Oxidation of some catecholamines by sodium N-chloro-p-toluenesulfonamide in acid medium: A kinetic and mechanistic approach

The kinetics of the oxidation of five catecholamines viz., dopamine (A), L-dopa (B), methyldopa (C), epinephrine (D) and norepinephrine (E) by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in presence of HClO₄ was studied at 30±0.1 °C. The five reactions followed identical kinetics with a first-order dependence on [CAT] _o , fractional-order in [substrate] _o , and inverse fractional-order in [H⁺]. Under comparable experimental conditions, the rate of oxidation of catecholamines increases in the order D>E>A>B>C. The variation of ionic strength of the medium and the addition of p-toluenesulfonamide or halide ions had no significant effect on the reaction rate. The rate increased with decreasing dielectric constant of the medium. The solvent isotope effect was studied using D₂O. A Michaelis-Menten type mechanism has been suggested to explain the results. Equilibrium and decomposition constants for CAT-catecholamine complexes have been evaluated. CH₃C₆H₄SO₂NHCl of the oxidant has been postulated as the reactive oxidizing species and oxidation products were identified. An isokinetic relationship is observed with β=361 K, indicating that enthalpy factors control the reaction rate. The mechanism proposed and the derived rate law are consistent with the observed kinetics.     

Kinetic and mechanistic studies on the complexation and anation of dioxotetracyanomolybdate (IV) with 2,2′-bipyridyl in aqueous solution

Dioxotetracyanomolybdate(IV) has been found to form a 1 : 2 complex with 2,2′-bipyridyl. The kinetics of the reaction has been studied over the pH range 5.3–8.7 by visible spectrophotometry under pseudo conditions. The effect of the 2,2′-bipyridyl and dioxotetracyanomolybdate(IV), temperature, ionic strength, and pH on the reaction rate was determined. The reaction follows first-order kinetics with respect to dioxotetracyanomolybdate(IV) ion and fractional-order kinetics with respect to 2,2′-bipyridyl. Values for the outer-sphere complex formation constant (K_os2) and rate constants (k₂) were also calculated from the kinetic data. It was found that rate of the reaction increases with the decreasing pH. The following rate equation based on the outersphere complexation equilibrium preceding the associative interchange has been derived. On the basis of the observed results probable mechanism has been proposed. © 1996 John Wiley & Sons, Inc.     

Vapor phase hydrogenation of o -chloronitrobenzene ( o -CNB) over alumina supported palladium catalyst — a kinetic study

Kinetics of the vapor phase hydrogenation of o-chloronitrobenzene (o-CNB) over Pd/Al₂O₃ catalyst has been studied in a downflow microreactor under atmospheric pressure. Reaction rates have been measured at three different temperatures with respect to the partial pressures of o-CNB and hydrogen. The order of the reaction with respect to o-CNB was 0.53 at 280°C and increased with increasing temperature. However, with respect to hydrogen, a negative order was observed at 280°C, which decreased further with increasing in temperature. The apparent activation energy (Ea), from the Arrhenius plot was found to be 41 kJ/mol. On the basis of kinetic results a surface mechanism is suggested.     

OH radical reactions with ethanolamines: formation of reducing as well as oxidizing radicals

The reaction of OH radicals with ethanolamine, diethanolamine and triethanolamine was studied at pH values below and above the pK_a values of these compounds. The rate constants were found to be lower for the protonated amines than those for their neutral forms. The OH radical reaction led to the formation of both oxidizing as well as reducing species, as observed by their reactions with methyl viologen and ascorbic acid. The oxidizing species formed by OH radical reaction at the amine site was not found to react with the parent molecules and thereby no secondary yield of reducing species was obtained as in the case of glycine (except in the case of triethanolamine at pH 9.2).     

Kinetics and mechanism of oxidation of aliphatic alcohols by tetrabutylammonium tribromide

Oxidation of nine primary aliphatic alcohols by tetrabutylammonium tribromide (TBATB) in aqueous acetic acid leads to the formation of the corresponding aldehydes. The reaction is first order with respect to TBATB. Michaelis-Menten type kinetics is observed with respect to alcohols. The reaction failed to induce the polymerization of acrylonitrile. Tetrabutylammonium chloride has no effect on the reaction rate. The proposed reactive oxidizing species is the tribromide ion. The oxidation of [1,1-²H₂]ethanol exhibits a substantial kinetic isotope effect. The effect of solvent composition indicates that the rate increases with increase in the polarity of the solvent. The reaction is susceptible to both polar and steric effects of substituents. A mechanism involving transfer of a hydride ion in the rate-determining step has been proposed.     

Kinetics and Mechanism of the Reduction of Enneamolybdomanganate(IV) by Citric Acid

The kinetics and mechanism of reduction of enneamolybdomanganate(IV) by citric acid in acidic solution was studied by spectrophotometry, and showed that the reaction, with respect to enneamolybdomanganate(IV) and to citric acid, is pseudo-second-order and first-order, respectively. The values of 1/k_obs increase with the values of [H⁺] and reaction rates increase with temperature. The activation parameters of the rate-determining steps were evaluated. A mechanism related to this reaction is proposed.     

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     

Kinetics and mechanism of the oxidation of some diols by benzyltrimethylammoniumtribromide

The kinetics of oxidation of five vicinal and four non-vicinal diols, and two of their monoethers by benzyltrimethylammonium tribromide (BTMAB) have been studied in 3:7 (v/v) acetic acid-water mixture. The vicinal diols yield the carbonyl compounds arising out of the glycol bond fission while the other diols give the hydroxycarbonyl compounds. The reaction is first-order with respect to BTMAB. Michaelis-Menten type kinetics is observed with respect to diol. Addition of benzyltrimethylammonium chloride does not affect the rate. Tribromide ion is postulated to be the reactive oxidizing species. Oxidation of [1,1,2,2-²H₄] ethanediol shows the absence of a kinetic isotope effect. The reaction exhibits substantial solvent isotope effect. A mechanism involving a glycol-bond fission has been proposed for the oxidation of the vicinal diols. The other diols are oxidized by a hydride ion transfer to the oxidant, as are the monohydric alcohols.     

Kinetics and mechanism of the oxidation of substituted benzaldehydes by hexamethylenetetramine‐bromine

The oxidation of thirty‐six monosubstituted benzaldehydes by hexa‐methylenetetramine‐bromine (HABR), in aqueous acetic acid solution, leads to the formation of the corresponding benzoic acids. The reaction is first order with respect to HABR. Michaelis‐Menten–type kinetics were observed with respect to aldehyde. The reaction failed to induce the polymerization of acrylonitrile. There is no effect of hexamethylenetetramine on the reaction rate. The oxidation of [²H]benzaldehyde (PhCDO) indicated the presence of a substantial kinetic isotope effect. The effect of solvent composition indicated that the reaction rate increases with an increase in the polarity of the solvent. The rates of oxidation of meta‐ and para‐substituted benzaldehydes showed excellent correlations in terms of Charton's triparametric LDR equation, whereas the oxidation of ortho‐substituted benzaldehydes correlated well with tetraparametric LDRS equation. The oxidation of para‐substituted benzaldehydes is more susceptible to the delocalization effect but the oxidation of ortho‐ and meta‐substituted compounds displayed a greater dependence on the field effect. The positive value of γ suggests the presence of an electron‐deficient reaction center in the rate‐determining step. The reaction is subjected to steric acceleration when ortho‐substituents are present. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 615–622, 2000     

The oxidation of 3,3′-dimethoxy benzidine with potassium bromate in acidic solutions

The kinetics and mechanism of the oxidation of 3,3′-dimethoxybenzidine (oda, o-dianisidine) by potassium bromate in aqueous acidic medium were studied by monitoring the formation rate of the reaction product, 3,3′-dimethoxy 4,4′-diphenoquinone at 447 nm. The reaction is, first order with respect to both the substrate and oxidant, and second order with respect to H⁺. The oda: bromate stoichiometric ratio is 1:1. Plausible mechanism and rate laws are proposed accounting the experimental findings. Computer simulations were done using the proposed mechanism.     

Ruthenium(III)‐catalyzed oxidation of cyclopentanol and cyclohexanol by N‐bromoacetamide

Kinetic investigations on Ru(III)‐catalyzed oxidation of cyclopentanol and cyclohexanol by acidic solution of N‐bromoacetamide (NBA) in the presence of mercury(II) acetate as a scavenger have been carried out in the temperature range of 30–45°C. Similar kinetics was followed by both the cyclic alcohols. First‐order kinetics in the lower concentration range of NBA was observed to tend to zero order at its higher concentrations. The reaction exhibits a zero‐order rate dependence with respect to each cyclic alcohol, while it is first order in Ru^III. Increase in [H⁺] and [Cl^?] showed positive effect, while successive addition of acetamide exhibited negative effect on the reaction rate. Insignificant effect of sodium perchlorate, D₂O, and mercury(II) acetate on the reaction velocity was observed. Cationic bromine has been proposed as the real oxidizing species. Various thermodynamic parameters have been computed. A suitable mechanism in agreement with the kinetic observations has been proposed. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 275–281, 2005     

Mechanistic investigations of oxidation of isatins by sodium N-chlorobenzenesulfonamide in alkaline medium: A kinetic study

Oxidation of isatins (isatin, 5-methylisatin, 5-bromoisatin and 5-nitroisatin) to their anthranilic acids was performed efficiently with sodium N-chlorobenzenesulfonamide or chloramine-B (CAB) in alkaline medium at 35±0.1°C. The reactions follow identical kinetics for all the isatins, being first-order dependence each in [CAB] _o and [Isatin] _o and inverse fractional-order on [NaOH]. Addition of halide ions and benzenesulfonamide, reduction product of CAB, do not significantly affect the rate. Variation of ionic strength of the medium had no effect on the rate, while the dielectric effect is negative. The solvent isotope effect was studied using D₂O. Activation parameters for the overall reaction have been computed. The rates satisfactorily correlate with the Hammett σ relationship and the reaction constant ρ is −0.31 signifies that electron releasing groups accelerate the reaction while the electron withdrawing groups retard the rate. Values of ΔH^≠ and ΔS^≠ are linearly related and an isokinetic relationship is observed with β=376 K, indicating the reaction is controlled by enthalpy. The stoichiometry of the title reaction is found to be 1∶1. Oxidation products of isatins were identified as their corresponding anthranilic acids and the yields were found to be around 90 %. The observed results have been explained by a plausible mechanism and the related rate law deduced. This method offers several advantages including high yield of the products, short reaction times, easier isolation of products, and stable, cost effective and relatively non-toxic reagents, which make the reaction process simple and smooth.     

Investigation of isothermal bulk polymerization ofo-alkyaryl methacrylates by DSC

The kinetics of the AIBN-initiated free radical bulk polymerization of fiveo-alkylphenyl methacrylates was studied by means of DSC in the temperature range 353–373 K, and the enthalpy of polymerization, the overall reaction rate constant and the activation energy were determined. The results were compared with those published recently on correspondingp-alkylaryl methacrylates. All measured reaction rate constants were found to increase with increasing temperature and to decrease with increasingo-alkyl substituent mass and size. It was shown thato-substituents influence the rates of polymerization to a greater extent thanp-substituents. At about 373 K, all differences in rate, most probably resulting from steric hindrance caused by the alkyl groups, disappear in both series, a phenomenon earlier observed for dimethyl phenyl methacrylates.This work was supported by the Ministry of Science of the Republic of Serbia.     

Cr(VI) oxidation using cetylpicolinium dichromate: Kinetics of oxidation of benzaldehydes with a green protocol

Oxidation kinetics of benzaldehyde and some of its ortho- and para-monosubstituted derivatives have been studied using cetylpicolinium dichromates, a class of novel phase transfer oxidants, in dichloromethane medium. The rate of reaction is first order with respect to oxidant and fractional order with respect to the substrates. The Michaelis–Menten type oxidation was observed with respect to the substrates. Benzaldehydes are found to be oxidized to their corresponding acids. The mechanism of oxidation reaction has been suggested based on the solvent isotope effect, Hammett plot, and thermodynamic study. The solvent isotope effect (k_CHCl3/k_CDCl3 = 1.57) indicates the involvement of hydrogen exchange with the medium during oxidation reactions. A strong influence of specific solute–solvent interactions on the rate of the reaction is observed. Both the electron-withdrawing and electron-releasing substituents on the substrates accelerate the rate of reaction.     

Kinetic study of the ruthenium(III)-catalyzed oxidation of glycine by <Emphasis Type="Italic">N</Emphasis>-bromophthalimide in acidic medium

The kinetics of ruthenium(III) chloride-catalyzed oxidation of glycine by N–bromophthalimide (NBP) was studied in aqueous perchloric acid at 35 °C. The results showed first- and zero-order behavior with respect to NBP and Gly, respectively. Ru(III) showed a catalytic effect on the reaction which followed first-order kinetics with respect to [Ru(III)] at a low concentration range and tended to zero order at high concentration range. The rates decreased with increase in the proton concentration, while chloride positively influenced the rate of the reaction. Two moles of NBP were required to oxidize one mole of Gly, and the products were identified as phthalimide (NHP), HCN, CO₂, and Br⁻. Neither added NHP nor Br⁻ influenced the reaction rate. Ionic strength and dielectric constant of the medium had no significant effect on the rate. Activation parameters were determined by studying the reaction at different temperatures. A reaction scheme of the catalytic oxidation is proposed.     

Kinetics and Mechanism of Oxidation of L-Leucine by Alkaline Diperiodatonickelate(IV) – A Free Radical Intervention, Deamination, and Decarboxylation

The kinetics of oxidation of L-leucine by alkaline diperiodatonickelate(IV) was studied spectrophotometrically using a rapid kinetic accessory. The reaction is first order with respect to [DPN] and apparently less than unit order, each in [L-leucine] and [alkali] under the experimental conditions. However, the order in [L-leucine] and [alkali] changes from first order to zero order as the concentrations change from lower to higher values. Addition of periodate has no effect on the reaction rate. A mechanism involving the deprotonated diperiodatonickelate(IV) (DPN) as the reactive oxidant is proposed. The reaction constants involved in the mechanism were evaluated. There is a good agreement between the observed and calculated rate constants under varying experimental conditions. The isokinetic temperature was determined and the activation parameters with respect to the slow step of the reaction scheme were evaluated and discussed.     

Free radical intervention,deamination and decarboxylation in the ruthenium(III)-catalysed oxidation of L-arginine by alkaline permanganate – a kinetic study

The kinetics of the Ru^III catalysed oxidation of L-arginine by alkaline permanganate was studied spectrophotometrically using a rapid kinetic accessory. The reaction follows a two stage process. In both the stages the reaction is first order with respect to [oxidant] and [catalyst] with an apparent less than unit order in [substrate] and [alkali]. The data suggest that oxidation proceeds via formation of a complex between the active Ru^III species and L-arginine, which then reacts with one mole of permanganate in a slow step to yield a L-arginine free radical, followed by a fast step to form the products. The reaction constants involved in the mechanism were evaluated. There is a good agreement between observed and calculated rate constants under different experimental conditions for both stages of reaction. The activation parameters for the slow step were calculated and are discussed.     

Kinetics of epoxidation of poly(trans-1,4-butadiene) crystals in suspension

The kinetics of the epoxidation reaction between metachloroperbenzoic acid (MCPBA) and poly-(trans-1,4-butadiene), (PTBD), crystals in toluene suspension was investigated in the 6–21°C range using infrared spectroscopy. Crystals of PTBD with M?_n = 36,000 grown from heptane and from toluene solutions and crystals of PTBD with M?_n = 5500 grown from heptane were studied. For toluene-grown crystals the total number of double bonds available for reaction increases with reaction temperature. For all preparations studied the epoxidation reaction is initially second-order—first-order with respect to [MCPBA] and first-order with respect to the concentration of the available double bonds. The second-order rate constant is found to be dependent on temperature, on molecular weight, and also on the crystal preparation conditions. The bromination of PTBD crystals was studied in CCl₄ suspension at 0°C; this reaction was found to be complete within 1 hr with the fraction of double bonds brominated consistent with the epoxidation results. The IR spectra for dried mats of brominated and of epoxidized PTBD crystals were obtained: Changes in the amorphous band at 1335 cm^?1 due to reaction of double bonds at crystal surfaces were observed. The results of this investigation are discussed in terms of the amorphous content of the PTBD lamellas.