Nirmala Vaz

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 [RuIII]. The reaction rate is inversely dependent on [H+]. In alkaline medium, the rate is first order in [BAB] and in [OsVIII] 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: OsVIII > RuIII. The proposed mechanisms and the derived rate laws are consistent with the observed kinetics.

Kinetics of oxidation of acidic amino acids by sodium N-bromobenzenesulphonamide in acid medium: A mechanistic approach

Kinetics of oxidation of acidic amino acids (glutamic acid (Glu) and aspartic acid (Asp)) by sodium N-bromobenzenesulphonamide (bromamine-B or BAB) has been carried out in aqueous HClO4 medium at 30°C. The rate shows first-order dependence each on [BAB]o and [amino acid]o and inverse first-order on [H+]. At [H+] > 0·60 mol dm−3, the rate levelled off indicating zero-order dependence on [H+] and, under these conditions, the rate has fractional order dependence on [amino acid]. Succinic and malonic acids have been identified as the products. Variation of ionic strength and addition of the reaction product benzenesulphonamide or halide ions had no significant effect on the reaction rate. There is positive effect of dielectric constant of the solvent. Proton inventory studies in H2O-D2O mixtures showed the involvement of a single exchangeable proton of the OH− ion in the transition state. Kinetic investigations have revealed that the order of reactivity is Asp > Glu. The rate laws proposed and derived in agreement with experimental results are discussed.

Oxidation of vanillin and related compounds by sodium N-chloro-p- toluenesulfonamide in acid medium: A kinetic and mechanistic approach

The kinetics of oxidation of vanillin, vanillyl alcohol, vanillylamine hydrochloride and vanillyl mandelic acid (hereafter abbreviated as substrate) by N-chloro-p-toluenesulfonamide or chloramine–T (CAT) in HClO4 medium has been investigated at 30±0.1°C. The oxidation reaction follows identical kinetics with first-order in [CAT]o, fractional order in [substrate]o, and inverse fractional-order in [H+]. Variation of ionic strength and addition of the reaction product, p-toluenesulfonamide, or halide ions had no significant effect on reaction rate. Decrease in dielectric constant of the medium decreases the rate. The solvent isotope effect was studied in D2O medium. The reaction does not induce polymerization of acrylonitrile. Michaelis-Menten type of kinetics has been proposed and activation parameters for the rate-limiting step as well as overall reaction have been computed. The decomposition constants of substrate-oxidant complexes have been evaluated. Under comparable experimental conditions, the rate of oxidation increases in the following order vanillin>vanillylamine hydrochloride>vanillyl alcohol>vanillylmandelic acid. An isokinetic relationship is observed with β=339 K, indicating enthalpy as a controlling factor. Vanillic acid was identified as the oxidation product of each substrate and was confirmed by IR and GC-MS data. A general mechanism involving the formation of a complex between substrate and the conjugate acid (CH3C6H4SO2NHCl) of the oxidant has been proposed. The derived rate law is in agreement with the experimental results.

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.

Os(VIII)-Catalyzed Oxidation of Some Aliphatic Ketones by Sodium N-Chlorobenzenesulfonamide (Chloramine-B) in the Presence of Sodium Hydroxide: Kinetic Mechanistic Chemistry

Kinetics of oxidation of 2-propanone, 2-butanone, and 2-pentanone by chloramine-B (CAB) in presence of NaOH and OsO4 catalyst has been investigated at 303 K. The reaction rate shows a first–order dependence each on [CAB]o and [Os(VIII)], fractional-order on [NaOH], and zero-order on [ketone]o. The solvent isotope effect k/(H2O) / k/(D2O) = 0.58. Activation parameters have been deduced. An isokinetic relation is obtained with β = 340 K. The rate of oxidation of ketones follows the order: 2-pentanone > 2-butanone > 2-propanone. The Os(VIII) catalyzed reactions are 7 to 9 fold faster than the uncatalyzed reactions. Suitable mechanism and rate law have been proposed.

Ruthenium(III) Catalyzed Kinetics of Oxidation of Substituted Ethanols by Bromamine-B in Hydrochloric Acid Solution

Abstract The kinetics of the ruthenium(III) catalyzed oxidation of ethanol and substituted ethanols, RCH2CH2OH (R ˭ H, OC2H5, OCH3, NH2, CI and Br) by sodium N-bromobenzenesulfonamide (bromamine-B or BAB) in HCI solution has been studied at 30°C. The reaction rate shows a first-order dependence each on [BAB], [alcohol] and [ruthenium(III)]. The reaction also shows an inverse fractional-order dependence on [acid]. Added halide ions and the reduction product of BAB (benzenesulfonamide), and variation of ionic strength of the solvent medium have no effect on the rate. Activation parameters have been evaluated. Proton inventory studies have been made in H2O-D2O mixtures for ethanol and 2-bromoethanol. A general mechanism consistent with the above kinetic data has been proposed. The protonation constant of monobromamine-B has been evaluated to be 24.2 M−1. The rates do not correlate satisfactorily with the Taft substituent constant. An isokinetic relationship is observed with β = 372 K indicating that enthalpy factors control the rate.