Abstract
The condensation reaction of lignin is believed to interfere with delignification in alkaline pulping processes, without any clear evidence, which has motivated us to quantitatively revisit it. This paper is the first of a series, and hence we employed the simplest model system using 4-hydroxymethyl-2-methoxyphenol (vanillyl alcohol, Va) and 2-methoxy-4-methylphenol (creosol, Cr) under soda cooking conditions. The α-5-type condensation product between these compounds [VaCr, 2-(4-hydroxy-3-methoxybenzyl)-6-methoxy-4-methylphenol] was identified and quantified as exclusive. VaCr was yielded with a mole amount of 24%, 46%, 62%, or 72% based on that of disappearing Va at a reaction time of 120 min when the ratio of the initial concentration of Cr to that of Va was 1.0, 2.5, 5.0, or 7.5, respectively. These yields and an HPLC analysis of the reaction solution obtained by a treatment of Va as the sole compound under the same soda cooking conditions suggested the formation of self-condensation products of Va even in the treatments containing Cr. The obtained results comprehensively suggested that the self-condensation of Va progresses more readily than the condensation between Va and Cr. The factors behind this will be the topic of our next paper.
Highlights
Condensation is a major reaction mode of phenolic substructures of lignin in alkaline pulping processes
Formaldehyde (HCHO) is often liberated from a γ-hydroxymethyl group during the processes, and reacts with the aromatic nucleus of a phenoxide substructure to introduce a hydroxymethyl group on the empty ortho-position. This ortho-hydroxymethylated substructure liberates the HO− to convert to the ortho-type Quinone methide structure (QM) (o-QM, Fig. 1), which is nucleophilically attacked by the aromatic nucleus of another phenoxide substructure to afford a 5-CH2-5-type product
Vanillyl alcohol as a label (Va) is the simplest lignin model compound that can convert to a QM, because it has the phenolic hydroxy group, the shortest side-chain with the α-hydroxy group, and the guaiacyl nucleus that is a characteristic aromatic nucleus of lignin
Summary
Condensation is a major reaction mode of phenolic substructures of lignin in alkaline pulping processes. The aromatic nucleus of another phenoxide substructure nucleophilically attacks the α-carbon of QM to afford an α-5- or α-1-type condensation product. Formaldehyde (HCHO) is often liberated from a γ-hydroxymethyl group during the processes, and reacts with the aromatic nucleus of a phenoxide substructure to introduce a hydroxymethyl group on the empty ortho-position. This ortho-hydroxymethylated substructure liberates the HO− to convert to the ortho-type QM (o-QM, Fig. 1), which is nucleophilically attacked by the aromatic nucleus of another phenoxide substructure to afford a 5-CH2-5-type product
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