Kinetics of iodate oxidation of lactose in presence of [IrCl₃(H₂O)₂OH]^-as homogeneous catalyst

Abstract

The kinetics and mechanism of oxidation of lactose by potassium iodate in the presence of Ir(III) chloride as homogeneous catalyst in aqueous alkaline medium has been investigated. The reaction is found to be first order with respect to sugar, iodate and hydroxyl ions at their low concentrations (sugar from 1.00×10^-2 to 3.00×10^-2 <i style="mso-bidi-font-style:normal">M</i> , iodate from 2.50×10^-4 to 15.00×10^-4 <i style="mso-bidi-font-style:normal">M</i> and hydroxyl ions from 8.00×10^-2 to 24.00×10^-2 <i style="mso-bidi-font-style:normal">M</i>) and zero order with respect to sugar, iodate and hydroxyl ions at their high concentrations (sugar from 3.50×10^-2 to 10.00×10^-2 <i style="mso-bidi-font-style:normal">M</i>, iodate from 17.50×10^-4 to 25.00×10^-4 <i style="mso-bidi-font-style: normal">M</i> and hydroxyl ions from 32.00×10^-2 to 80.00×10^-2 <i style="mso-bidi-font-style:normal">M</i>). Rate of oxidation is directly proportional to [Ir(III)] throughout its variation. Ionic strength of the medium and [Cl^–] have no effect on the rate of oxidation. From the values of observed rate constants at four different temperatures, the activation parameters have been calculated. Lyxonic acid, arabinonic acid and formic acid have been identified as the main oxidation products of the reaction. The results are in accord with the proposed mechanism in which the most reactive activated complex, [C₆H₁₁O₉I.IrCl₃(H₂O)₂]^2- in the rate determining step, decomposes into an intermediate, which finally forms the products through fast steps with regeneration of the effective species of catalyst.