Studies on the reaction of p-hydroxybenzyl alcohol and hydroxyl radicals

The reaction of p-hydroxybenzyl alcohol and hydroxyl radicals generated by the Fenton reaction is studied. The products of the reaction are separated and identified by high-performance liquid chromatography (HPLC)-diode-array detection and HPLC-mass spectrometry. According to the structures of the products, a mechanism of the reaction is proposed.     

Kinetics of solute adsorption at solid/aqueous interfaces: searching for the theoretical background of the modified pseudo-first-order kinetic equation

It is shown that the modified pseudo-first-order (MPFO) kinetic equation proposed recently by Yang and Al-Duri simulates well the behavior of the kinetics governed by the rate of surface reaction and described by our general kinetic equation, based on the statistical rate theory. The linear representation with respect to time, offered by the MPFO equation seems to be a convenient tool for distinguishing between the surface reaction and the diffusional kinetics. Also, a method of distinguishing between the surface reaction and the intraparticle diffusion model based on analyzing the initial kinetic isotherms of sorption is proposed. The applicability of these procedures is demonstrated by the analysis of adsorption kinetics of the reactive yellow dye onto an activated carbon.     

Unraveling the Mechanism for the Sharp‐Tip Enhanced Electrocatalytic Carbon Dioxide Reduction: The Kinetics Decide

The electrocatalytic reduction reaction of carbon dioxide can be significantly enhanced by the use of a sharp‐tip electrode. However, the experimentally observed rate enhancement is many orders of magnitudes smaller than what would be expected from an energetic point of view. The kinetics of this tip‐enhanced reaction are shown to play a decisive role, and a novel reaction‐diffusion kinetic model is proposed. The experimentally observed sharp‐tip enhanced reaction and the maximal producing rate of carbon monoxide under different electrode potentials are well‐reproduced. Moreover, the optimal performance shows a strong dependence on the interaction between CO₂ and the local electric field, on the adsorption rate of CO₂, but not on the reaction barrier. Two new strategies to further enhance the reaction rate have also been proposed. The findings highlight the importance of kinetics in modeling electrocatalytic reactions.     

Purine N-oxides—XLI : The 3-acyloxypurine 8-substitution reaction: On the mechanism of the reaction

The 3-acyloxypurine 8-substitution reaction involves elimination of the 3-acyloxy group and nucleophilic substitution at C-8 to yield 8-substituted xanthines or guanines. In aqueous solutions the reaction of 3-acetoxyxanthine proceeds slowly below pH 2, but is greatly accelerated with an increase of the pH from 3 to 7. It is proposed that the slow reaction involves heterolytic cleavage of the 3-acetoxy moiety from 3-acetoxyxanthine to yield a nitrenium ion at N-3 followed by intermolecular nucleophilic substitution of the incipient carbonium ion at the allylic C-8 position, also the most probable mechanism in polar aprotic solvents. The beginning of the fast reaction coincides with the beginning of ionization of the imidazole hydrogen of 3-acetoxyxanthine. It is proposed that this ionization induces a similar but more rapid departure of the 3-acetoxy group from the anion of 3-acetoxyxanthine to produce dehydroxanthine. The latter, upon protonation, yields the same reactive carbonium ion at C-8 that is formed in the slow reaction. Some reduction of 3-acetoxyxanthine to xanthine accompanies the fast reaction. That reduction has the characteristics of a free-radical mediated reaction. It is proposed that reduction results from a homolytic cleavage of the NO bond in the 3-acetoxyxanthine anion to produce a radical-anion, which abstracts hydrogen from water to yield xanthine. These reaction mechanisms and possible alternatives are evaluated.     

Extraction of the contribution of kinetic profile for reaction of a chemical species in unknown matrices by standard addition method

A new and simple strategy is applied to resolve kinetic profile for the reaction of an analyte in unknown matrices, using standard addition method (SAM). The proposed method uses kinetic spectrophotometric data obtained by standard addition of analyte into unknown mixtures followed by the reaction of analyte with a proper reagent. The proposed method extracts kinetic profile for the reaction of an analyte by averaging the kinetic profiles obtained by subtraction of kinetic profiles after and before standard addition. The rate constant can be obtained using computational curve fitting. The performance of method was evaluated by using synthetic data as well as several experimental data sets. The proposed method can be applied to obtain kinetic profiles of the reactions in the presence of additive interference as well as multiplicative interferences. Hydroxylation reaction of diphenylcarbazide (DPCI) in the presence of diphenylcarbazone (DPCO) as a real system at various pHs was also studied by the present method. The rate constant and the order of the hydroxylation reaction were determined from extracted kinetic profiles.     

杂多酸为催化剂的丙烯与醋酸气-液酯合成的反应模型初探

杂多酸对有机羧酸与低级烯烃的直接加成反应具有催化作用,但一般需在高压下进行。作者研究了较低压力(0.5～2.0 MPa)下的醋酸与丙烯、丁烯的加成酯化反应,结果较令人满意。本文初步探讨了醋酸与丙烯加成反应模型,为预测最佳反应条件提供了证据。     

微量热法研究过氧化氢酶底物抑制动力学

过氧化氢酶是需氧生物体内抗氧化酶系的重要组分。过氧化氢酶催化过氧化氢分解是一个两底物酶促反应,依照Chance提出的机理,反应速率方程具有一级反应方程的形式。此反应在高浓度底物存在的情况下,表现出明显的不可逆底物抑制。本研究用热动力学方法研究了这一反应,提出了一种不可逆底物抑制机理,并应用该机理求出了相关动力学参数。在310.15K,pH=7.0时k0=9.6×10^5L·mol^-1^·s^-1,k1/k2=2.9×10^6。实验结果证明此机理正确有效。     

Alkynes From Furans: A General Fragmentation Method Applied to the Synthesis of the Proposed Structure of Aglatomin B

Furans are versatile synthons in organic chemistry. Described is a general method for transforming furans into alkynes by dual C?C double‐bond cleavage. The reaction is proposed to proceed by sequential [4+2] cycloaddition between furan and singlet oxygen and a formal retro‐(3+2) fragmentation of the endoperoxide intermediate. A wide array of furans, including those derived from sapogenins, are amenable to this reaction, thus providing the corresponding alkynoic acids in up to 88 % yields. The synthetic utility was demonstrated by a seven‐step synthesis of the proposed structure of a pregnane natural product, aglatomin B, from a known intermediate.     

Kinetics and Mechanism of the Autocatalytic Oxidation of Mn(II) by Bromine

The oxidation of Mn(II) by bromine is an autocatalytic reaction, which seems to be important for a detailed elucidation of chemical oscillators, based on manganese chemistry. With regard to the mechanism proposed previously, an alternative reaction mechanism is proposed, based on a micro-heterogeneous oxidation of Mn(II) ion, adsorbed on a surface of the MnO₂ colloid. This revised version was published online in June 2006 with corrections to the Cover Date.     

Evidence for the formation of a mo-h intermediate in the catalytic cycle of formate dehydrogenase

DFT/BP86/TZVP and DFT/B3LYP/TZVP have been used to investigate systematically the reaction pathways associated with the H-transfer step, which is the rate-determining step of the reaction HCOO(-) ? CO(2) + H(+) + 2e(-), as catalyzed by metalloenzyme formate dehydrogenase (FDH). Actually, the energetics associated with the transfer from formate to all H (proton or hydride) acceptors that are present within the FDH active site have been sampled. This study points to a viable intimate mechanism in which the metal center mediates H transfer from formate to the final acceptor, i.e. a selenocysteine residue. The Mo-based reaction pathway, consisting of a β-H elimination to metal with concerted decarboxylation, turned out to be favored over previously proposed routes in which proton transfer occurs directly from HCOO(-) to selenocysteine. The proposed reaction pathway is reminiscent of the key step of metal-based catalysis of the water-gas shift reaction.     

Investigation of the Influence of Diffusion Control on the Curing Reaction Using DSC and Temperature Modulated DSC

The curing reaction of a thermosetting system is investigated by DSC and temperature modulated DSC (TMDSC). When the material vitrifies during curing, the reaction becomes diffusion controlled. The phase shift signal measured by TMDSC includes direct information on the reaction kinetics. For long periods the phase shift is approximately proportional to the partial temperature derivative of the reaction rate. This signal is very sensitive for changes in the reaction kinetics. In the present paper an approach to determine the diffusion control influence on the reaction kinetics from the measured phase shift is developed. The results are compared with experimental data. Further applications of this method for other reactions are proposed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalysis of the arsenic(III)-potassium bromate reaction--II: catalytic determination of osmium

A simple visual titrimetric procedure is proposed for the catalytic determination of osmium, based on the arsenic(III)-bromate reaction. A detailed kinetic study of the catalytic reaction allows a mechanism to be proposed and optimum conditions for analysis to be selected. The analytical method has a detection limit of 10(-4) ppm osmium and possesses a higher degree of freedom from interference by ruthenium than other similar systems.     

Kinetic studies of the oxidative coupling of methane over the Mn/Na2WO4/SiO2 catalyst

Oxidative coupling of methane is a direct way to obtain C2 hydrocarbon,and Mn-Na-W/SiO2 catalyst is the most promising among all the catalysts.The 2%Mn/5%Na2WO4/SiO2 catalyst was prepared by the incipient wetness impregnation method.A 7-step heterogeneous reaction model of the oxidative coupling of methane to C2 hydrocarbons was conducted by co-feeding methane and oxygen at a total pressure of 1 bar over the catalyst.The kinetic measurements were carried out in a micro-catalytic fixed bed reactor.The kinetic data were obtained at the appropriate range of reaction conditions (4 kPa＜Po2 ＜20 kPa,20 kPa＜PCZH4 ＜80 kPa,800℃＜T＜900℃).The proposed reaction kinetic scheme consists of three primary and four consecutive reaction steps.The conversions of hydrocarbons and carbon oxides were evaluated by applying Langrnuir-Hinshelwood type rate equations.Power-law rate equation was applied only for the water-gas shift reaction.In addition,the effects of operating conditions on the reaction rate were studied.The proposed kinetic model can predict the conversion of methane and oxygen as well as the yield of C2 hydrocarbons and carbon oxides with an average accuracy of ±15%.     

Transannular Diels-Alder studies on the asymmetric total synthesis of chatancin: the pyranophane approach

[reaction: see text] A pathway is proposed for the biosynthesis of (+)-chatancin and (+)-sarcophytin linking these tetracycles to cembranoids by a pyranophane transannular Diels-Alder reaction. Preliminary synthetic results in this direction to reach macrocyclic dienedione 28 from farnesol are reported. Major synthetic steps include a Prins reaction, two enantioselective hydrogenations, and a macrocyclization via a beta-ketosulfoxide Michael-addition on an enone.     

On the complexity of kinetics and the mechanism of the thiosulfate-periodate reaction

The thiosulfate-periodate reaction has been studied spectrophotometrically in a slightly acidic medium at 25.0 ± 0.1 °C in an acetate/acetic acid buffer by monitoring the absorbance in the 250-600 nm wavelength range at a constant ionic strength adjusted by the buffer component sodium acetate. In agreement with a previous study, we found that the reaction cannot be described by a single stoichiometric equation, tetrathionate and sulfate are simultaneously formed, and its ratio strongly depends on the pH. As expected at certain initial concentration ratios of the reactants, the reaction behaves as a clock reaction, but after its appearance, iodine is slowly consumed mainly because of the moderate tetrathionate-iodine reaction. It is also enlightened that the initial rate of the reaction is completely independent of the pH, which apparently contradicts a previous study, which postulates a "supercatalytic" behavior of the hydrogen ion on the title reaction. Significant buffer assistance that may change the absorbance-time profiles was also observed. On the basis of the kinetic data, a robust 28-step kinetic model with 22 fitted parameters is proposed and discussed to explain adequately all of the important characteristics of the kinetic curves.     

Kinetic photometric determination of copper at the nanograms per millilitre level in real samples

A highly sensitive and selective kinetic method is proposed for the determination of nanogram amounts of copper(II) based on its catalytic effect on the oxidation of 3-hydroxybenzaldehyde azine by potassium peroxydisulphase in an ammoniacal medium. The rate of the reaction is monitored spectrophotometrically at 465 nm and the method allows the determination of copper concentrations in the range 0.5–18 ng ml^?1 with a relative standard deviation of about 2%. The proposed method is virtually specific to copper and has been applied satisfactorily to its determination in a variety of food and industrial samples. A possible mechanism for the catalysed reaction is proposed.     

The influence of the adsorption of surface active substance on the rate constant of the electrode reaction

The electrode reaction of copper-EDTA complex was studied polarographically in the presence of polyvinylalcohol. The d.c. polarographic current-time curves were analyzed by using the equation given by Matsuda for the diffusion process of the depolarizer influenced by the adsorbates. The rate constants affected by the adsorption are assumed to be composed of two parts at relatively high coverage, i.e., one is the rate constant of the electrode reaction at free surface of DME and the other the rate constant of the electrode reaction through the adsorded layer. The rate constant at the uncovered surface was analyzed by a formula analogous to that proposed by Parsons and it was shown that one of the parameters involved in the formula depends on the electrode potential. On the other hand, it was shown that the electrode reaction at the covered surface is irreversible, and its cathodic rate constant depends on the electrode potential exponentially.     

Toward the total synthesis of methyl isosartortuoate: construction of the backbone of the diene unit

The backbone of diene precursor in the proposed biogenesis of methyl isosartortuoate through a Diels-Alder reaction has been constructed via dehydration to establish the conjugated system; and double cyclization by using Horner-Wittig-Emmons reaction and the chiral epoxy ring opening.     

Study of the base-catalysed oxidation of the anti-bacterial and anti-protozoal agent metronidazole by permanganate ion in alkaline medium

The mechanism of dismutation of MnO₄ ^2? via the complex [MTZ–MnO₄·OH]^2?, formed during the oxidation of metronidazole (MTZ), has been investigated spectrophotometrically at different temperatures. The stoichiometry of the reaction is 1:1, i.e. 1 mol MTZ reacts with 1 mol Mn(VII).The reaction is first order in permanganate, less than first order in [MTZ] and [alkali]. The effects of added products and the dielectric constant and ionic strength of the reaction medium were investigated. The main products were identified by spot test and FT-IR. A mechanism involving a free radical has been proposed. In the equilibrium step MTZ binds to the MnO₄ ^? species to form a complex (C). Investigation of the reaction at different temperatures enabled determination of the activation data for the slow step of proposed mechanism. The proposed mechanism and the derived rate laws are consistent with the observed kinetics.