Oxidation of alcohol by lipopathic Cr(VI): a mechanistic study

The oxidation kinetics of various aliphatic primary and secondary alcohols having varied hydrocarbon chain length were studied using cetyltrimethylammonium dichromate (CTADC) in dichloromethane (DCM) in the presence of acetic acid and in the presence of a cationic surfactant. The rate of the reaction is highly sensitive to the change in [CTADC], [alcohol], [acid], [surfactant], polarity of the solvents, and reaction temperature. A Michaelis-Menten type kinetics was observed with respect to substrate. The chemical nature of the intermediate and the reaction mechanism were proposed on the basis of (i) observed rate constant dependencies on the reactants, that is, fractional order with respect to alcohol and acid and a negative order with respect to oxidant, (ii) high negative entropy change, (iii) inverse solvent kinetic isotope effect, k(H2O)/k(D2O) = 0.76, (iv) low primary kinetic isotope effect, kH/kD = 2.81, and (v) the k(obs) dependencies on solvent polarity parameters. The observed experimental data suggested the self-aggregation of CTADC giving rise to a reverse micellar system akin to an enzymatic environment, and the proposed mechanism involves the following: (i) formation of a complex between alcohol and the protonated dichromate in a rapid equilibrium, equilibrium constant K = 5.13 (+/-0.07) dm(3) mol(-1), and (ii) rate determining decomposition (k(2) = (7.6 +/- 0.7) x 10(-3) s(-1)) of the ester intermediate to the corresponding carbonyl compound. The effect of [surfactant] on the rate constant and the correlation of solvent parameters with the rate constants support the contribution of hydrophobic environment to the reaction mechanism.     

Deoximation by cetyltrimethylammonium dichromate: A kinetic study

The deoximation kinetics of some oximes was studied by using cetyltrimethylammonium dichromate (CTADC) in dichloromethane in the presence of acetic acid and a cationic surfactant. The rate of reaction is highly sensitive to the change in [CTADC], [oxime], [acid], [surfactant], polarity of the solvents, and reaction temperature. The reaction is found to be catalyzed by acid with an appreciable uncatalytic rate. The reaction is first order with respect to substrate. With increase in CTADC concentration, rate of the reaction increases with a fractional order dependency with respect to oxidant. Consistent to the observation, a mechanism has been proposed in which the substrate forms a complex with CTADC in the rate determining step followed by decomposition with a fast process to yield corresponding carbonyl compounds. The structure of the substituents has also a significant effect on the rate constant. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 482–488, 2011     

Oxidation of Antitubercular Drug Isoniazid by a Lipopathic Oxidant,Cetyltrimethylammonium Dichromate: A Mechanistic Study

The oxidation of an antitubercular drug isoniazid by a lipopathic oxidant cetyltrimethylammonium dichromate (CTADC) in a nonpolar medium generates isonicotinic acid both in the presence and the absence of acetic acid. The conventional UV–vis spectrophotometric method is used to study the reaction kinetics. The occurrence of the Michaelis–Menten–type kinetics with respect to isoniazid confirms the binding of oxidant and substrate to form a complex before the rate‐determining step. The existence of the inverse solvent kinetic isotope effect, k(H₂O)/ k(D₂O) = 0.7, in an acid‐catalyzed reaction proposes a multistep reaction mechanism. A decrease in the rate constant with an increase in [CTADC] reveals the formation of reverse micellar–type aggregates of CTADC in nonpolar solvents. In the presence of different ionic and nonionic surfactants, CTADC forms mixed aggregates and controls the reaction due to the charge on the interface and also due to partition of oxidant and substrate in two different domains. High negative entropy of activation (ΔS^? = –145 and –159 J K^?1 mol^?1 in the absence and presence of acetic acid) proposes a more ordered and highly solvated transition state than the reactants. Furthermore, the solvent polarity‐reactivity relationship reveals (i) the presence of less polar and less ionic transition state compared to the reactants during the oxidation, (ii) differential contribution from nonpolar and dipolar aprotic solvents toward the reaction process, and (iii) the existence of polarity/hydrophobic switch at log P = 0.73. A suitable mechanism has been proposed on the basis of experimental results. These results may provide insight into the mechanism of isoniazid oxidation in hydrophobic environment and may assist in understanding the drug resistance in different location.     

Kinetic and mechanistic studies on the oxidation of Norfloxacin by Chloramine‐B and N‐Chlorobenzotriazole in acidic medium

The kinetics of oxidation of Norfloxacin [1‐ethyl‐6‐fluoro‐1,4‐dihydro‐4‐oxo‐7‐(l‐piperazinyl)‐3‐quinoline carboxylic acid] by chloramine‐B and N‐chlorobenzotriazole has been studied in aqueous acetic acid medium (25% v/v) in the presence of perchloric acid at 323 K. For both the oxidants, the reaction follows a first‐order dependence on [oxidant], a fractional‐order on [Norfloxacin], and an inverse‐fractional order on [H⁺]. Dependence of reaction rate on ionic strength, reaction product, dielectric constant, solvent isotope, and temperature is studied. Kinetic parameters are evaluated. The reaction products are identified. The proposed reaction mechanism and the derived rate equation are consistent with the observed kinetic data. Formation and decomposition constants for substrate–oxidant complexes are evaluated. ©1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 153–158, 1999     

Kinetics and mechanism of oxidation of aspirin by bromamine-T,N-bromosuccinimide,and N-bromophthalimide

The kinetics of the oxidation of aspirin (ASP) by bromamine-T (BAT), N-bromosuccinimide (NBS), and N-bromophthalimide (NBP) has been studied in aqueous perchloric acid at 303 K. The oxidation reaction follows identical kinetics with first-order in [oxidant], fractional-order in [ASP], and inverse fractional-order in [H⁺]. Under identical experimental conditions the extent of oxidation with different oxidizing agents is in the order: NBS>BAT>NBP. The rate decreased with decreasing dielectric constant of the medium. The variation of ionic strength and the addition of the reaction products and halide ions had no significant effect on the reaction rate. The solvent isotope effect was studied using D₂O. Kinetic parameters were evaluated by studying the reaction at different temperatures. The reaction products were identified by GC–MS. The proposed reaction mechanism and the derived rate law are consistent with the observed kinetic data. Formation and decomposition constants for ASP-oxidant complexes have been evaluated. Decarboxylation, bromination, and loss of acetic acid gave 2,4,6-tribromophenol. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 407–414, 1998     

Cooxidation of anilides and oxalic acid by chromic acid: A one‐step,three‐electron oxidation

The chromic acid oxidation of a mixture of oxalic acid and anilides proceeds much faster than that of either of the two substrates alone. The oxidation kinetics of acetanilide, p‐methyl‐, p‐chloro‐, and p‐nitroacetanilides by Cr(VI) in the presence of oxalic acid in aqueous acetic acid medium follows first‐order, zero‐order, and second‐order dependence in [oxidant], [substrate], and in [oxalic acid], respectively, while the oxidation kinetics of benzanilide, p‐methyl‐, p‐chloro‐, and p‐nitrobenzanilides follow first order in [oxidant] and fractional order each in [substrate] and [oxalic acid] and yields corresponding azobenzenes and benzaldehydes in the case of benzanilide and substituted benzanilides as the main products of oxidation. Aluminium ions suppress the reaction. The intermediate is believed to be formed from the anilide and a chromic acid‐oxalic acid complex. In the proposed mechanism, the rate‐limiting step involves the direct reduction of Cr(VI) to Cr(III). © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 33: 21–28, 2001     

A kinetic and mechanistic study on the oxidation of indole‐3‐acetic acid by peroxodisulfate

The kinetics of oxidation of indole‐3‐acetic acid (IAA) by peroxodisulfate (PDS) has been carried out in aqueous acetic acid medium. First‐order dependence of rate each with respect to [IAA] and [PDS] was observed. The reaction rate was unaffected by added [H⁺]. Increase of percentage of acetic acid decreased the rate. Variation of ionic strength (μ) had negligible influence on the rate. A suitable kinetic scheme based on these observations involving a nonradical mechanism is proposed. The reactivity of peroxodisulfate toward indole‐3‐acetic acid was found to be lower than that with peroxomonosulfate. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 355–360, 2005     

Effect of Anionic Surfactant on the Oxidation of DL-Aspartic Acid by N-Bromophthalimide: A Kinetic Study

The kinetics of oxidation of DL-Aspartic acid (Asp) by N-bromophthalimide (NBP) was studied in the presence of sodium dodecyl sulfate (SDS) in acidic medium at 308 K. The rate of reaction was found to have first-order dependence on [NBP], fractional order dependence on [Asp] and inverse fractional order dependence on [H⁺]. The addition of reduced product of the oxidant, that is, [Phthalimide] has decreased the rate of reaction. The rate of reaction increased with increase in inorganic salts concentration, whereas a change in [Cl^?], ionic strength of the medium and [Hg(OAc)₂] had no effect on the oxidation velocity. The rate of reaction decreased with a decrease in dielectric constant of the medium. COOH-CH₂-CN was identified as the main oxidation product of the reactions. The various activation parameters have been computed. A suitable reaction mechanism consistent with the experimental findings has been proposed. The micelle-binding constant has been calculated.     

Oxidation of Some α-Amino Acids by Pyridinium Bromochromate in an Aquo-Acetic Acid Medium—A Kinetic and Mechanistic Study

Oxidation of -amino acids by pyridinium bromochromate (PBC) was studied in acetic acid–water mixture containing perchloric acid. The reaction rate is first order in [PBC] and inverse first order in [H⁺] and has aldehyde as a product. The results are contrary to that of Karim and Mahanti, who observed first order with [H⁺] and cyanide as the product in the oxidation of amino acids by quinolinium dichromate. Michaelis–Menten type kinetics has been observed with respect to -amino acids. The rate of reaction increases with a decrease in the polarity of solvent indicating an ion–dipole interaction in the slow step. The reactions exhibit no primary kinetic isotope effect. The activation parameters have been evaluated. The reaction mechanism involving the formation of chromate-ester between unprotonated PBC and unprotonated amino acid followed by C–C bond fission in the slow step has been suggested. The value of the Michaelis constant (substrate–oxidant complex formation constant) increases as the number of carbon atoms increases in the amino acid.     

Mn(III) oxidation of peptides: A mechanistic investigation of kinetic variations in hydrophobic‐induced oxidation of tetrapeptides of elastin sequences

Four tetrapeptide analogues of elastin sequences, glycyl‐glycyl‐alanyl‐proline (GGAP), glycyl‐glycyl‐valyl‐proline (GGVP), glycyl‐glycyl‐isoleucyl‐proline (GGIP), and glycyl‐glycyl‐phenylalanyl‐proline (GGFP) were synthesized, based on their increasing order of hydrophobicity, by a classical solution phase method and were characterized. These tetrapeptides (TETPs) were oxidized using Mn(OAc)₃ in 25% acetic acid at 298 K, and the kinetics of the reaction was monitored spectrophotometrically at λ_max = 400 nm. A first‐order dependence of rate on each of [Mn(OAc)₃], [OAc^?], and substrate [TETP], an inverse order dependence on [H⁺], has been observed. The rate is independent of [Mn(II)]. However, an inverse order dependence on varying the dielectric constant using various percentages (v/v) of acetic acid has also been observed, and but addition of anions such as Cl^? and ClO₄^? has insignificant effect on the rate. Activation parameters have been evaluated using the Arrhenius and Erying plots. The oxidation products were isolated and characterized. Based on the results obtained, a plausible mechanism involving [Mn(OAc)₄]^? has been proposed. An apparent correlation was noted between the rate of oxidation of these TETPs by Mn(III) in the presence of sulfate ions in sulfuric acid medium and Mn(OAc)₃ in the acetic acid medium. The rate of oxidation with Mn(OAc)₃ was observed to be slower than with the former. The rate of oxidation of GGFP was found to be higher than GGIP, GGVP, and GGAP. This may be due to the presence of an aromatic side chain and/or because of the increased hydrophobicity. The overall order of rate of oxidation of TETPs is GGFP > GGIP > GGVP > GGAP, which also represents an increasing order of their hydrophobicity. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 115–123, 2006     

Mechanism of the OH-initiated oxidation of hydroxyacetone over the temperature range 236-298 K

The mechanism of the gas-phase reaction of OH radicals with hydroxyacetone (CH3C(O)CH2OH) was studied at 200 Torr over the temperature range 236-298 K in a turbulent flow reactor coupled to a chemical ionization mass-spectrometer. The product yields and kinetics were measured in the presence of O2 to simulate the atmospheric conditions. The major stable product at all temperatures is methylglyoxal. However, its yield decreases from 82% at 298 K to 49% at 236 K. Conversely, the yields of formic and acetic acids increase from about 8% to about 20%. Other observed products were formaldehyde, CO2 and peroxy radicals HO2 and CH3C(O)O2. A partial re-formation of OH radicals (by approximately 10% at 298 K) was found in the OH + hydroxyacetone + O2 chemical system along with a noticeable inverse secondary kinetic isotope effect (k(OH)/k(OD) = 0.78 +/- 0.10 at 298 K). The observed product yields are explained by the increasing role of the complex formed between the primary radical CH3C(O)CHOH and O2 at low temperature. The rate constant of the reaction CH3C(O)CHOH + O2 --> CH3C(O)CHO + HO2 at 298 K, (3.0 +/- 0.6) x 10(-12) cm3 molecule(-1) s(-1), was estimated by computer simulation of the concentration-time profiles of the CH3C(O)CHO product. The detailed mechanism of the OH-initiated oxidation of hydroxyacetone can help to better describe the atmospheric oxidation of isoprene, in particular, in the upper troposphere.     

Preparation and characterization of manganese(IV) in aqueous acetic acid

Mn(IV) acetate was generated in acetic acid solutions and characterized by UV-vis spectroscopy, magnetic susceptibility, and chemical reactivity. All of the data are consistent with a mononuclear manganese(IV) species. Oxidation of several substrates was studied in glacial acetic acid (HOAc) and in 95:5 HOAc-H(2)O. The reaction with excess Mn(OAc)(2) produces Mn(OAc)(3) quantitatively with mixed second-order kinetics, k (25.0 °C) = 110 ± 4 M(-1) s(-1) in glacial acetic acid, and 149 ± 3 M(-1) s(-1) in 95% AcOH, ΔH(?) = 55.0 ± 1.2 kJ mol(-1), ΔS(?) = -18.9 ± 4.1 J mol(-1) K(-1). Sodium bromide is oxidized to bromine with mixed second order kinetics in glacial acetic acid, k = 220 ± 3 M(-1) s(-1) at 25 °C. In 95% HOAc, saturation kinetics were observed.     

Ru(III)-catalyzed oxidation of pyruvic acid by iodate -A kinetic study

Ru(III) acts as a catalyst in the oxidative decarboxylation of pyruvic acid by iodate. The reaction is found to be first order with respect to [oxidant] and [catalyst] and fractional order in [pyruvic acid]. Increase in the concentration of H₂SO₄ and decrease in the dielectric constant of the medium retard the oxidation process. The product of oxidation is acetic acid. A mechanism involving the formation of a complex between the substrate and the catalyst, which reacts with the oxidant in the slow step is proposed. The formation constant of the complex and the rate constant of the slow step are determined. This revised version was published online in June 2006 with corrections to the Cover Date.     

Oxidative kinetics of aminoacids by N-bromoacetamide : A study of solvent effect and general base catalysis

The kinetic results of the oxidation of aminoacids by N-bromoacetamide in acid and alkaline media are presented. It is noticed that the order with respect to substrate is dependent on the nature of the medium, 0.30 – 0.80 in perchloric acid (except phenylalanine in which case the order is 1.0) and alkaline media and zero order in aqueous acetic acid. Irrespective of the medium, the reaction is first order in [oxidant]. The rate of oxidation increases with [OH^-] and decreases with [H⁺]. The changes observed in the direction of the pH-rate profile correspond to the ionization constants of the aminoacids. The reaction is inhibited by the addition of acetamide in perchloric acid medium but, is independent of it in alkaline medium. The rate of oxidation is susceptible to changes in the composition of acetic acid and is maximum at 25% aqueous acetic acid. The oxidation of aminoacids by N-bromoacetamide is also catalysed by carboxylate anions. The catalytic constants of propionate, butyrate, acetate, chloropropionate and chloroacetate are measured and Brønsted coefficients (β) so evaluated are in the range 0.38 – 0.46. The proposed mechanism and the derived rate laws are consistent with the observed kinetics.     

硫酸存在时N-溴代苯二甲酰亚胺对甘露糖的胶束催化氧化

The kinetics of micellar-catalyzed oxidation of mannose by N-bromophthalimide was studied in the presence of sulfuric acid at 313 K. The orders of reaction with respect to [mannose], [oxidant], and [H+] were found to be fractional, first, and negative fractional order, respectively. Anionic micelles of sodiumdodecyl sulfate showed a partial inhibitory effect, while cationic micelles of cetyltrimethylammonium bromide increased the reaction rate with the same kinetic behavior. The reaction was catalyzed by cationic micelles, because of favorable electrostatic/thermodynamic/hydrophobic/hydrogen bonding between reactants and cationic micelles. Their catalytic roles are best explained by Berezin’s model. A variation of [phthalimide] showed that the rate of reaction decreased with increasing [phthalimide]. It was observed that, an increase of [mercuric acetate] had no effect on reaction velocity. The influence of salts on the reaction rate was also studied. The rate constant (kW ), binding constants (KS+KO), and corresponding activation parameters (Ea, ⊿H#, ⊿S#, and ⊿G#) were determined. A detailed mechanism with associated reaction kinetics is presented and discussed.     

Kinetic Study of Oxidation of DL-Serine by N-Bromophthalimide in the Presence of Sodium Dodecyl Sulfate

The kinetics of oxidation of DL-serine (Ser) by N-bromophthalimide (NBP) was studied in the presence of an anionic surfactant, sodium dodecyl sulfate, in acidic medium at 308 K. The rate of reaction was found to have first-order dependence on [NBP], fractional order dependence on [Ser] and inverse fractional order dependence on [H⁺]. The addition of reduced product of the oxidant [Phthalimide] and [Hg(OAc)₂] has no effect on the rate of reaction. The change in ionic strength of the medium had no effect on oxidation velocity. The rate of reaction increased with increasing [Br^?] and decreased with increasing [Cl^?]. The rate of reaction decreased with decrease in dielectric constant of the medium. OHCH₂CN was identified as the main oxidation product of the reactions. The various activation parameters have been computed. A suitable mechanism consistent with the experimental findings has been proposed. The micelle-binding constant has been calculated.     

Kinetics and mechanism of Ru(III)-catalyzed oxidation of aliphatic alcohols by trichloroisocyanuric acid in HClO4 medium

The kinetics and mechanism of Ru(III)-catalyzed oxidation of some aliphatic alcohols by trichloroisocyanuric acid (TCICA) has been studied in aqueous HOAc-HClO₄ medium. The reaction is zero order in [TCICA], fractional order in [alcohol] and first order in [Ru(III)]. The reaction is insensitive towards changes in acid concentration. The rate is not affected by an increase in [Cl⁻]. The polar reaction constant (ρ*) was found to be −1.27 at 308 K. A mechanism involving complex formation between the substrate and catalyst in the fast equilibrium step followed by its decomposition in a slow step is proposed.     

Oxidation of aniline and some para-substituted anilines by benzimidazolium fluorochromate in aqueous acetic acid medium – A kinetic and mechanistic study

The oxidation kinetics of some para-substituted anilines by benzimidazolium fluorochromate (BIFC) have been studied in aqueous acetic acid media in the presence of perchloric acid. The reaction is first order with respect to both aniline and BIFC and is catalysed by a hydrogen ion. The rate data obey Hammett relationship. The products of oxidation are the corresponding azo benzenes. Based on the kinetic results and product analysis, a suitable mechanism has been proposed for the reaction of BIFC with anilines.     

N‐bromosuccinimide oxidation of dipeptides and their amino acids: Synthesis,kinetics and mechanistic studies

Dipeptides (DP), namely valyl–glycine (Val–Gly), alanyl–proline (Ala–Pro), and valyl–proline (Val–Pro) were synthesized by classical solution phase methods and characterized. The kinetics of oxidation of amino acids (AA) and DP by N‐bromosuccinimide (NBS) was studied in the presence of perchlorate ions in acidic medium at 28°C. The reaction was followed spectrophotometrically at λ_max = 240 nm. The reactions follow identical kinetics, being first order each in [NBS], [AA], and [DP]. No effect on [H⁺], reduction product [succinimide], and ionic strength was observed. Effects of varying dielectric constant of the medium and addition of anions such as chloride and perchlorate were studied. Activation parameters have been computed. The oxidation products of the reaction were isolated and characterized. The proposed mechanism is consistent with the experimental results. An apparent correlation was noted between the rate of oxidation of AA and DP. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 376–385, 2006     

Ruthenium(III)- and osmium(VIII)-catalyzed oxidation of 2-thiouracil by bromamine-B in acid and alkaline media: a kinetic and mechanistic study

The kinetics of oxidation of 2-thiouracil (TU) by sodium N-bromobenzenesulphonamide or bromamine-B (BAB) have been studied in an HCl medium, catalyzed by RuCl₃, and in a NaOH media with OsO₄ as catalyst, at 313 K. The stoichiometry and oxidation products are the same in both cases, but their kinetic patterns were found to be different. In acid medium the rate shows a first order dependence in each of [BAB] and [TU], and is dependent on [Ru^III]. The reaction rate is inversely dependent on [H⁺]. In alkaline medium, the rate is first order in [BAB] and in [Os^VIII] and zero order in [TU]. The reaction rate is dependent on [NaOH]. Activation parameters have been evaluated, solvent isotope effects have been studied in D₂O medium, and equilibrium constants were calculated. The activation parameters and rate constants indicate that the catalytic efficiency is: Os^VIII > Ru^III. The proposed mechanisms and the derived rate laws are consistent with the observed kinetics.