008.     

Kinetics and mechanism for the oxidation of alkanes by peroxynitrous acid in the gas phase: the slow step involving reaction with oh radicals

The relative rate constants (kRH/kEtH), the temperature dependence of these constants at from 5 to 55 °C, and the activation parameters were found for reactions of propane, butane, pentane, hexane, isobutane, cyclopentane, and cyclohexane with peroxynitrous acid (HOONO) in water. The similarity of these results to the data for the reaction of alkanes with OH radicals confirms that the active species in the reactions of HOONO with hydrocarbons in water are OH radicals formed in the homolysis of the HO—ONO bond.     

Reactions of carbonate radical with cobalt(II) aminopolycarboxylates

Reactions of carbonate (CO ₃ ^–· ) and bicarbonate (HCO ₃ ^· ) radicals generated by photolysis of a carbonate or bicarbonate solution at pH 11.2 and 8.5, respectively, with Co(II) complexes of iminodiacetic acid (IDA) and ethylenediaminetetraacetic acid (EDTA) have been studied. The rate constants for the reactions were in the order of 10⁶–10⁷ dm³mol^–1s^–1. From the time-resolved spectroscopy of the products formed after reaction of CO _3· ^–· or HCO ₃ ^· , it is observed that CO ₃ ^–· or HCO ₃ ^· oxidize the metal center to its higher oxidation state.     

Formation and decay of phenoxyl radicals: Variation with pH and pulse dose

Phenoxyl type radicals were produced from tyrosine methyl ester (TME) using azide (N ₃ ^. ) radicals. The rate constant of formation increased from 2·10⁸ dm³·mol^–1·s^–1 at pH 7 to 4·10⁹ dm³·mol^–1·s^–1 at pH 11, whereas that of the decay, 2k=(6±1)·10⁸ dm³·mol^–1·s^–1, remained constant. The maximum yield of the radicals varied with pH and pulse dose consistently with the kinetic scheme, which involved a competition of the oxidation of TME by azide radicals with the natural decay of N ₃ ^. .     

The N—H and O—H bond dissociation energies in 4-hydroxydiphenylamine and its phenoxyl and aminyl radicals

The N—H and O—H bond dissociation energies in 4-hydroxydiphenylamine Ph—NH—C₆H₄—OH (D _NH= 353.4, D _OH=339.3 kJ mol^–1) and its semiquinone radicals D _NH(Ph—NH—C₆H₄—O^·) = 273.6, D _OH(Ph—N^·—C₆H₄—OH) = 259.5 kJ mol^–1 were first estimated using the parabolic model and experimental data (rate constants) on two elementary reactions with participation of N-phenyl-1,4-benzoquinonemonoimine (2). One of the reactions, namely, that of 2 with aromatic amines, was studied in this work using a specially developed method.     

Effect of the chemical structure of nitroxyl radicals on their reactivity in the reaction with hydrazobenzene and tetranitromethane

The reaction rate constants of reduction of stable radicals of different classes by hydrazobenzene in hexane at 20C, in the range of 0.4–5·10³ M^–1sec^–1, were determined; a single scale of the oxidative properties of stable nitroxyl radicals was constructed. The rate constants of oxidation of a series of nitroxyl radicals by tetranitromethane in aqueous medium at 20C, in the range of 0.06–10 M^–1·sec^–1, were determined. It was shown that the oxidative properties of the nitroxyl group decrease slightly with an increase in its reducing properties for nitroxyl radicals of the piperidine and imidazoline series in reactions with ascorbic acid and tetra-nitromethane in aqueous medium, respectively.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2575–2582, November, 1990.     

Electron transfer. 153. Internal electron transfer to bound cobalt(III) induced by hydroxyl radical

Hydroxyl radicals, generated in aqueous solution from Fe²⁺ and H₂O₂, react with the formato, glycolato, lactato and mandelato complexes of (NH₃)₅Co^III, extracting H·, releasing CO₂ and inducing the internal reduction of Co^III to Co²⁺; decomposition of peroxynitrous acid (O=N—OOH) in the presence of these complexes also yields Co²⁺, indicating partial utilization (15% at 22°C and pH 1) of a path involving OH·.     

Mechanism of the Oxidation of Cyclohexane by Peroxynitrous Acid in Media with and without Oxygen

A study was carried out on the products and kinetics of the reaction of cyclohexane with peroxynitrous acid (HOONO) in a three-phase system containing an aqueous solution, cyclohexane, and a gaseous phase (O₂ or He). In both the helium and oxygen atmosphere, cyclohexanol is formed with a constant initial rate, while cyclohexane is formed after a considerable induction period related to the release of O₂ upon the decomposition of HOONO.     

Interaction of the natural antioxidant echinochrome with hydroperoxides under anaerobic conditions

The action of cumene hydroperoxide and oxidized ethyl esters of the fatty acids of linseed oil on echinochrome (2,3,5,7,8-pentahydroxy-6-ethylnaphthalene-1,4-dione), an active scavenger of peroxide radicals, was investigated under anaerobic conditions (argon). The dependence of the rate of consumption of echinochrome on the initial concentrations of the components indicates the occurrence of two competitive reactions: a) with hydroperoxides and b) with the radicals formed in the thermolysis of hydroperoxides. The upper limit of the values of the rate constants of the reaction of echinochrome with cumene hydroperoxide (5·10^–7 M^–1·sec^–1) and with the total hydroperoxides of linseed oil (3·10^–6 M^–1·sec^–1) was determined.The numeration of the reaction used corresponds to the scheme of free-radical oxidation from [5].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 329–333, February, 1991.The authors would like to express sincere gratitude to V. A. Roginskii for participating in the discussion of the results.     

Kinetics of Conversions of Peroxynitrous Acid in Reactions of Isomerization,Decomposition, and Reduction by Cyclohexane

We have studied the kinetics of decomposition and reduction of peroxynitrous acid HOONO by cyclohexane in aqueous solutions. We used the measured effective rate constants of the reactions to compare the reactivities of the acid in the processes of isomerization, decomposition, and reduction.     

Phenazinee di-N-oxide radical cation and its reactions with hydrocarbons

The absorption spectra of phenazine di-N-oxide radical cation (OPO^+·) in dichloromethane were recorded by the spectroelectrochemical method in the range from 300 to 700 nm. The reactions of the electrochemically generated OPO^+· withpara-substituted toluenes, ethylbenzene, and cumene were studied. Using differential cyclic voltammetry, relative rate constants of reactions of OPO^+· with substrates were determined; their correlations with ⁺ para constants of substituents gives p = -2.7, which attests to the nonradical character of the reaction of OPO^+· with RH. This conclusion is confirmed by the study of the effect of O₂ on oxidation of ethylbenzene and cumene.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1992–1998, August, 1996.     

QM–MM investigation of the reaction products between nitroxyl and O2 in aqueous solution

By means of combined quantum-classical molecular (QM/MM) dynamics simulations, we investigated the structural properties and the vibrational spectra in aqueous solution of four possible intermediate species of the oxidation of nitroxyl (HNO/NO⁻) by dioxygen, the cis- and trans-isomers of peroxynitrous acid and nitroxyl-dioxygen adduct. Our study suggests that Raman spectroscopy will provide signals for discrimination between the peroxynitrous acid isomers from both adducts, and that infrared spectroscopy will probably assist the discrimination between the cis-peroxynitrous acid isomer from the other compounds considered. This application of hybrid simulation methods highlights the benefits of explicit solvent representations.     

Gamma radiolysis of nitrilotriacetic acid (NTA) in aqueous solutions

Aqueous solutions of nitrilotriacetic acid (NTA) were irradiated with gamma-rays. In deaerated acidic solutions G (IDA, iminodiacetic acid) was found to be 3.0 and in aerated solutions 2.7. Both H and OH radicals abstracted alpha hydrogen from the NZA molecule. The dehydrogenated radical disproportionated to NTA and IDA; however in presence of air, the radical added with O₂ to give peroxy intermediate which was hydrolyzed to IDA and HO₂. The rate constants, for the reaction of OH-radical with NTA at pH 2.0, 6.0 and 10.0 as determined by competition kinetic methods were 0.61·10⁸, 5.5·10⁸ and 42·10⁸ dm³·mol^–1·s^–1, respectively. These indicated that the unprotonated form of NTA is more reactive than its protonated form. This has been attributed to the deactivation of alpha-hydrogen centers by protons through inductive effect.     

Pulse radiolysis of Triton X-100 aqueous solution

Pulse radiolysis of deaerated aqueous solutions of 4·10^–5–2.4·10^–3 mol dm^–3 Triton X-100 gives rise to a transient species originating from the reactions of OH radicals and H atoms. The rate constants of these reactions were found to be 8.8·10⁹ mol^–1·dm³·s^–1 and 1.25·10⁹ mol^–1·dm³·s^–1, respectively, for Triton X-100 concentrations below CMC. The corresponding transient species were found to decay according to second order kinetics. The mechanism of the reactions involved including concentration effects is discussed.     

Treatment of Red Tide in Ocean Using Non-Thermal Plasma Based Advanced Oxidation Technology*

A Pilot-scale experiment for the treatment of red tide in the enclosure was done in sea area of Shandong Province, P.R. China on Aug. 25, 2002. With the method of micro-gap discharge, O₂ in air and H₂O in seawater are ionized and dissociated into large numbers of OH^• radicals, and then dissolved into a part of seawater to form OH^• solution of high concentration. With OH^• concentration of 0.68 mg/L, the kill efficiencies of 29 kinds of red tide organisms such as Chaetoceros lorenzianus and so on reached 99.89%, in which bacterium and vibrio were reduced below the detection limit, also Gonyaulax cysts and Prei. Cysts with the crust were done the lowest limit of test. At the same time, the content of chlorophyll-a was decreased below the detection limit. DO (dissolved oxygen) saturation of seawater was greatly increased to 100% because the residual OH^• radical was decomposed into H₂O and O₂ after 20 min. Therefore the treatment of red tide using OH^• radicals is a kind of advanced oxidation technology, which realizes zero pollution, zero emission and zero residual in the process of the production of OH^• radicals and the treatment of red tide.*Key project of National Foundation Research from Science and Technology Ministry of China, (2002CCC00900); key project of National Natural Science Foundation of China (NSFC; 60031001).     

Superoxide radical dismutation by copper proteins

The catalytic effects of human superoxide dismutase and bovine ceruloplasmin on superoxide radical dismutation were studied by pulse radiolysis at room temperature. The rate constants for the disappearance of superoxide radicals were found to be (8.1±1.2)·10⁵M^–1·s^–1 for spontaneous disproportionation, (9.8±1.5)·10⁶ M^–1·s^–1 in the presence of ceruloplasmin, and (2.1±0.3)·10⁹M^–1·s^–1 in the presence of superoxide dismutase.