Composite Activation Parameters Research Articles

Synthesis Ofsome Benzothiazole Derivatives and Kinetic Studies of Their Oxidation Using Chloramine-T in Acid Medium

The kinetics of oxidation of 2-phenyl-benzothiazole (BzlH), 2-(4-methoxyphenyl) benzothiazole (OMeBzlH), 2-(4-nitrophenyl)-benzothiazole (NO2BzlH) by chloramine-T; (CAT) in presence of HClO4 has been investigated at 296 K. Under similar experimental conditions, the oxidation reactions follow identical kinetics for all the three benzothiazoles with first order dependence each on [CAT]0 and [substrate]0 and inverse fractional order dependence on [H+]. Solvent composition shows negative effect indicating the involvement of negative ion-dipolar molecule in the rate determining step. Variation of ionic strength of the medium and addition of halide ions had no effect on the reaction rate. Addition of p-toluenesulphonamide (PTS), the reduction product also has no effect on the rate of reaction. The reactions were studied at different temperatures and the composite activation parameters have been computed. Relative reactivity of oxidation of these follow the order: OMeBzlH>BzlH> NO2BzlH. This trend may be attributed to inductive effects. The observed results have been explained by a plausible mechanism and the related rate law has been deduced.

Conversion of imidazoles to imidazolones with chloramine-B: kinetic and mechanistic study

Oxidative conversion of 1H-imidazole (IzlH) and 2-substituted imidazoles viz., 2-ethyl-1H-imidazole (2-EtIzlH), 2-methyl-1H-imidazole (2-MeIzlH), 1H-imidazole-2-ester (2-EsIzlH), and 1H-imidazole-2-carbaldehyde (2-CaIzlH) to the corresponding imidazolones with sodium N-chlorobenzenesulfonamide or chloramine-B (CAB) were established. This reaction was kinetically investigated at 303 K in HClO4 medium. Under comparable experimental conditions, the oxidation reactions followed identical kinetics for all the five imidazoles with a first-order dependence of rate on [CAB]o and fractional-order dependence each on [imidazole]o and [H+]. Effects of added benzenesulfonamide, halide ions, NaClO4, and MeOH on the rate of reaction have been studied. Reaction products were identified and characterized by GC–MS and NMR spectral analysis. The rates are measured at different temperatures and the composite activation parameters have been computed from the Arrhenius plots. Isokinetic temperature (β) was found to be 449 K which is higher than the experimental temperature (303 K), indicating that the rate has been under enthalpy control. Relative reactivity of these imidazoles is in the order: 2-EtIzlH > 2-MeIzlH > IzlH > 2-EsIzlH > 2-CaIzlH. H2O+Cl have been postulated as the reactive oxidizing species. The reaction mechanism and the derived rate law are in good agreement with the observed experimental results. Spectroscopic studies have been made for an intermediate complex formation between CAB and imidazole. Advantages of the present method include: mild reaction conditions, cost-effectiveness, short reaction time, ease of isolation of products, and the use of eco-friendly reagents. Hence, the methodology developed could be adopted for the facile oxidative conversion of imidazoles to imidazolones and can be scaled up to industrial operation with suitable modification.

Kinetics and Mechanisms of the Chromium(III) Reactions with 2,4‐ and 2,5‐Dihydroxybenzoic Acids in Weak Acidic Aqueous Solutions

The reactions of 2,4- and 2,5-dihydroxybenzoic acids (dihydroxybenzoic acid, DHBA) with chromium(III) in weak acidic aqueous solutions have been shown to take place in at least two stages. The first stage of the reactions has an observed rate constant k 1(obs) = k 1[DHBA] + C and the corresponding activation parameters are ΔH1(2,4) ≠ = 49, 5 kJ/mol−1, ΔS 1(2,4) ≠ = −103, 7 J mol−1 K−1, ΔH1(2,5) ≠ = 60, 3 kJ/mol−1, and ΔS 1(2,5) ≠ = −68, 0 J mol−1 K−1. These are composite activation parameters and the breaking of the strong intramolecular hydrogen bonding in the two ligands is suggested to be the first step of the (composite) first stage of the reactions. The second stage is ligand concentration independent and is thus attributed to a chelation process. The corresponding activation parameters are ΔH2(2,4) ≠ = 45, 13 kJ/mol−1, ΔS 2(2,4) ≠ = −185, 9 J mol−1 K−1, ΔH2(2,5) ≠ = 54, 55 kJ/mol−1, and ΔS 2(2,5) ≠ = −154, 8 J mol−1 K−1. The activation parameters support an associative mechanism for the second stage of the reactions. The various substitution processes are accompanied by proton release, resulting in pH decrease.

Oxidative conversion of thiourea and N-substituted thioureas into formamidine disulfides with acidified chloramine-T: a kinetic and mechanistic approach

The kinetics of oxidation of thiourea and N-substituted thioureas namely: N-methylthiourea, N-allylthiourea N-phenylthiourea and N-tolylthiourea to the corresponding formamidine disulfides by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in the presence of HClO4 has been investigated at 278 K. The reactions follow identical kinetics for all thioureas, being first order each with respect to [CAT]o, [Thiourea]o and [H+]. Ionic strength of the medium and addition of p-toluenesulfonamide or halide ions have negligible influence on the rate. The solvent isotope effect has been studied using D2O in the case of the oxidation of thiourea. Decrease in the dielectric constant of the medium by adding methanol decreases the rate. The reactions were studied at different temperatures, and the composite activation parameters have been computed. An isokinetic relationship was observed with β=314 K, indicating that enthalpy factors control the reaction rate. Under comparable experimental conditions, the rate of oxidation of thioureas increases in the order: N-allylthiourea>N-phenylthiourea>N-methylthiourea>thiourea>N-tolylthiourea. A mechanism involving the interaction of conjugate acid (CH3C6H4SO2NHCl) and substrate giving an intermediate complex, in a slow step, has been suggested. The derived rate law is in agreement with the observed kinetics.

Reaction of Chromium(III) with 3,4‐Dihydroxybenzoic Acid: Kinetics and Mechanism in Weak Acidic Aqueous Solutions

The interactions between chromium(III) and 3,4-dihydroxybenzoic acid (3,4-DHBA) were studied resulting in the formation of oxygen-bonded complexes upon substitution of water molecules in the chromium(III) coordination sphere. The experimental results show that the reaction takes place in at least three stages, involving various intermediates. The first stage was found to be linearly dependent on ligand concentration k 1(obs)′ = k 0 + k 1(obs)[3, 4-DHBA], and the corresponding activation parameters were calculated as follows: ΔH 1(obs) ≠ = 51.2 ± 11.5 kJ mol−1, ΔS 1(obs) ≠ = −97.3 ± 28.9 J mol−1 K−1 (composite activation parameters) . The second and third stages, which are kinetically indistinguishable, do not depend on the concentrations of ligand and chromium(III), accounting for isomerization and chelation processes, respectively. The corresponding activation parameters are ΔH 2(obs) ≠ = 44.5 ± 5.0 kJ mol−1, ΔS 2(obs) ≠ = −175.8 ± 70.3 J mol−1 K−1. The observed stages are proposed to proceed via interchange dissociative (I d, first stage) and associative (second and third stages) mechanisms. The reactions are accompanied by proton release, as is shown by the pH decrease.

Os(VIII)-catalyzed mechanistic investigation of oxidation of some benzimidazoles by chloramine-T in alkaline medium: A kinetic approach

Abstract The kinetics of Os(VIII)-catalyzed oxidation of benzimidazole (BzlH) and 2-substituted benzimidazoles viz., 2-methylbenzimidazole (MeBzlH), 2-hydroxybenzimidazole (2-HyBzlH), 2-aminobenzimidazole (2-AmBzlH) and 2-phenylbenzimidazole (2-PhBzlH) by sodium-N chloro- p -toluenesulfonamide (chloramine-T; CAT) in presence of NaOH has been investigated at 303 K. Under comparable experimental conditions, the oxidation reactions follow identical kinetics for all the five benzimidazoles with first order dependence each on [CAT] 0 and [substrate] 0 and fractional order dependence each on [OH − ] and [Os(VIII)]. The reactions were studied at different temperatures and the composite activation parameters have been computed. The calculated isokinetic temperature ( β ) of 380 K obtained from enthalpy-entropy relationships and Exner correlations was much higher than the experimental temperature of 303 K, indicating that the rate is under enthalpy control. Relative reactivity of oxidation of these follow the order: 2-MeBzlH > 2-HyBzlH > 2-AmBzlH > BzlH > IzlH > 2-PhBzlH. This trend may be attributed to inductive effects. Further, the kinetics of Os(VIII)-catalyzed oxidation of these benzimidazoles have been compared with those of their uncatalyzed reactions (in the absence of catalyst) and found that the catalyzed reactions are 10–15 fold faster than the uncatalyzed reactions. The observed results have been explained by a plausible mechanism and the related rate law has been deduced.

Ru(III)-catalysed oxidation of some N-heterocycles by chloramine-T in hydrochloric acid medium: a kinetic and mechanistic study

The kinetics of the ruthenium(III) chloride (Ru(III))-catalysed oxidation of five N-heterocycles (S) viz. imidazole (IzlH), benzimidazole (BzlH), 2-hydroxybenzimidazole (2-HyBzlH), 2-aminobenzimidazole (2-AmBzlH) and 2-phenylbenzimidazole (2-PhBzlH) by sodium- N-chloro- p-toluenesulfonamide (chloramine-T; CAT) in the presence of HCl has been studied at 313 K. The oxidation reaction follows the identical kinetics for all the five N-heterocycles and obeys the rate law, rate = k [CAT] 0 [ S ] 0 x [H +] y [Ru(III)] z , where x, y and z are less than unity. Addition of p-toluenesulfonamide (PTS) retards the reaction rate. Variation of ionic strength of the medium and the addition of halide ions show negligible effect on the rate of the reaction. The rate was found to increase in D 2O medium and showed positive dielectric effect. The reaction products are identified. The rates are measured at different temperatures for all substrates and the composite activation parameters have been computed from the Arrhenius plots. From enthalpy–entropy relationships and Exner correlations, the calculated isokinetic temperature ( β) of 392 K is much higher than the experimental temperature (313 K), indicating that, the rate has been under enthalpy control. Relative reactivity of these substrates are in the order: 2-HyBzlH > 2-AmBzlH > BzlH > IzlH > 2-PhBzlH. This trend may be attributed to resonance and inductive effects. Further, the kinetics of Ru(III)-catalysed oxidation of these N-heterocycles have been compared with uncatalysed reactions (in the absence of Ru(III) catalyst) and found that the catalysed reactions are 16–20 times faster. The catalytic constant ( K C) was also calculated for each substrate at different temperatures. From the plots of log K C versus 1/ T, values of activation parameters with respect to the catalyst have been evaluated. H 2O +Cl has been postulated as the reactive oxidizing species. The reaction mechanism and the derived rate law are consistent with the observed experimental results.

Mechanistic investigation of oxidation of atenolol by N-chloro-p-toluenesulfonamide in alkaline medium a kinetic approach

Kinetic studies of the oxidation of atenolol (ATN) by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in presence of NaOH at 26.0 ′ 0.1°C follows the rate law -d[CAT]/dt = k[CAT][ATN] x [OH] - y , where x and y are less than unity. The variation of ionic strength and addition of the reaction product, p-toluenesulfonamide and Cl - ion had no pronounced effect on the reaction rate. Decrease of dielectric constant of the medium by increasing the MeOH content decreased the rate. Composite activation parameters for the reaction have been computed from Arrhenius plot. Michaelis-Menten type of kinetics is observed and activation parameters for rate limiting step have been computed. Formation and decomposition constants of CAT-ATN complexes in the reaction scheme have been determined. The conjugate acid, CH 3 C 6 H 4 SO 2 NHCl is assumed to be the reactive species. 4-Acetamido-benzene-oxy-acetic acid and isopropyl amine were found to be the main oxidation products of atenolol. The proposed mechanism and derived rate law are consistent with the observed kinetic data.

Chlorine Dioxide Reduction by Aqueous Iron(II) through Outer-Sphere and Inner-Sphere Electron-Transfer Pathways

The reduction of ClO(2) to ClO(2)(-) by aqueous iron(II) in 0.5 M HClO(4) proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 x 10(6) M(-1) s(-1). The inner-sphere electron-transfer reaction takes place via the formation of FeClO(2)(2+) that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 x 10(5) M(-1) s(-1)) is controlled by ClO(2) substitution of a coordinated water to give an inner-sphere complex between ClO(2) and Fe(II) that very rapidly transfers an electron to give (Fe(III)(ClO(2)(-))(H(2)O)(5)(2+))(IS). The composite activation parameters for the ClO(2)/Fe(aq)(2+) reaction (inner-sphere + outer-sphere) are the following: DeltaH(r)++ = 40 kJ mol(-1); DeltaS(r)++ = 1.7 J mol(-1) K(-1). The Fe(III)ClO(2)(2+) inner-sphere complex dissociates to give Fe(aq)(3+) and ClO(2)(-) (39.3 s(-1)). The activation parameters for the dissociation of this complex are the following: DeltaH(d)++= 76 kJ mol(-1); DeltaS(d)++= 32 J K(-1) mol(-1). The reaction of Fe(aq)(2+) with ClO(2)(-) is first order in each species with a second-order rate constant of k(ClO2)- = 2.0 x 10(3) M(-1) s(-1) that is five times larger than the rate constant for the Fe(aq)(2+) reaction with HClO(2) in H(2)SO(4) medium ([H(+)] = 0.01-0.13 M). The composite activation parameters for the Fe(aq)(2+)/Cl(III) reaction in H(2)SO(4) are DeltaH(Cl(III))++ = 41 kJ mol(-1) and DeltaS(Cl(III))++ = 48 J mol(-1) K(-1).

Ruthenium(III)-catalyzed oxidation of substituted ethanols by sodium N-bromo-p-toluenesulfonamide in hydrochloric acid medium

The kinetics of oxidation of substituted alcohols, RCH2CH2OH (R = H-, OEt-, OMe-, NH2-, Cl- and Br-) by sodium N-bromo-p-toluenesulfonamide or bromamine-T (BAT), catalyzed by ruthenium(III) chloride in the presence of HCl, has been studied at 303K. The reaction rate shows first order dependence each on [BAT], [alcohol] and [RuIII]. The reaction rate is inversely dependent on [H+]. Addition of halide ions and the reduction product, p-toluenesulfonamide has no significant effect on the rate. Composite activation parameters ΔH‡, ΔS‡ and ΔG‡ were computed by studying the reaction at different temperatures (298–313K). The rate decreased in D2O medium and the solvent isotope effect k′(H2O)|k′(D2O) = 1.63 and 1.68 for EtOH and BrCH2CH2OH respectively. Proton inventory studies have been made in H2O–D2O mixtures for both alcohols. The conjugate acid, TsNHBr, is assumed to be the reactive species. The rates do not correlate satisfactorily with Taft substituent constants. The protonation constant (25.3) of monobromamine-T has been evaluated. From enthalpy-entropy relationships and Exner correlations, the isokinetic temperature (β) was found to be 368 K, which is much higher than the experimental temperature, indicating that enthalpy factors control the rate. The proposed reaction mechanisms and the derived rate laws are consistant with the observed kinetic data. The rate of oxidation of alcohols RCH2CH2OH follows the order: H > Br > OEt > OMe > Cl > NH2.

Mechanistic Investigations of Oxidation of Amino Sugars by Sodium N-Chloro-P-Toluenesulfonamide in Alkaline Medium

The kinetics of oxidation of D-glucosamine and D-galactosamine (S) to the respective aldonic acids by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in the presence of alkali at 40 °C follows the rate law, -d [CAT] / dt = k [CAT] [S] [HO−]x, where x <1. The products, p-toluenesulfonamide and Cl− ions had no influence on rate. Increase of ionic strength increased the rate and the Debye-Hückel plot had a slope of 0.6. The rate decreased when the dielectric constant of medium was decreased and values of dAB, the size of activated complex, were calculated by using the Scatchard equation. The rate increased in D2O solution and the inverse solvent isotope effect k(D2O) / k(H2O) was 1.94 and 1.53, respectively, for the two amines. Proton inventory was studied in H2O-D2O mixtures. Composite activation parameters for the reaction were determined from Arrhenius plots. The mechanism assumes the formation of sugar alkoxy anion, a rate limiting complexation with oxidant, formation of the respective pentose through decarboxylation and deamination, and further oxidation to D-arabonic and D-lyxonic acids, respectively. The thermodynamic parameters for sugar-alkoxy anion formation and activation parameters for the rate limiting step were evaluated from double reciprocal plots of 1/k obsvs.1/ [HO−].

Inner-sphere oxidation of ethylenediaminetetraacetatocobaltate(II) by copper(III)Oxime complexes

The oxidation of the Co IIEDTA complex by the imineoxime copper(III) complexes [Cu III(Enio) + and [Cu III(Pre)] + {EnioH 2 = N,N 0ethylenbis(isonitrosoacetylacetoneimine) and PreH 2 = N,N o-propylenebis(isonitrosoacetylacetoneimine)} proceeds via rapid electron-transfer leading to the formation of long-lived intermediates which undergo slow conversion to the final products. It is believed that these intermediates arise from the coordination of one or both of the oxime oxygens to the cobalt(II) complex prior to electron-transfer. The kinetics of the fast step showed firs-order dependence on each reactant concentration. The second-order rate constant varied with [H +] according to the eqn. (i): k 2= k 2 0 + k′ [H +] 1 + K 1[H +] where k 2 0 k′ refer to the hydrogen-independent and hydrogen-dependent pathways respectively, and K 1 to the protonation constant of [Co II(EDTA) 2−. Composite activation parameters associated with k 2 0 and k′ have been determined.