Kinetics and mechanism of the reduction of chromium(VI) by D-ribose

Abstract

The reaction between Cr^VI and D-ribose in aqueous perchloric acid, studied by application of the initial-rates method, is first order in both oxidant and reductant, whereas the apparent order of H⁺ is a little lower than 2. At [HClO₄]=0.366 M and 25.0°C the reaction with monomeric Cr^VI is 12.7 times faster than that with dimeric Cr^VI. Under the same acidity conditions, the apparent activation parameters are ΔH^0,=40.1±0.6 kJ mol^-1 and ΔS^0,=-155±2 J K^-1 mol^-1. At low [Cr^VI]₀ the reaction is catalyzed by Mn^II. The catalytic reaction pathway is zero order in Cr^IV and first order in both D-ribose and H⁺. At high [Cr^VI]₀ the reaction is inhibited by Mn^II because it traps the intermediate Cr^VI. In the presence of Mn^II the formation of Mn^III as an intermediate has been detected. Mechanisms for the uncatalyzed and catalyzed reaction pathways are proposed.