Oxidation of some aldopentoses by chloramine-B in alkaline medium: a kinetic and mechanistic study

Abstract

The oxidation kinetics of d-xylose, d-ribose, and l-arabinose by chloramine-B (CAB) in alkaline medium at 35° obey the rate law, rate = [CAB][HO −l[Pentose]. Based on the results, a mechanism is suggested involving the anion of the β anomer of the pentose. The addition of reaction products, benzenesulphonamide and NaCl, had no effect on the rate of reaction whereas increase in the ionic strength and dielectric constant of the medium increased the rate. Proton-inventory studies were made in H 2OD 2O mixtures. The rate of oxidation of pentoses follows the order 1 1 A referee queried whether sugars containing the same percent of β anomer react at the same rate, taking d-xylose, d-fructose, and d-glucose as the examples. We note that d-fructose has the highest rate because of the reactive carbonyl group (keto). As a group, the pentoses react faster than the hexoses 16 as the latter are presumably stabilized by the -I effect of the CH 2OH group. : d-xylose > d-ribose > l-arabinose. Kinetic and thermodynamic parameters for the reaction were computed from the Arrhenius plots. The isokinetic temperature β = 416 K, which is much above the experimental temperature, suggests enthalpy-controlled reactions.