Cerium(IV) Oxidations Of Sulfated Polysaccharides In Aqueous Perchlorate Solutions: A Kinetic And Mechanistic Approach

Abstract

The kinetics of oxidation of iota- and lambda-carrageenans as sulfated polysaccharides by cerium(IV) was studied spectrophotometrically in aqueous perchlorate solutions at a fixed ionic strength of 2.0 mol dm -3 and a temperature of 25 o C. The reactions showed a first order dependence on (Ce IV ), whereas the orders with respect to each carrageenan concentration were less than unity. The reactions exhibited fractional-first order kinetics with respect to (H + ). Increasing ionic strength increased the oxidation rates. The oxidation products of carrageenans were characterized by elemental analysis and IR spectra as their diketo-acid derivatives. The oxidation products were found to have high tendencies to form coordination polymer complexes with some metal cations such as Ba II , Cd II , Pb II and Ag I . Kinetic evidences for the formation of 1:1 complexes were revealed. Plausible mechanistic scheme for cerium(IV) oxidations of carrageenans were proposed. The activation parameters with respect to the slow step of the reactions were evaluated and discussed. The rate laws have been derived and the reaction constants involved in the different steps of the mechanisms were calculated. The activation parameters associated with the rate- determining step of the mechanism along with thermodynamic quantities of the equilibrium constants were computed and discussed. Copyright © 2016 VBRI Press.