The kinetics and mechanism of gas-phase N-oxidation of pyridine with hydrogen peroxide

The kinetics of N-monooxidation of 4-vinylpyridine as a π-deficient heteroaromatic compound under the conditions of gas-phase free-radical chain oxidation was studied. The experimental interference kinetic curves of synchronous hydrogen peroxide decomposition and 4-vinylpyridine N-oxidation reactions were obtained. The region of selective N-oxidation was determined and optimum conditions of N-oxide preparation found. The most probable mechanism was suggested. According to this mechanism, a key role in the free-radical N-oxidation of the substrate and its synchronization with the H₂O₂ decomposition reaction was played by HO₂ radicals.     

Décomposition du peroxyde de benzoyle dans les mélanges benzéne/X-4 pyridines (X = CH3, H,CN). etude cinétique et influence des substituants sur la réactivité nucléophile et radicalaire de l'atome d'azote

Effect of a 4-substituent in the pyridine ring upon the decomposition kinetics of benzoyl peroxide in 4-X pyridine/benzene binary mixtures(X = CH₃,H,CN) has been studied. The second-order rate constant for the pyridine-induced decomposition was 2xl0^-6l mol^-1 s^-1 and in 4-methylpyridine it was l0^-5 l mol^-1 s^-1, a five-fold increase, whereas there was no nucleophilic attack on the peroxide oxygen atoms of benzoyl peroxide by 4-cyanopyridine. The surprisingly high increase of the radical-induced decomposition in 4- cyanopyridine might result from the attack at the nitrogen atom of the pyridine ring by the phenyl radical, the 1-phenylpyridinyl radical being stabilized by the cyano group.     

Study On The Catalytic Reaction Of β-Cd-Hemin Using 2, 3, 4-Trichlorophenol As Substrate And Analytic Application

ABSTRACT|The catalytic activity of various mimetic enzymes instead of the peroxidase have been investigated by 4-aminoantipyrine (4-AAP) and 2, 3, 4-trichlorophenol (TCP) to form a dye utilizing hydrogen peroxide as hydrogen acceptor. The different Chlorophenolic derivatives, which act as a substrate in β-CD-hemin-H₂O₂-4-AAP catalytic reaction, have been systematically studied.|Meanwhile, the relationship of structure-effect for the β-CD-hemin as catalyst, and chlorphenols as substrate has been respectively discussed. The mechanism of catalytic reaction has been investigated. The results showed that β-CD-hemin was the best mimetic enzyme for peroxidase among those tested and TCP was a good substrate for the determination of hydrogen peroxide with β-CD-hemin. The method for the determination of hydrogen peroxide was proposed using 4-AAP-TCP system with β-CD-hemin as catalyst. A linear calibration graph was obtained over the H₂O₂ concentration of 4.8×10-^?8-7.7×10-^?5M, and the relative standard deviation at a H₂O₂ concentration of 2.8×10-^?5M was 2.5%. The apparent molar absorptivity of the chromogenic reaction for H₂O₂ was 1.54× 10⁴ L.mol-^?1.cm^?1. Satisfactory results were obtained in the determination of H₂O₂ in synthetic samples by this method.

Also, the method was coupled with the glucose oxidation reaction to determination glucose in human serum.     

Catalytic behaviour of a Cu(II) complex sorbed on zirconium molybdate in the decomposition of hydrogen peroxide

Summary Zirconium molybdate has been used as a support on which [Cu(NH₃)₄]²⁺ has been sorbed. Its catalytic activity has been studied through hydrogen peroxide decomposition at different temperatures, using different concentrations of hydrogen peroxide and different amounts of catalyst. A probable mechanism is suggested based on kinetic data.     

Effect of the hydrogen peroxide concentration in stereospecific oxidation of alkanes by models of non-heme oxygenases

The biomimetic oxidation of alkanes (cyclohexane, adamantane, cis-1,2-dimethylcyclohexane) with hydrogen peroxide catalyzed by Fe(II) complexes containing tetradentate nitrogen ligands (M = [Fe(bpmen)(MeCN)₂](ClO₄)₂ (bispicolyl-1,2-dimethylethylenediamine), [Fe(bpen)(MeCN)₂](ClO₄)₂ (bispicolylethylenediamine), and [Fe(tpcaH)(MeCN)₂]₂(ClO₄)₄ (tripyridylcarboxamide) is studied. The effects of the hydrogen peroxide concentration on the alcohol/ketone (A/K) ratio and on the regioselectivity of oxidation (3/2) are discovered. Rather high stereospecificity (RC = 96–99%) persisting at high hydrogen peroxide concentrations is hardly consistent with the participation of the HO^. radical, inferred from the rather low regioselectivity and low A/K ratio observed under these conditions. The molecular mechanism of oxygen transfer from hydrogen peroxide, which was earlier proved reliably for low concentrations of hydrogen peroxide ([H₂O₂]/[M] ? 10), can be applied to high peroxide concentrations ([H₂O₂]/[M] > 10) if a new ferryl species containing two equivalents of the oxidant is assumed to be involved in the process. This assumption is confirmed by the direct stereospecific formation of alkyl hydroperoxide from alkane at a high concentration of hydrogen peroxide.     

New water-soluble hydrogen donors for the enzymatic spectrophotometric determination of hydrogen peroxide

Eight N -alkyl-N-V-sulphopropylaniline derivatives have been synthesized and assessed as water-soluble hydrogen donors for the spectrophotometric determination of hydrogen peroxide in the presence of peroxidase. The sodium salts of N-ethyl-N-sulphopropylaniline (ALPS), N-ethyl-N-sulphopropyl-m-toluidine (TOPS) and N -ethyl-N-sulphopropyl-m-anisidine (ADPS) are recommended. They have excellent water solubilities, and the optimum pH range for oxidative condensation with 4-aminoantipyrine in the presence of hydrogen peroxide and peroxidase is 5.5–9.5. The absorbances of the resulting chromogens are 2–3 times higher than that achieved with phenol. The molar absorptivities of the chromogens with 4-aminoantipyrine are 41300 (ALPS, λ_max 561 nm), 37400 (TOPS, λ_max 550 nm) and 27900 (ADPS, λ_max 540 nm). Calibration graphs for the determination of hydrogen peroxide in the presence of a control serum are linear for 7–40 × 10^-6 mol H₂O₂ l^-1.     

Kinetics and mechanisms of decomposition reaction of hydrogen peroxide in presence of metal complexes

Hydrogen peroxide was discovered in 1818 and has been used in bleaching for over a century [ 1 ]. H₂O₂ on its own is a relatively weak oxidant under mild conditions: It can achieve some oxidations unaided, but for the majority of applications it requires activation in one way or another. Some activation methods, e.g., Fenton's reagent, are almost as old [ 2 ]. However, by far the bulk of useful chemistry has been discovered in the last 50 years, and many catalytic methods are much more recent. Although the decomposition of hydrogen peroxide is often employed as a standard reaction to determine the catalytic activity of metal complexes and metal oxides [ 3 , 4 ], it has recently been extensively used in intrinsically clean processes and in end‐of‐pipe treatment of effluent of chemical industries [ 5 , 6 ]. Furthermore, the adoption of H₂O₂ as an alternative of current industrial oxidation processes offer environmental advantages, some of which are (1) replacement of stoichiometric metal oxidants, (2) replacement of halogens, (3) replacement or reduction of solvent usage, and (4) avoidance of salt by‐products. On the other hand, wasteful decomposition of hydrogen peroxide due to trace transition metals in wash water in the fabric bleach industry, was also recognized [ 7 ]. The low intrinsic reactivity of H₂O₂ is actually an advantage, in that a method can be chosen which selectively activates it to perform a given oxidation. There are three main active oxidants derived from hydrogen peroxide, depending on the nature of the activator; they are (1) inorganic oxidant systems, (2) active oxygen species, and (3) per oxygen intermediates. Two general types of mechanisms have been postulated for the decomposition of hydrogen peroxide in the presence of transition metal complexes. The first is the radical mechanism (outer sphere), which was proposed by Haber and Weiss for the Fe(III)‐H₂O₂ system [ 8 ]. The key features of this mechanism were the discrete formation of hydroxyl and hydroperoxy radicals, which can form a redox cycle with the Fe(II)/Fe(III) couple. The second is the peroxide complex mechanism, which was proposed by Kremer and Stein [ 9 ]. The significant difference in the peroxide complex mechanism is the two‐electron oxidation of Fe(III) to Fe(V) with the resulting breaking of the peroxide oxygen‐oxygen bond. It is our intention in this article to briefly summarize the kinetics as well as the mechanisms of the decomposition of hydrogen peroxide, homogeneously and heterogeneously, in the presence of transition metal complexes. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 643–666, 2000     

The monooxidation of ethylene with hydrogen peroxide on the per-FTPhPFe<Superscript>3+</Superscript>OH/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> biomimetic

The gas-phase monooxidation of ethylene by hydrogen peroxide on a biomimetic heterogeneous catalyst (per-FTPhPFe³⁺OH/Al₂O₃) was studied under comparatively mild conditions. The biomimetic oxidation of ethylene with hydrogen peroxide was shown to be coherently synchronized with the decomposition of H₂O₂. Depending on reaction medium conditions, one of two desired products was formed, either ethanol or acetaldehyde. The kinetics and probable mechanism of ethylene transformation were studied.     

N-alkyl-4, 4'-Bipyridyl. A new efficient electron carrier in the photochemical hydrogen producing system

Various pyridine derivatives were found to be effective electron carriers in the photochemical hydrogen generating system consisting of ascorbic acid as a reproducible electron donor, zinc meso-tetra (4-hydroxysulphophenyl) porphyrin as a photocatalyst. and colloidal platinum. Among them the system using an N-alkyl-4, 4'-bipyridyl, especially the n-hexyl derivative (C₆Py⁺Py), was most effective, where the quantum yield for the hydrogen generation was 0.1 and the theoretical recycling number of the photocatalyst reached 100,000. The mechanism of the hydrogen generating reaction is discussed. Based on a number of standard experiments, C₆Py⁺Py is shown to be effective only in the specific combination with the present components.     

Direct Synthesis in the Undergraduate Laboratory: Synthesis of Copper Complexes from Zero-Valent Metals as a Demonstration of Base-Catalyzed Direct Synthesis

Direct synthesis is an important and active research field for scientists and technologists involved with the use of elemental metals. An undergraduate laboratory demonstration is presented that exposes students to this important synthetic technique. The direct synthesis of [Cu(NH₃)₄]²⁺ and [Cu(en)₂]²⁺ complexes in aqueous solution from zero-valent Cu metal is employed as an experiment illustrating the oxidizing properties of alkaline hydrogen peroxide solutions. The experiment also shows the decomposition of hydrogen peroxide catalyzed by the copper complexes. Finally, students can learn that the direct oxidation of metallic copper by alkaline hydrogen peroxide solution is an efficient and novel alternative approach to synthesize these and other copper complexes.     

Activation of cyclohexylhydroperoxide by diiron complexes: a new route for selective peroxide decomposition

The catalytic and selective decomposition of cyclohexylhydroperoxide has been demonstrated using dinuclear iron catalysts in acetonitrile. The complex, Fe₂OL₂(ClO₄)₄, [L=N,N′-dimethyl-2,11-diaza[3,3](2,6)pyridinophane] was found to be the most stable (up to 9 000 T.N.) and selective catalyst. Furthermore, cyclohexylhydroperoxide was found efficient as an oxidant during alkane oxidation. A free radical mechanism has been proposed for both peroxide decomposition and alkane oxidation, implicating the formation of iron–alkylhydroperoxo species.     

The question of the oxidation of α-hydroxypyridine and nicotinic acid by Fenton's reagent

The oxidation of nicotinic acid and -hydroxypyridine by Fenton's reagent (H₂O₂ + Fe⁺⁺) takes place with the decomposition of the pyridine ring to form oxalic acid. In this process, half the label from the -hydroxypyridine passes into one of the oxygen atoms of the oxalic acid and the other oxygen atoms are derived from the hydrogen peroxide. The oxalic acid isolated from the oxidation of the nicotinic acid contains oxygen from the oxidizing agent. The results obtained show that the hydroxylation of the pyridine ring, apparently preceding its decomposition, takes place with the addition of an OH group containing oxygen from the oxidizing agent to the nucleus.     

The kinetics and mechanism of oxidation of isopropanol with the hydrogen peroxide-vanadate ion-pyrazine-2-carboxylic acid system

The vanadate anion in the presence of pyrazine-2-carboxylic acid (PCA) was found to effectively catalyze the oxidation of isopropanol to acetone with hydrogen peroxide. The electronic spectra of solutions and the kinetics of oxidation were studied. The conclusion was drawn that the rate-determining stage of the reaction was the decomposition of the vanadium(V) diperoxo complex with PCA, and the particle that induced the oxidation of isopropanol was the hydroxyl radical. Supposedly, the HO^· radical detached a hydrogen atom from isopropanol, and the Me₂ C^· (OH) radical formed reacted with HOO^· to produce acetone and hydrogen peroxide. The electronic spectra of solutions in isopropanol and acetonitrile and the dependences of the initial rates of isopropanol oxidation without a solvent and cyclohexane oxidation in acetonitrile on the initial concentration of hydrogen peroxide were compared. The conclusion was drawn that hydroxyl radicals appeared in the oxidation of alkanes in acetonitrile in the decomposition of the vanadium diperoxo complex rather than the monoperoxo derivative, as was suggested by us earlier.     

Microwave activation in the synthesis of nitrogen heterocycles, N-oxides of pyridine series

Oxidation of heterocycles of the pyridine series under conditions of microwave irradiation was studied. It is established that oxidation of pyridine and 4-methylpyridine with hydrogen peroxide results in the corresponding N-oxides, and oxidation of 3-(piperidin-2-yl)pyridine gives δ-oximino-δ-(pyridyl-3-N-oxide)-valeric acid.     

Studies on substrates for the spectrophotometric determination of hydrogen peroxide based on the catalytic effects of peroxidase-like metalloporphyrins

Summary The effects have been studied of ten phenol derivatives as chromogenic substrates for the hydrogen peroxide oxidation of 4-aminoantipyrine, catalysed by horse radish peroxidase or peroxidase-like metalloporphyrins. Sodium 2-hydroxy-3,5-dichlorobenzene-sulfonate was found to be the most suitable substrate for the determination of H₂O₂.     

Oxidations with alkaline permanganate using monovalent thallium for the back titration: Estimation of hydrogen peroxide

Owing to the instability of hydrogen peroxide in alkaline solutions, direct oxidation with KmnO₄ did not yield accurate results.The back titration of KmnO₄ in the presence of IN NaOH and Ba⁺² ions also gave inaccurate results. The reaction could not be checked at the manganate state. However, in the presence of 2N NaOH and telluric acid quantitative data were obtained, which is not the case if telluric acid is absent.Another advantageous method is the oxidation of hydrogen peroxide with excess KmnO₄ in the presence of 1N NaOH and telluric acid, followed by back titrating excess oxidant with monovalent thallium.     

Kinetic studies on acid- and base-catalyzed aquation of bis-alaninatochromium(III) and on oxidation of tris- and bis-Aa–chromium(III) complexes (Aa = Gly,Ala, Asn) by H2O2

Two aqua derivatives of [Cr(Ala)₃] were characterized in solution. Acid-catalyzed aquation of cis-[Cr(Ala)₂(H₂O)₂]⁺ leads to inert [Cr(Ala)(H₂O)₄]²⁺, whereas base hydrolysis of cis-Cr(Ala)₂(OH)₂]^? causes dissociation of both the Ala ligands and formation of chromates(III). Kinetics of these processes have been studied spectrophotometrically in both 0.1–1.0 M HClO₄ and 0.2–0.9 M NaOH under first-order conditions. A linear dependence of the k _obs,H on [H⁺] and a small dependence of the (k _obs)_OH on [OH^?] were established. In the proposed mechanism, the rate determining step is Cr–N bond breaking in the reactive form of the substrate, i.e., in the protonated aqua- or dihydroxo complex. The effect of pH on the complex reactivity is discussed. The kinetic results are compared with those determined previously for analogous glycine and asparagine complexes. Additionally, oxidation of tris- and bis-Aa–chromium(III) complexes, where Aa = Gly, Ala or Asn, by hydrogen peroxide in alkaline medium was studied. Two reaction products were detected: thermodynamically stable CrO₄ ^2? and [Cr(O₂)₄]^3? that under a large excess of hydrogen peroxide is metastable. The rate-limiting stage of this process is an inner sphere two-electron transfer within the peroxido intermediate.     

Grape tissue-based electrochemical sensor for the determination of hydrogen peroxide

The use of grape tissue as a source of catalase for the determination of hydrogen peroxide is reported. A slice of grape tissue attached to the membrane of a Clark-type oxgen sensor was used to monitor the oxidation of hydrogen peroxide by catalase. At the steady state, the sensor responds linearly to hydrogen peroxide in the concentration range 1 × 10^?5–5 × 10^?4 M. The response time (T₉₀) was of the order of 1 min for this sensor. No interference was observed from ethanol, amino acids, glucose and lactic acid. The long-term stability of the grape tissue sensor was much better than previously reported immobilized enzyme and liver tissue-based hydrogen peroxide sensors.     

Some reactions of 2-chloro-3-(<Emphasis Type="Italic">β</Emphasis>-chloroethyl)-4,6-dihydroxypyridine

The chemical properties of 2-chloro-3-(β-chloroethyl)-4,6-dihydroxypyridine (I) have been studied. It has been shown that this compound, which is relatively stable in acids and in neutral and, particularly, in alkaline media, readily splits off hydrogen chloride under mild conditions and is converted into derivatives of 2, 3-dihydro-5-azabenzofuran. The dehalogenation of I in an acid medium yielded 3-(β-chloroethyl)-4, 6-dihydroxypyridine, which was converted into 4, 6-dichloro-3-(β-chloroethyl)pyridine and into 6-chloro-4-methoxy-3-vinylpyridine.     

Dihaloiodates(I): synthesis with hydrogen peroxide and their halogenating activity

Tetraalkylammonium and pyridinium dichloro- and dibromoiodates(I) were efficiently prepared from iodine and tetralkylammonium chloride or pyridine by oxidation with hydrogen peroxide in the presence of an equimolar amount of a hydrogen halide. Their halogenating activity was tested on 1,3-dimethoxybenzene and styrene as model substrates. The results show that ICl₂^? salts acted as iodinating reagents, while IBr₂^? salts brominated both substrates.