Effects of lignin structure and solvent on the formation rate of quinone methide under alkaline conditions

Abstract

Abstract The purpose of this study was to examine how differences in the type of aromatic nucleus, side-chain structure and type of solvent affect the formation rate of quinone methide (QM) in the alkaline reaction of lignin. The reaction was done at a NaOH concentration of 1.0 mol l−1 and temperature of 75–140°C under an anaerobic condition. The formation rates of QM were in the order: syringyl > guaiacyl > p-hydroxyphenyl, when model compounds with lignin-type aromatic nuclei were compared. This and other results on various phenolic compounds suggested that the formation of QM is rapid from compounds having a high electron density in the aromatic π-electron system. The formation of QM was faster from a C6-C2-type than from a C6-C1-type lignin model compound, which was attributed to the fact that QM is an alkene and hence more stable when an unsaturated carbon in QM has an alkyl substituent, like the C6-C2-type compound. When aqueous 1,4-dioxane solutions with different 1,4-dioxane contents were used, the formation of QM became slower with increasing 1,4-dioxane content. This can be explained by the variation in the negative charge density in the rate-determining step, where the density is larger at the transition than at the initial state and consequently the activation energy is lower in a solvent with higher polarity.