PHENOXYL SUBSTITUENT EFFECT ON THE ALKALINE-HYDROLYSIS RATES OF BETA-0-4 LIGNIN MODELS - A REVIEW

This article is a review of our recent alkaline hydrolysis work in whi ch a number of nonphenolic beta-O-4 models were synthesized with diffe rent substituents on the leaving phenoxyl and the substituent influenc e on the alkaline hydrolysis rate determined For benzylic hydroxyl com pounds with 2- or 3-carbon side chains, the presence of a 4'-electron- withdrawing carbonyl increased the hydrolysis rates more than 100 time s compared to an unsubstituted derivative. The activation parameters w ere significantly different for the unsubstituted (H) versus 4'-CHO su bstituted models, bur labeling experiments suggested that most benzyli c hydroxyl compounds hydrolyzed by the neighbouring group mechanism. H owever an unusual base-catalyzed internal SNAr reaction apparently occ urs for all 4'-CHO compounds. Comparison of alpha-carbonyl and benzyli c hydroxyl analogues showed that the disappearance rates in alkali wer e similar. However the ketone compounds probably underwent a variety o f reactions and thus the disappearance rate is the sum of the hydrolys is rate plus other reactions. The greatly enhanced hydrolysis rates of model compounds,vith a 4'-CHO suggest that, if benzylic hydroxyls can be oxidized to electron-withdrawing ketones, then this oxidative modi fication prior to alkaline delignification or bleaching might decrease the pulping time and/or give greater delignification.