Revisiting the Mechanism of β-O-4 Bond Cleavage During Acidolysis of Lignin: Part 5: On the Characteristics of Acidolysis Using Hydrobromic Acid

Abstract

The disappearance of a C6-C3 dimeric non-phenolic β-O-4 type lignin model compound, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)propane-1,3-diol (VG), and formations of reaction products, 2-methoxyphenol (G) and 1-hydroxy-3-(3,4-dimethoxyphenyl)propan-2-one (HK), were examined in acidolysis using HBr to characterize an unknown mechanism observed in our previous study in which the formation of HK is suppressed at the initial stage of the reaction. The presence of bromide anion was necessary to observe this unknown mechanism, but the relative contribution of this mechanism to the overall reaction seemed to be greater when the concentration of bromide anion was lower. The contribution of the unknown mechanism seemed to accelerate the disappearance of VG and the formation of G and cause the disappearance of VG to deviate from a pseudo-first-order reaction. The disappearances of VG were approximated well to pseudo-first-order reactions when data points after a reaction time of 2 h were applied to the approximations. The rate constants obtained by this approximation were larger with increased amount of bromide anion, which suggests the participation of bromide anion in the general acidolysis mechanism.