The Effect of Liquor Composition on the Rate of Reaction of Lignin Model Compound Acetoguaiacone in Oxygen and Alkali

Abstract

Abstract Acetogualacone (AV; 4-hydroxy-3-methoxyacetophenone) was reacted with oxygen (1.02 mPa) at 80°C in different alkaline solutions. All reactions exhibited a pseudo first order dependence on the initial AV concentration and showed a faster reaction rate in systems buffered with carbon dioxide than in other buffered alkaline systems. At pH values greater than eight, the reactions of AV in oxygen/noncarbonate buffered solutions had very similar reaction rates. The rate of degradation of AV was much less when reacted with oxygen at a pH lower than the pKa of AV. The composition of the buffer and the pH had a significant effect on the yield of oxidation products. The existence of carbonate radicals (HCO3) and radical anions (CO3) is postulated to account for the faster reaction rate of AV with oxygen in carbonate buffers. These radicals are known to be generated in aqueous carbonate solutions when hydroxyl radicals are present. The addition of DMSO (a known hydroxyl radical scavenger) to AV-oxygen-sodium hydroxide, AV-oxygen-borate buffer, and AV-oxygen-sodium bicarbonate buffer led to slower reaction rates, with the latter exhibiting the greatest rate reduction. These results suggest both hydroxyl radicals and carbonate radicals can play important roles during the oxygen-alkali reactions of AV.