Oxidation of d-glucose by N-bromophthalimide in the presence of chlorocomplex of iridium(III): a kinetic and mechanistic study

Abstract

The kinetics of Iridium (III) catalyzed oxidation of d-glucose [d-glu] has been studied by N-bromophthalimide [NBP] in the acidic medium (pH 1.3) at 303 K. The reaction followed first-order kinetics with respect to [NBP] from 0.5 × 10−4 to 4.0 × 10−4 mol dm−3. The results indicate the first-order kinetics in [iridium(III)] chloride at lower concentrations from 0.75 × 10−6 to 5.8 × 10−6 mol dm−3 tends towards a zero order at its higher concentrations (up to 13.36 × 10−6 mol dm−3). Zero-order kinetics with respect to [d-glu] from 2.0 × 10−3 to 20.0 × 10−3 mol dm−3 was observed throughout its variation. A positive effect on the oxidation rate was observed for [Cl−] from 0.2 × 10−5 to 2.4 × 10−5 mol dm−3 and the rate of reaction decreased with increase in dielectric constant (decreasing acetic acid from 35 to 15%) of the medium. A negative effect was observed on the reaction rate for [H+] from 1.5 × 10−2 to 12.0 × 10−2 mol dm−3. Addition of reduced product of oxidant, i.e., phthalimide from 1.0 × 10−4 to 8.0 × 10−4 mol dm−3 did not show a significant effect on the oxidation velocity. Rate of reaction increased with increase in ionic strength from 0.2 × 10−1 to 4.0 × 10−1 mol dm−3 of the medium, i.e., increase in [KNO3]; the rate of reaction was increased. From the linear Arrhenius plot of log k 1 versus 1/T, the activation energy (Ea = 50.91 kJ mol−1) was calculated. With the help of the energy of activation, parameters such as enthalpy of activation (ΔH # = 48.39 kJ mol−1), entropy of activation (ΔS # = −321.63 JK mol−1), Gibbs free energy of activation (ΔG # = 145.84 kJ mol−1) and frequency factor (Log A = −3.97) were also calculated. A suitable mechanism in conformity with kinetic observations was proposed to explain reaction stoichiometry and product analysis.