Kinetics and mechanism of oxidation of tetrahydrofurfuryl alcohol by dihydroxydiperiodatonickelate (IV) complex in aqueous alkaline medium

The kinetics of oxidation of tetrahydrofurfuryl alcohol by dihydroxydiperiodatonickelate (IV) complex in the temperature range of 20–35°C has been studied by spectrophotometry in aqueous alkaline medium. The reaction order in [Ni(IV)] was found to be unity and that in [alcohol] to be 1.64–1.69. The rate of oxidation increases with increase in [OH^?] and decreases with increase in [IO₄^?], indicating that dihydroxymonoperiodatonickelate (IV) complex is the reactive species of oxidant. Salt effect studies indicated that the reaction is of ion-dipole type. Under the protection of nitrogen the reaction system does not induce polymerization of acrylonitrile or acrylamide, which indicates that a one-step two-electron transfer mechanism without free radical intermediate may be in operation. A mechanism involving a preequilibrium of an adduct formation between Ni(IV) and alcohol has been proposed. All the experimental phenomena can be explained by the equation derived from the mechanism. The activation parameters of the rate-determining step have been calculated. © John Wiley & Sons, Inc.     

Simple Procedure for the Synthesis of 5,7-Diarylpyrido[2,3-d]pyrimidine Derivatives catalyzed by KF-Alumina

A simple KF/Al₂O₃-catalyzed reaction of 1,3-diaryl-2-propen-1-one and 2,6-diamino-4-hydroxylpyrimidine in ethyl alcohol gave aromatized 5,7-diarylpyrido[2,3-d]pyrimidine derivative by air oxidation. On the other hand, the unaromatized intermediate products were isolated under dry nitrogen successfully. A possible reaction mechanism with two pathways to lose water was proposed based on the further experimental results; one of them was confirmed by ¹H NMR spectra of isolated intermediate product.     

Kinetics and mechanism of oxidation of secondary alcohols by potassium bromate

The oxidation by Br(V) of propan-2-ol follows the rate law (?d[Br(V)]/dt) = k₄ [alcohol][Br(V)][H⁺]². The initial reaction is complicated by the presence of the product bromide ion. The reaction is composed of two second order reactions—the first, a comparatively slow one and the second stage, a faster reaction which is mainly bromine oxidation. The pure bromate oxidation can be followed by the initial addition of mercuric acetate which prevents the accumulation of bromine in the system under these conditions. The reaction rate does not depend on the nature and structure of the alcohol. A mechanism involving a slow rate-determining formation of an alkyl-bromate ester followed by a fast decomposition to the products is in accord with the observed results.     

Kinetic Studies of Catalyzed Chromic Acid Oxidation of Propionaldehyde by Picolinic Acid

The chromic acid oxidation of propionaldehyde in the presence of picoIinic acid proceeds much faster than that of propionaldehyde alone. It is shown that picolinic acid is an effective catalyst. The rate law governing this catalysis is rate=k[HCrO₄^?][PA]-[CH³CH²CHO][H⁺]²/(a+b[H⁺]+c[H⁺]²). A mechanism consistent with the rate law was proposed: picolinic acid reacts with chromic acid to form a cyclic intermediate complex C¹, C¹ further reacts with a molecule of hydrated aldehyde to give a termolecular complex C² and a hydrogen ion, C² then undergoes oxidative decomposition in the next step to give products.     

Water-catalyzed racemisation of lactide

The reactions of lactide racemisation, hydrolysis, or alcoholysis in the presence of water or anhydrous alcohol have been investigated separately. A small amount of water leads to lactide racemisation, hydrolysis, or both. Water acts as a catalyst for the racemisation of lactide. The racemisation mechanism has been studied by substituting D₂O instead of H₂O and measuring the substituent by gas chromatography-mass spectrometry (GC-MS). The experimental results show that the hydrogen atom on a chiral carbon of lactide is substituted by a ²H atom of D₂O. The reaction of lactide with water is in good agreement with the mechanism of addition-elimination. The addition of water to the carbonyl group produces an intermediate with a pair of hydroxyls connected to a carbon atom. If a hydroxyl hydrogen atom is transferred to the ester bond (CO-O), the hydrolysis of lactide generally occurs. Any of these hydroxyls could also be dehydrated with the close methine, thus producing an enolate, and the transfer of hydrogen from the enolic hydroxyl group results in lactide racemisation. The conversion of d- or l-lactide into meso-lactide is a reversible and endothermic reaction when catalyzed by water. When lactide reacts with alcohol, its alcoholysis occurs more readily than its racemisation.     

Kinetic Studies of Picolinic Acid Catalyzed Chromic Acid Oxidation of Cyclohexanone

The chromic acid oxidation of cyclohexanone catalyzed by picolinic acid in water undergoes a change from first-to zero-order dependence in both cyclohexanone and acidity. The mechanism proposed indicates the formation of an intermediate C₁ by picolinic acid and chromic acid. Then C₁ would react with enol form of cyclohexanone to give another intermediate C₂. C₂ finally cleaves into products.     

Kinetics and mechanism of the oxidation of butane-2,3-diol by alkaline hexacyanoferrate (III), catalyzed by ruthenium trichloride

The kinetics of oxidation of butane-2,3-diol by alkaline hexacyanoferrate (III), catalyzed by ruthenium trichloride has been studied spectrophotometrically. The reaction rate shows a zero-order dependence on oxidant, a first-order dependence on |Ru(III)|_T, a Michaelis-Menten dependence on |diol|, and a variation complicated on |OH⁻|. A reaction mechanism involving the existence of two active especies of catalyst, Ru(OH)₂⁺ and Ru(OH)₃, is proposed. Each one of the active species of catalyst forms an intermediate complex with the substrate, which disproportionates in the rate determining step. The complex disproportionation involves a hydrogen atom transfer from the α C(SINGLE BOND)H of alcohol to the oxygen of hydroxo ligand of ruthenium, to give Ru(II) and an intermediate radical which is then further oxidized. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 1–7, 1997.     

C–H bond activation versus ring cleavage in the gas phase ion-molecule reactions of Nb+ and Ta+ with toluene and picoline

The reactions of Nb⁺ and Ta⁺ with toluene and picoline and their deuterium-labelled analogues were studied in a Fourier transform ion cyclotron resonance mass spectrometer. Methyl substitution completely changes the reactivities relative to benzene and pyridine. Both metals react to dehydrogenate toluene exclusively. In contrast to benzene, no ring cleavage is observed in the Ta⁺/toluene reaction. A simple explanation for this difference in reactivities is proposed based on the relative energies of the Hückel orbitals of benzene and toluene. The (b₁) symmetric antibonding orbital is higher in energy for toluene. Population of this orbital is necessary for formation of the metallanorbornadiene intermediate and does not occur at thermal energies. Reaction with ring labelled toluene-d₅ shows exclusive H₂ or D₂ elimination in reaction with Nb⁺ and H₂, HD, and D₂ elimination in reaction with Ta⁺. Reactions with the picolines show both dehydrogenation and ring cleavage. Isotope labelling studies show facile H/D scrambling occurs in the intermediate ion-molecule complexes with HCN and DCN both eliminated from the methyl-d₃-2-picoline and 4-picoline. The metals react with picoline and pyridine by different mechanisms. The isotope labelling results suggest a metal-hydrido-azepinium structure for the intermediate complex.     

Mechanism of Polymerization of α-Olefins on Oxide Catalysts

The polymerization of ethylene on a chromic oxide catalyst with and without a solvent has been studied. It was found that the active catalyst surface is formed exclusively as a result of its interaction with ethylene. This interaction is accompanied by the formation of products which poison the surface of the catalyst when they are sorbed on it in the absence of a solvent. A catalyst which contains no Cr⁺⁶ atoms as a result of reduction by alcohol is inactive. On the other hand, a catalyst which contains only Cr⁺⁶ atoms becomes active only after it has been partially reduced. The most probable product of this reduction is trivalent chromium atoms. The results obtained have given grounds for the assumption that the active complex contains Cr⁺⁶ and Cr⁺³ atoms. A possible mechanism of the reaction is discussed. Owing to the oxidative action of CrO₃ on the ethylene molecules, part of the Cr⁺⁶ is reduced to Cr⁺³, and the trivalent chromium becomes alkylated. The monomer molecule is added at the Cr⁺³—C bond thus formed. A strong Lewis acid, CrO₃, lowers the electron density on the Cr⁺³ atom. This increases the strength of the Cr⁺³—C bond and the ability of the Cr⁺³ atom to coordinate with the monomer molecule. The monomer molecule enters the chain at the moment when the strength of the Cr^?3—C bond is weakened due to coordination of this molecule with the Cr⁺³ atom.     

Kinetics and mechanism of ethylenediaminetetraacetic acid-, 2,2′-bipyridyl-, and 1,10-phenanthroline-assisted chromium(VI) oxidation of 2-propanol

Complexing agents ethylenediaminetetraacetic acid (EDTA), 2,2-bipyridyl (bpy), and 1,10-phenanthroline (phen) influence the rate of chromic acid oxidation of 2-propanol in aqueous HClO₄ solutions. EDTA and bpy catalyse the oxidation rate, whereas phen has no effect, but at higher concentration it acts as an inhibitor. Dependent on conditions, the reactions follow differing order kinetics. In the presence of EDTA and bpy, at the end of the reaction, chromium(III) is present as the EDTA–Cr^III and bpy–Cr^III complexes. Probable mechanisms are proposed involving a negatively charged termolecular intermediate complex formed from chromic acid, complexing agent, and 2-propanol.     

Kinetic Studies of Chromic Acid Oxidation of Glyoxal

The kinetics of chromic acid oxidation of glyoxal is reported. The reaction is first-order in glyoxal and indicates a gradual change from a first-order to a zero-order dependence on acidity. The kinetic nature of this reaction has been studied and the rate law is consistent with a proposed mechanism as follows; rate = kK_b, K[Cr⁺⁶][Gx][H⁺]/(l+0.238[H⁺]) at 25°C. The product analysis indicates that formic acid is the oxidation product under similar kinetic condition.     

The reaction of the hydroxyl radical with acetylene

The reaction of OH with acetylene was studied in a discharge flow system at room temperature. OH was generated by the reaction of atomic hydrogen with NO₂ and was monitored throughout the reaction using ESR spectroscopy. Mass-spectrometric analysis of the reaction products yielded the following results: (1) less than 3 molecules of OH were consumed, and less than 2 molecules of H₂O were formed for every molecule of acetylene that reacted; (2) CO was identified as the major carbon-containing product; (3) NO, formed in the generation of OH, reacted with a reaction intermediate to give among other products N₂O. These observations placed severe limitations on the choice of a reaction mechanism. A mechanism containing the reaction OH + C₂H₂ → HC₂O + H₂ better accounted for the experimental results than one involving the abstraction reaction OH + C₂H₂ → C₂H + H₂O. The rate constant for the initial reaction was measured as 1.9 ± 0.6 × 10^?13 cm³ molecule^?1 sec^?1.     

Unimolecular decomposition mechanism of vinyl alcohol by computational study

Although vinyl alcohol(CH₂=CHOH)molecule was found to be an important intermediate in the combustion flames of hydrocarbon (Taatjes et al. in Science 308:1887, 2005), the removal mechanism of vinyl alcohol has not been established yet. The removal mechanism is critical to characterize the kinetics behavior of hydrocarbon in combustion chemistry and to develop the chemical models of hydrocarbon oxidation. In this work, the potential energy surface for the unimolecular decomposition of syn-CH₂=CHOH reaction has been first studied by ab initio. The kinetics and product branching ratios for the decomposition reaction are evaluated by Variflex code in the temperature range of 500–3,000 K at 0.1, 1.0, and 100.0 atmosphere pressure. The results show that the formation of CH₃ + CHO via the CH₃CHO intermediate is dominant in the decomposition reaction and its branching ratios at 0.1, 1.0, and 100.0 atm are more than 99.90, 99.30, and 89.20%, respectively, through the whole temperature range investigated.     

Contribution to the study of the sorption of radionuclides on hydrated oxides

The mechanism of¹⁰⁶Ru(III),¹⁰⁶RuNO(II),¹⁴⁴Ce(III),¹⁴⁷Pm(III),⁸⁵Sr(II),¹³¹I⁻,³⁵SO ₄ ²⁻ , and H₂ ³²PO ₄ ²⁻ radionuclide sorption on hydrated ferrous, ferric, aluminium and chromic hydrated oxides was studied. The dependence of sorption on the pH has shown that in a certain range of pH values it is the ion exchange of the radionuclide for a proton or a hydroxyl group of the oxide that probably plays a decisive role in the sorption process. The sorption depends considerably on the pH in the whole range of studied, but its decrease with cations in alkaline media and its increase with I⁻ ion in acidic media does not agree with the above sorption mechanism. Similarly, the course of the dependence of sorption on the sorbent concentration does not indicate ion exchange to be the only mechanis, but it indicates a more complicated sorption process. Probably the sorption of colloidal forms of the radionuclides proceeds, too.     

Evidence of protonation during the oxidation of some aryl alcohols by permanganate in perchloric acid medium and mechanism of the oxidation processes

The oxidation of benzyl alcohol and methoxy-, chloro-, and nitro- substituted benzyl alcohols by permanganate has been studied in aqueous and acetic acid medium in presence of perchloric acid. The reaction is first-order in [MnO₄^?] and [XC₆H₄CH₂OH], but the order is complex with respect to [H⁺]. Different thermodynamic parameters have been evaluated. The reaction occurs through the protonation of alcohol in a fast preequilibrium followed by a slow rate-determining oxidation step. A two-electron transfer oxidation step has been suggested for benzyl alcohol and chloro- and nitro- substituted alcohols, while the oxidation of methoxy compounds involves a one-electron transfer via a free-radical mechanism. © 1995 John Wiley & Sons, Inc.     

Rh-Catalyzed Oxidation of Anthracenes with tert-Butyl Hydroperoxide. Kinetic Aspects

The polar effects of substituents on reactivity in oxidation of 2-substituted anthracenes with tert-butyl hydroperoxide (TBHP)/Rh(PPh₃)₃Cl have been investigated and compared with those obtained with TBHP/VO(acac)₂ and chromic acid. The anthracene reactivities obtained from competition experiments are correlated with Hammett's σ_p-constants. The P-values are -2.60 for chromic acid and 0.72 for TBHP/VO(acac)₂. A poor correlation with p = ?0.17 (r = 0.756) was obtained for TBHP/Rh (PPh₃)₃Cl. It is concluded that the Rh-catalyzed reaction does not consist in electrophilic oxygen transfer to the anthracene.     

Role of Graphitic Nitrogen and π‐Conjugated Functional Groups in Selective Oxidation of Alcohols: A DFT based Mechanistic Elucidation

Selective oxidation of alcohols to aldehydes is challenging reaction due to its accessibility to overoxidation. In this study, we have made an attempt to unravel the mechanistic aspects of selective oxidation of allyl alcohols that contain multiple functional groups catalyzed by N‐doped graphene. The role of graphitic nitrogen and the presence of π‐conjugated functional groups are demonstrated using the state‐of‐the‐art density functional theory calculations. The detailed reaction mechanism for aerobic oxidation of allyl alcohol (AA) and cinnamyl alcohol (CA) are investigated. The formation of activated oxygen species (AOS) over N‐doped graphene (NG) has been adopted from our previous report. The results revealed that ketonic AOS oxidizes allyl alcohols into aldehydes selectively with a relatively lower activation barrier of 20.1 kcal mol^?1. The oxidation of alcohols with the AOS formed at the edge results in high activation barriers owing to its high thermodynamic stability. Similarly, AOS formed at the center leads to the formation of H₂O₂ along with high activation barriers. As a consequence, AOS formed at the center is less active when compared to ketonic AOS. The overoxidation of aldehyde is only possible due to the formation of H₂O₂. However, it is unlikely to happen due to unfavorable ambient conditions. The presence of multiple π‐conjugated functional groups is responsible for the significant reduction in the activation barriers of the second hydrogen transfer step due to the stabilization of intermediate by increasing the acidic nature of the intermediates. On the basis of the results, a generalized reaction mechanism has been proposed. These results would definitely shed light on the effective fabrication of catalysts for oxidation of alcohol and sustainable energy.     

Kinetics and mechanism of the oxidation of primary alcohols by sodium N-chloroethylcarbamate

The oxidation of primary alcohols by sodium N-chloroethylcarbamate in acid solution, results in the formation of corresponding aldehydes. The reaction is first order with respect to the oxidant and alcohol. The rate increases with an increase in acidity. The oxidation of α,α-dideuterioethanol exhibited a primary kinetic isotope, k_H/k_D = 2.11 at 298 K. The value of solvent isotope effect k(H₂O)/k(D₂O) = 2.23 at 298 K. Addition of ethyl carbamate does not affect the rate. (EtOC(OH)NHCl)⁺ has been postulated as the reactive species. Plots of (log k₂ + Ho) against (Ho + log[H⁺]) are linear with the slope, ?, having values from 1.78–1.87. This suggested a proton abstraction by water in the rate-determining step. The rates of oxidation of alcohols bearing both electron-withdrawing and electron-donating groups are more than that of methanol. A concerted mechanism involving transfer of a hydride ion from the C? H bond of the alcohol tothe oxidant and removal of a proton from the O? H group by a water molecule has been proposed.     

Detection of metaphosphate intermediates in reaction solutions of phosphate esters: Chromatographic and spectroscopic studies

This study was undertaken to gain a better understanding of uncatalysed phosphoryl transfer reactions under neutral pH conditions in aqueous media. A combination of chromatographic (Ultra high performance liquid chromatography, UHPLC) and spectroscopic (Quadrupole time-of-flight-mass spectrometry- qTOF-MS;) UV-vis; and FTIR techniques were utilized to probe the mechanism of reaction under physiological conditions (aqueous medium; pH 7.0; 25°C). Mass spectroscopic data from these studies reveal that the phosphate monoesters, phenyl phosphate (PP) and 4-nitrophenyl phosphate (NPP) undergo collision-induced dissociation dynamics through the formation of the metaphosphate intermediate, [PO₃]^? – m/z 79, a process that was found to be independent of the pKa of the leaving group. Under conditions of the present experiment, the metaphosphate intermediate does not undergo protonation to either metaphosphoric acid, HPO₃ or the dibasic phosphonic acid, H₃PO₃.     

Mechanism for the direct synthesis of tryptophan from indole and serine: a useful NMR technique for the detection of a reactive intermediate in the reaction mixture

The reaction mechanism for the biomimetic synthesis of tryptophan from indole and serine in the presence of Ac₂O in AcOH was investigated. Although the time‐course ¹H‐NMR spectra of the reaction of 5‐methoxyindole with N‐acetylserine were measured in the presence of (CD₃CO)₂O in CD₃CO₂D, the reactive intermediate could not be detected. This reaction was conducted without 5‐methoxyindole in order to elucidate the reactive intermediate, but the intermediate could not be isolated from the reaction mixture. Since the intermediate would be expected to have a very short life time, and therefore be very difficult to detect by conventional analytical methods, the structure of the intermediate was elucidated using a 2D‐NMR technique, diffusion‐ordered spectroscopy (DOSY). Two intermediates were detected and confirmed to be 2‐methyl‐4‐methyleneoxazol‐5(4H)‐one and 2‐methyl‐4‐hydroxymethyloxazol‐5(4H)‐one. The present results demonstrated that DOSY is a powerful tool for the detection of unstable intermediates. Copyright © 2010 John Wiley & Sons, Ltd.