It has been reported that degradation of carbohydrates by alkaline hydrogen peroxide can take place by the Fenton reaction1-3, the Bayer-Villiger reaction4, the alpha hydraxy hydroperoxide (aHHP) cleavage-reaction>', the ester reaction', the dihydroxy-epoxide reaction"', and the peroxy-radical reaction'@". In addition, mechanisms for the degradation of carbohydrates by hydrogen peroxide in the presence and in the absence of4Eerrous ion have been described1:'3.
Originally, Fenton's reagent was used under acidic or neutral conditions. Isbell and co-workers' found that hydrogen peroxide and hydroperoxide anion, in both the presence and absence of ferrous ion, oxidatively degrade alditols, aldoses, and aldonic acids nearly completely to formic acid and water. Both systems involve the formation of hydroxyl radical, and reaction of this with the carbohydrate substrate. The similarity of the two systems was noted. With Fenton's reagent, hydroxyl radical, hydroxyl ion, and ferric ion are produced from hydrogen peroxide and ferrous ion (Eq. I). In the absence of ferrous ion, hydroxyl radical, hydroxyl ion, and hydroperoxide radical are formed from hydrogen peroxide and hydroperoxide anion (Eq. 2). It was of interest to compare the effectiveness of the two systems.
Oxidation of an alditol to an aldose proceeds according to Eq. 3 and 4. Addition of hydrogen peroxide to the resulting carbonyl form of the aldose produces an adduct (Eq. 5) capable of decomposing by several mechanisms13. Important among these is the aHHP cleavage reaction. This may take place either heterolytically (Eq. 6) or homolytically (Eq. 7). Heterolytic cleavage is slow because the O-O bond is stable to hydrolysis. Homolytic cleavage is catayzed by hydroxyl radical, which abstracts the hydrogen of the 2-hydroxyl group, releasing an electron for homolytic cleavage of the C-2-C-l bond. This produces the next lower aldose, formic acid, and water. Repetition of the process leads to complete conversion of the aldose into formic acid and water. The reaction is rapid, because it includes both the addition and elimination of the high-energy constituent, hydroxyl radical.
The experimental work described here shows that the oxidative reaction is significantly more rapid in the presence of a catalytic amount of ferrous ion than in its absence. The difference may be ascribed to the regeneration of ferrous ion from ferric