Kinetics of the oxidation of lactose by copper(II) complexed with bipyridyl in alkaline medium using chloro-complex of rhodium(III) in its nano-concentration range as homogeneous catalyst: a spectrophotometric study

Kinetics of the oxidation of lactose by Cu(II) complexed with bipyridyl have been investigated at 40 °C for the first time spectrophotometrically using Rh(III) chloride as homogeneous catalyst in aqueous alkaline medium in its nano-concentration range. The order of reaction was found to be fractional positive-order, when the concentration of Rh(III) chloride was varied from 0.30 × 10−9 M to 6.00 × 10−9 M. The reaction shows fractional positive-order kinetics with respect to [lactose] and [OH−] and zeroth-order kinetics with respect to [Cu(II)]. The reaction also shows slight increase in the rate by decreasing dielectric constant of the medium and remains unaffected by the change in ionic strength of the medium. The reaction was carried out at four different temperatures and observed values of rate constants were utilized to calculate various activation parameters specially the entropy of activation (ΔS#). The species, [RhCl3(H2O)2OH]−, was postulated as the main reactive species of Rh(III) chloride for the oxidation of lactose by Cu(II) in alkaline medium. On the basis of kinetic and equivalence studies together with spectrophotometric information for the formation of a complex, the most appropriate mechanism for the aforesaid reaction has been proposed. Support to the proposed mechanism was also given by the observed activation parameters and multiple regression analysis. Sodium salts of formic acid, arabinonic acid and lyxonic acid were identified as the main oxidation products of the reaction under investigation.

Kinetic data for the oxidation of lactose by Cu(Bip)22+ in alkaline medium using [RhCl3(H2O)2OH]− as homogeneous catalyst have been collected at 40 °C spectrophotometrically. The reaction path with the following rate determining step has been proposed.Figure optionsDownload as PowerPoint slideHighlights
► The oxidative behavior of Cu(Bip)22+ in alkaline medium.
► Role of reducing sugar as reductant in the presence of Rh(III) as catalyst in alkaline medium.
► The possibility of formation of complex or complexes during the course of reaction.
► Catalytic potential of Rh(III) in the oxidation of lactose by Cu(Bip)22+.