Tp. Schultz et Th. Fisher, PHENOXYL SUBSTITUENT EFFECT ON THE ALKALINE-HYDROLYSIS RATES OF BETA-0-4 LIGNIN MODELS - A REVIEW, Journal of pulp and paper science, 24(8), 1998, pp. 242-246
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.