Structural/Reactivity Studies (II): Reactions of Lignin Model Compounds with Pulping Additives

Lignin model compounds, capable of forming quinonemethides and having different substituents on the quinonemethide (A) rings and β-aryl ether (B) rings, have been degraded under a variety of conditions in the presence of NaOH (soda), NaOH/Na2S (kraft), NaOH/glucose, and NaOH/AHQ (soda/AQ). Substituent changes on ring B had no effect on the extent of model fragmentation in the presence of excess AHQ and NaSH. Substituent changes on ring A had large effects on fragmentation under soda/AQ and kraft conditions. These substituent-reactivity relationships indicate that the slow step in the mechanism for model fragmentation under soda/AQ and kraft conditions is quinonemethide formation. Conditions aimed at demonstrating the existence of electron-transfer fragmentation of the model compounds gave results which were inconclusive, but demonstrated that equilibrium reactions between quinonemethide intermediates exist.