The alkaline anthraquinone-2-sulfonate-H2O2-catalyzed oxidative degradation of lactose: an improved Spengler-Pfannenstiel oxidation.

The alkali-catalyzed oxidative degradation of lactose (1) to potassium O-beta-D-galactopyranosyl-(1----3)-D-arabinonate (2) has been studied and compared with that of D-glucose to D-arabinonate and D-galactose to D-lyxonate. A mechanism for the degradation of 1 catalyzed by alkali only is presented and discussed, taking into consideration the main reaction products. Increasing the reaction temperature from 293 to 318 K resulted in a drastic decrease of the selectivity for 2. Increasing the oxygen pressure from 1 to 5 bar did not significantly influence the selectivity. The overall reaction kinetics followed first-order behavior with respect to lactose, D-glucose, or D-galactose. The simultaneous addition of catalytic, equimolar amounts of sodium 2-anthraquinonemonosulfonate and H2O2 showed a pronounced effect on the selectivity. A reaction mechanism for this type of alkali-catalyzed oxidative degradation of carbohydrates is presented and discussed. Lactose could be oxidized up to almost complete conversion with a selectivity of 90-95% (mol/mol), whereas D-glucose was oxidized to D-arabinonate with a selectivity of 98%. This increased selectivity was maintained at temperatures from 293 up to 323 K, allowing a reduction of the batch time necessary for almost complete conversion from 50 to 1.5 h. The overall reaction kinetics still followed first-order behavior with respect to lactose, D-glucose or D-galactose. The apparent activation energy amounted to 114 +/- 2 kJ mol-1 for lactose, to 109 +/- 2 kJ mol-1 for D-glucose, and to 104 +/- 9 kJ mol-1 for D-galactose.