Formation rate of benzyl cations in various aqueous solutions containing different concentrations of acid but with a specific proton activity in lignin acidolysis

Abstract

Abstract This study aimed to clarify the effect of different solvents, aqueous 1,4-dioxane, ethanol, or ethylene glycol at various mol%, on the formation rate of the benzyl cation, which forms via two steps, the protonation of the benzyl hydroxy or alkoxy group as a pre-equilibrium step and subsequent liberation of the water or corresponding alcohol molecule, respectively, in a model system of the lignin acidolysis, focusing on distinguishing the effects on both steps. For this aim, the acidolysis reactions were conducted in various aqueous solutions and organic solvents that showed a specific proton activity to establish the pre-equilibrium at a specific position. The formation rate of the benzyl cation showed minima in the intermediate mol% range. However, these rate differences were much smaller than those observed in the previous report where the aqueous solutions were prepared to contain a specific acid concentration and show different proton activities and thus the net effect of the solvent type on both steps was indistinguishable. These results clearly showed that the solvent type affects the first step (the proton activity) much more than the last step (the solvent-dissociating power). This paper also describes the preparation of the aqueous solutions to demonstrate a specific proton activity.