Oxidation of L-rhamnose and D-mannose by Cr(VI) in perchloric acid. A comparative study

Abstract

The oxidation of L-rhamnose and D-mannose by Cr(VI) in perchloric acid follows the rate law:[Formula: see text]K = 5.40, k0 = 0.00138 M−1 s−1, k1 = 6.71 × 10−4 M−3 s−1 for D-mannose and K = 2.63, k0 = 2.51 × 10−3 M−1 s−1k1 = 2.74 × 10−3 M−3 s−1 for L-rhamnose. This rate law corresponds to the reaction leading to the formation of L-1,4-rhamnonelactone and D-1,4-mannonelactone. No cleavage to carbon dioxide takes place when a 20-fold or higher excess of aldose over Cr(VI) is employed. Relative values of kinetic constants are interpreted in terms of primary hydroxyl group participation in the chromic ester formed in the first reaction step. The free radicals formed during the reaction react with Cr(VI) to yield Cr(V). Two signals (giso = 1.977 and giso = 1.970) ascribed to intermediate Cr(V) complexes were observed by electron paramagnetic resonance (epr) of 0.25–1 N HClO4 solutions saturated with L-rhamnose and Cr2O72−. At higher pH (3–7), only the signal at g = 1.977 was detected. For D-mannose only one signal at g = 1.978 was observed over the whole range of HClO4 concentrations.