Interconversion Between D-glucuronic Acid and D-glucuronolactone in Subcritical Aqueous Ethanol

D-glucuronic acid (G1cA) and D-glucuronolactone (G1cL) were treated with subcritical aqueous ethanol in the range of 0% to 80% (w/w) at 180 degrees C in order to examine the effect of ethanol on the interconversion between G1cA and G1cL. When G1cA was treated at higher ethanol concentrations, less G1cA disappeared and more G1cL was formed compared to treatments at lower ethanol concentrations. For comparison, in the treatment of G1cL at higher ethanol concentrations, disappearance of G1cL was slower and less G1cA was formed. The degradation and interconversion of G1cA and G1cL were kinetically analyzed under the assumptions of first-order kinetics in order to evaluate the rate constants for each process. The rate constants were smaller at higher ethanol concentrations. This was particularly manifested in the significantly low rate constants observed at ethanol concentrations higher than 60%. The treatments of GIcA and G1cL were also examined at 200 degrees C, and the effect of ethanol at 200 C was similar to that at 180 degrees C.