On the mechanism of the OH initiated oxidation of acetylene in the presence of O2 and NO x

The mechanism of the oxidation of acetylene, in the presence of O₂ and NO _x , has been studied. Different levels of theory have been tested for the first step of the mechanism: the acetylene + OH radical reaction. Based on these results the meta-hybrid functional MPWB1K has been chosen for modeling all the other steps involved in the oxidation of acetylene. Different reaction paths have been considered and the one leading to glyoxal formation and OH regeneration is predicted to be the main channel, independently of the presence of NO _x . Two different mechanisms were modeled to account for formic acid formation, both of them involving cyclic intermediates. According to the computed activation free energies, the three-membered intermediate seems to be more likely to occur than the four-membered one. However, reaction barriers are very high and only a very small proportion of formic acid is expected to be formed through such intermediates. In the presence of NO _x , considered in this work for the first time, the main product of the tropospheric oxidation of acetylene is also expected to be glyoxal.     

Modeling competitive reaction mechanisms of peroxynitrite oxidation of guanine

5-Guanidino-4-nitroimidazole is a stable product from the peroxynitrite induced one-electron oxidation of guanine. Reaction mechanisms to form the 5-guanidino-4-nitroimidazole as well as 8-nitroguanine, through the combination of the guanine radical cation and nitrogen dioxide radical and through the combination of the deprotonated neutral guanine radical and nitrogen dioxide radical, have been investigated by the use of the B3LYP method of density functional theory. Our calculations suggest that the guanine radical cation mechanism is preferred over the neutral guanine radical mechanism and that a water molecule is involved in the reaction as a catalyst or as a reactant.     

Mechanism of oxidation of diphenyl sulfide by peroxyanions

Reactions of diphenyl sulfide with the two oxidants peroxydisulfate and peroxydiphosphate are described. With each oxidant, the reaction is first order in the oxidant and zero order in the substrate. The rate of the reaction is also independent of the effect of the substituent. Hydrogen ion catalyzes the reactions. The lack of inhibition of the reaction rate by added acrylamide rules out the possibility of a radical reaction initiated by the homolysis of the peroxyanions. From the effect of [H⁺] on the oxidation rates, the active species involved in the reactions have been determined. The redox reaction is essentially proceeding via hydrolysis of the protonated peroxy anions in a rate-determining step, followed by a fast step involving the oxidation of diphenyl sulfide to diphenyl sulfoxide. This has been confirmed by estimating the hydrolyzed products in each case and also by the product analyses.     

Aminoboranes: Their use in the synthesis of sulphenamides

Sulphenamides have been prepared in good yields and under mild conditions from the reaction of an aminoborane with a sulphenic ester.     

The kinetics of oxidation of selenosemicarbazide by oxygen in the presence of cupric ions

In acidic medium selenosemicarbazide undergoes oxidation by oxygen, dissolved in water, in the presence of copper(II) as a catalyst. The oxidation product exhibits an activating effect. The kinetics of the reaction have been investigated and the kinetic equations deduced both for small and high concentrations of the oxidation product. The constants involved in the kinetic equations deduced have been calculated. The proposed mechanism of the reaction agrees with the experimental results. The oxidation product is supposed to be the bis-selenosemicarbazidinium cation. The postulation of formation of this cation is based on the electronic structure and reactivity of selenosemicarbazide. The reaction can be used for copper determination.     

Mechanism of oxygen transfer in the epoxidation of an olefin by molecular oxygen in the presence of an aldehyde

[reaction: see text] The reaction pathway for peroxide-initiated aldehyde-mediated oxidation of olefins to epoxides by molecular oxygen has been studied. The pathways of reaction via a peroxy acid or an acyl peroxy radical have been differentiated by investigation of the reaction of 4 with oxygen to provide 6 via 8.     

Kinetics and mechanism of the oxidation of halotoluenes by acidic hexacyanoferrate(III)

The oxidation of halotoluenes by hexacyanoferrate(III) in aqueous acetic acid containing perchloric acid (0.5M) at 50°C gave the corresponding aldehyde as the major product, and a small amount of polymeric material. The order with respect to each of the reactants—substrate, oxidant, and acid—was found to be unity. Increasing proportions of acetic acid increased the rate of the reaction. The reaction was influenced by changes in temperature, and the activation parameters have been evaluated. The Hammett plot yielded a ρ⁺ value of ?1.8. A kinetic isotope effect k_H/k_D = 6.0 has been observed. The pathway for the conversion of the halotoluenes to the products has been mechanistically visualized as proceeding through the benzylic radical intermediate, formed in the rate-determining step of the reaction. The radical undergoes rapid conversion to the products.     

Oxidation of kojic acid catalyzed by manganese peroxidase from Ceriporiopsis subvermispora in the absence of hydrogen peroxide

We have previously reported the oxidation of kojic acid catalyzed by manganese peroxidase (MnP) from Ceriporiopsis subvermispora. This reaction is strictly dependent on Mn(II), although it does not require the addition of hydrogen peroxide. We have extended these studies because this reaction can be considered as a model system for the in situ generation of hydrogen peroxide in natural environments. We show here that oxidation of kojic acid with horseradish peroxidase (HRP) plus hydrogen peroxide or with manganic acetate rendered a product with identical chromatographic and spectral properties as the one obtained in the reaction catalyzed by MnP. The initial lag observed in the latter reaction decreased significantly upon UV irradiation of the substrate. On the other hand, ascorbic acid increased the lag and did not affect the yield of the reaction. The superoxide anion trapping agents glutathione, nitroblue tetrazolium, and superoxide dismutase markedly affected the reaction. In contrast, addition of the hydroxyl radical scavengers mannitol and salicylic acid had no effect. Based on these results, a mechanism for the MnP-catalyzed reaction is proposed.     

Radical formation during phenol oxidation in aqueous media

Kinetics of oxygen consumption, reaction product formation, and chemiluminescence during polyphenol oxidation by molecular oxygen in alkaline aqueous media with additions of l ‐ascorbic acid (AscH₂) and homocysteine (HCys) has been investigated. In these processes, AscH₂ and HCys have been shown to act as typical radical‐reaction inhibitors that can be used for determinations of the radical formation rates. The rates of radical formation during oxidation of hydroquinone (p‐QH₂), chlorohydroquinone (Cl‐QH₂), 2,5‐dichlorohydroquinone (2,5Cl‐QH₂), catechol (PK), 4‐methylcatechol (4CH₃‐PK), pyrogallol (PG), gallic acid (GK) have been estimated. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 414–422, 2012     

氮氧自由基的研究(Ⅳ)——氮氧自由基对半胱氨酸的氧化反应

氮氧自由基是目前最广泛采用的自旋标记物^[1-2]。2,2,6,6-四甲基哌啶氮氧自由基虽然在许多情况下是很稳定的,但是仍然可以发生歧化、单电子氧化还原等反应^[3-4]。     

Kinetics and mechanism of the oxidation of α-amino acids by n-bromoacetamide

The kinetics of the oxidation of eight α-amino adds by N-bromoacltanude have been studied in aqueous perchloric acid solution. The main products of the oxidation are the corresponding carbonyl compounds. The reaction is of first order with respect to the oxidant and the amino acid. The rate of oxidation decreases linearly with an increase in hydrogen ion concentration. The rate is decreased by the addition of acetamide. The oxidation of deuteriated glycine indicated the absence of a primary kinetic isotope effect. The reaction rate has been determined at different temperatures and activation parameters have been calculated. Hypobromous add has been postulated as the reactive oxidizing species. A rate-determining reaction of the neutral amino add and hypobromous add to give an N-bromo derivative has been proposed. The slow step is followed by a fast decomposition of the N-bromo derivative to yield the ultimate product.     

Kinetics and mechanism of the dehydration reaction of sarcosine to a bislactame through diacyclperoxide intermediate in strong acidic medium

The influence of substitution on the amine functional group of glycine in the permanganic oxidation of such an α‐amino acid in moderately concentrated sulfuric acid medium has been investigated. Reaction products analysis has revealed that contrary to the usual α‐amino acid oxidation product, which is an aldehyde species, a valuable compound, namely 1,4‐dimethylpiperazine‐2,5‐dione, has been obtained as the main product via a cheap, simple, efficient, and novel method. Sarcosine has been chosen as a substituted derivative of glycine, and the kinetics and mechanism of its permanganic oxidation have been investigated using a spectrophotometric technique. Conclusive evidence has proven delayed autocatalytic activity for Mn(II) in this reaction, analogous to some α‐amino acids. It has been revealed that such activity can show up when a certain concentration ratio of Mn(II) to sarcosine is built up in the medium, which we call the “critical ratio.” The magnitude of the latter ratio depends on the sulfuric acid concentration. Considering the “delayed autocatalytic behavior” of Mn(II) ions, rate equations satisfying observations for both catalytic and noncatalytic routes have been presented. The reaction shows first‐order dependence on permanganate ions and sarcosine concentrations in both catalytic and noncatalytic pathways, and apparent first‐order dependence on Mn²⁺ ions in catalytic pathways. The correspondence of pseudo‐order rate constants of the catalytic and noncatalytic pathways to Arrhenius and Eyring laws has verified “critical ratio” as well as “delayed autocatalytic behavior” concepts. The activation parameters associated with both pathways have been computed and discussed. Mechanisms for both catalytic and noncatalytic routes involving radical intermediates as well as a product having a diketopiperazine skeleton have been reported for the first time. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 689–703, 2009     

间溴苯甲酸在Pt电极上的电氧化反应

采用循环伏安法和原位傅里叶变换红外(FTIR)光谱法研究了碱性介质中间溴苯甲酸(3-BBA)在Pt 电极上的电化学氧化反应. 3-BBA 在Pt 电极上有良好的电化学活性. 在较低的氧化电位作用下(1000 mV), 3-BBA自由基负离子先在Pt 电极上失去一个电子, 生成对应的自由基, 继而发生脱羧反应生成溴苯自由基阳离子和二氧化碳. 随着电位的正移, 吸附在电极表面的羟基自由基进攻溴苯阳离子, 发生脱溴反应, 生成苯酚. 随着电位的升高, 苯酚继续氧化生成苯二酚以及苯醌等, 部分苯环还可经开环得到马来酸和富马酸.     

Electron spin resonance analysis of the redox indicator variamine blue II

The oxidation of the redox indicator Variamine Blue in an aqueous medium has been studied by the ESR method. The optimum conditions for the formation of the radical have been determined. The spectra have in some cases shown hyperfine splitting, from which conclusions could be drawn for the nature of the oxidation reaction and structure of the oxidized product. The coupling constants have also been determined for the radicals. The correctness of the assumptions for the structure of the radicals has been supported by the simulation of spectra and the investigation of deuterated derivatives.     

A rationale of the Baeyer-Villiger oxidation of cyclohexanone to ε-caprolactone with hydrogen peroxide: unprecedented evidence for a radical mechanism controlling reactivity

We demonstrate, for the first time, in the Baeyer-Villiger oxidation of cyclohexanone with aqueous hydrogen peroxide under conditions aimed at obtaining ε-caprolactone, that a thermally activated radical reaction leads to the concurrent formation of adipic acid, even when a stoichiometric amount of the oxidant is used. In fact, ε-caprolactone is the primary reaction product, but it is more reactive than cyclohexanone, and quickly undergoes consecutive transformations. When titanium silicalite-1 (TS-1) is used as a catalyst, the high concentration of hydroxy radicals within its pores accelerates the reaction rates, and the consecutive formation of adipic acid (and of lighter diacids as well) becomes largely kinetically preferred. The proper choice of the solvent, which also may act as a radical scavenger, both without catalyst and with TS-1, is a powerful tool for controlling the rates of the various reactions involved.     

Partial oxidation of 4-tert-butyltoluene catalyzed by homogeneous cobalt and cerium acetate catalysts in the Br-/H2O2/acetic acid system: insights into selectivity and mechanism

The partial oxidation of 4-tert-butyltoluene to 4-tert-butylbenzaldehyde by hydrogen peroxide in glacial acetic acid, catalyzed by bromide ions in combination with cobalt(II) acetate or cerium(III) acetate, has been studied in detail. Based on the observed differences in reaction rates and product distributions for the different catalysts, a reaction mechanism involving two independent pathways is proposed. After the initial formation of a benzylic radical species, either oxidation of this intermediate by the metal catalyst or reaction with bromine generated in situ occurs, depending on which catalyst is used. The first pathway leads to the exclusive formation of 4-tert-butylbenzaldehyde, whereas reaction of the radical intermediate with bromine leads to formation of the observed side products 4-tert-butylbenzyl bromide and its hydrolysis and solvolysis products 4-tert-butylbenzyl alcohol and 4-tert-butylbenzyl acetate, respectively. The cobalt(II) catalysts Co(OAc)(2) and Co(acac)(2) are able to quickly oxidize the radical intermediate, thereby largely preventing the bromination reaction (i.e., side-product formation) from occurring, and yield the aldehyde product with 75-80 % selectivity. In contrast, the cerium catalyst studied here exhibits an aldehyde selectivity of around 50 % due to the competing bromination reaction. Addition of extra hydrogen peroxide leads to an increased product yield of 72 % (cerium(III) acetate) or 58 % (cobalt(II) acetate). Product inhibition and the presence of increasing amounts of water in the reaction mixture do not play a role in the observed low incremental yields.     

Studies of the Anodic Oxidation of 1,4-Diazabicyclo

The title compound (1) was studied at platinum and gold electrodes in acetonitrile. A reversible oxidation peak occurs at +0.30 V vs the standard potential for ferrocenium ion/ferrocene. This process is followed by a second irreversible anodic peak that is due to the oxidation of the initially formed radical cation to the dication. The principal ultimate product of the first oxidation, the conjugate acid of 1, is also oxidized over the range of potentials corresponding to the second anodic peak. The rate of disappearance of the radical cation of 1 has been determined by cyclic voltammetry. The results are best interpreted in terms of parallel pseudo-first-order decay (k(1) = 0.6 s(-)(1)) and second-order reactions. The first of these second-order reactions is either proton transfer from the radical cation to neutral 1 or hydrogen atom abstraction by the radical cation from neutral 1, reactions that give the same products (k(2) = 100 M(-)(1) s(-)(1)) and are kinetically indistinguishable. The other second-order reaction is the hydrogen-atom-transfer disproportionation of the radical cation giving the conjugate acid of 1 and the immonium ion (k(3) = 100 M(-)(1) s(-)(1)). Both second-order processes must be included to account for the results. The present results are thought to be the first experimental evidence for the occurrence of hydrogen-atom-transfer disproportionation of amine radical cations.     

The kinetics of the chromic acid oxidation of diphenylmethane

The kinetics of the chromic acid oxidation of diphenylmethane in aqueous acetic acid solution has been studied. The rate law is v = k[φCH₂φ][CrO₃]h₀ a kinetic isotope effect, k_H/k_D = 6·4 at 30°, was noted, and electron releasing groups were found to moderately facilitate the reaction (⁺ = −1·17). These, and related data, suggest that the initial reaction is the abstraction of a hydrogen atom forming a benzhydryl radical. The latter may then be further oxidized to give the product, benzphenone. It is noted that the chromic acid oxidations which must involve hydrogen abstraction all show a kinetic dependence on the total chromium (VI) concentration, whereas those which are believed to proceed via an ester mechanism have a kinetic dependence on only the acid chromate ion. This difference is suggested as a possible method of distinguishing between these two mechanisms. The effect of the water content of the solvent on the rate of the reaction is discussed, and a tentative, relative, H₋ scale for some of these solutions is suggested. This may permit one to determine the number of molecules of water which are involved in a reaction.     

Antioxidant synergism between synthesised alkylated diphenylamine and dilauryl thiodipropionate in polyolefin base fluid

An oil-soluble additive alkylated diphenylamine (ADPA) was synthesized. The antioxidant activities of ADPA and dilauryl thiodipropionate (DLTDP) as well as their mixtures in PAO10 were evaluated by rotary bomb oxidation test, pressurized differential scanning calorimetry, hot oil oxidation test, and thermogravimetry. All the tests showed that the anti-oxidation stability of ADPA was significantly improved with the addition of DLTDP. The kinetic parameters of thermal-oxidative process were also evaluated using the Kissinger method which has been widely used for the determination of activation energy and pre-exponential factor. The results indicated that ADPA could significantly improve the activation energy of PAO10 and reduce the rate of the thermal-oxidative reaction when combining with DLTDP antioxidant. All these results demonstrated that the combination of ADPA and DLTDP revealed a good synergistic effect and could effectively enhance the thermal-oxidation stability of PAO10. The proposed mechanism of the inhibition involved a synergy between ADPA and DLTDP.     

Reactivity and acid-base behavior of ring-methoxylated arylalkanoic acid radical cations and radical zwitterions in aqueous solution. Influence of structural effects and pH on the benzylic C-H deprotonation pathway

A product and time-resolved kinetic study of the one-electron oxidation of ring-methoxylated phenylpropanoic and phenylbutanoic acids (Ar(CH2)nCO2H, n = 2, 3) has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations (Ar.+(CH2)nCO2H) or radical zwitterions (Ar.+(CH2)nCO2-) depending on pH, and pKa values for the corresponding acid-base equilibria have been measured. In the radical cation, the acidity of the carboxylic proton decreases by increasing the number of methoxy ring substituents and by increasing the distance between the carboxylic group and the aromatic ring. At pH 1.7 or 6.7, the radical cations or radical zwitterions undergo benzylic C-H deprotonation as the exclusive side-chain fragmentation pathway, as clearly shown by product analysis results. At pH 1.7, the first-order deprotonation rate constants measured for the ring-methoxylated arylalkanoic acid radical cations are similar to those measured previously in acidic aqueous solution for the alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations. In basic solution, the second-order rate constants for reaction of the radical zwitterions with (-)OH (k-OH)) have been obtained. These values are similar to those obtained previously for the (-)OH-induced alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations, indicating that under these conditions the radical zwitterions undergo benzylic C-H deprotonation. Very interestingly, with 3,4-dimethoxyphenylethanoic acid radical zwitterion, that was previously observed to undergo exclusive decarboxylation up to pH 10, competition between decarboxylation and benzylic C-H deprotonation is observed above pH 11.