Revisiting the condensation reaction of lignin in alkaline pulping with quantitativity. Part 2: evaluation of self-condensation of vanillyl alcohol compared with its condensation with creosol under soda cooking conditions

Abstract

Condensation of lignin is believed to interfere with delignification in alkaline pulping processes. The lack of clear evidence motivated us to quantitatively revisit it. This article evaluates the self-condensation of 4-hydroxymethyl-2-methoxyphenol (vanillyl alcohol, Va) in the simplest model system using Va and 2-methoxy-4-methylphenol (creosol, Cr) under the same soda cooking conditions employed in our previous paper. Dimeric and trimeric self-condensation products of Va consisting of the α-5 bond (VaVa, 2-(4-hydroxy-3-methoxybenzyl)-4-hydroxymethyl-6-methoxyphenol), α-1 bond (VaVa’, bis(4-hydroxy-3-methoxyphenyl)methane), and α-5 & α-1 bonds (VaVaVa’, 2,4-bis(4-hydroxy-3-methoxy-benzyl)-6-methoxyphenol) were identified and quantified. The formation rate of VaVa from Va was suggested to be about twice of that of VaVa’, and to be similar to that of the condensation product between the α-carbon of Va and the aromatic C5-carbon of Cr (VaCr), which was identified in the first paper. Among all these detected condensation products, only VaVa that can be converted to the corresponding quinone methide (QM) readily underwent further condensation to be VaVaVa’ and others. Only the monomers (Va and Cr) possessing the small steric factors condense with electrophilic QM’s as nucleophiles. These results suggest that condensation of lignin in actual soda cooking processes, with steric factors much larger than the model system, progresses much less frequently than generally believed.