Conversion of 2,3:4,6-di-O-isopropylidene-L-sorbose to 2,3:4,6-di-O-isopropylidene-2-keto-L-gulonic acid by catalytic oxidation with oxygen on platinum

Abstract

The kinetics of the oxidation of 2,3:4,6-di-O-isopropylidene-L-sorbose to 2,3:4,6-di-O-isopropylidene-2-keto-L-gulonic acid by oxygen were determined in the aqueous alkaline medium and over platinum on carbon as the catalyst. The most adequate kinetic equation suggested on the basis of non-linear regression was that of the Langmuir-Hinshelwood type. The corresponding most probable reaction mechanism involved an equilibrium single-site adsorption of the substrate, a dissociative dual-site adsorption of oxygen and a surface reaction between oxygen and the substrate to form aldehyde in the rate determining step; in the aqueous alkaline medium the aldehyde is oxidized in the next step to give the anion of 2,3:4,6-di-O-isopropylidene-2-keto-L-gulonic acid. This mechanism appears to be generally applicable to the catalytic transformations of primary alcohols into carboxylic acids by oxygen or air under the above reaction conditions. A linear correlation of the relative reactivities of a series of water-soluble primary aliphatic alcohols with the Taft σ constants shows the polar substituent effect to play the predominant role. However, ethanol is approximately twenty times as reactive as 2,3:4,6-di-O-isopropylidene-L-sorbose and this indicates that in the oxidation of the latter compound also strong steric effects should apply.