Effect of type of aromatic nucleus in lignin on the rate of the β-O-4 bond cleavage during alkaline pulping process

Several C6–C3 and C6–C2-type dimeric non-phenolic β-O-4 lignin model compounds, whose aromatic nuclei consisting of the carbon skeleton and β-O-4 linkage are named the A- and B-rings, respectively, were treated under alkaline pulping conditions to examine whether or not the substitution of methoxyl group on the B-ring or of guaiacyl for syringyl A-ring accelerates the β-O-4 cleavage as a further study of our previous reports. It was suggested that either first or second substitution of methoxyl group on the B-ring accelerates the β-O-4 cleavage in both C6–C3 and C6–C2 compounds although the former compounds are more sensitive to the substitution than the latter, suggesting that the lack of the γ-hydroxymethyl group makes model compound insensitive to the substitution. It was confirmed that the substitution of guaiacyl for syringyl A-ring accelerates the β-O-4 cleavage in both C6–C3 and C6–C2 compounds with the degrees similar to each other regardless of the type of the B-ring. It was clarified that the leaving ability of the leaving B-ring phenoxides in the β-O-4 cleavage does not correlate well with the pKa values of the conjugate acids of the phenoxides, which is not in accordance with the common property of a nucleophilic substitution reaction.