Cleavage of Cα-Cβ Bonds of Lignin Model Compounds by Nitrite and Nitric Acid

Abstract

Nitrous acid treatment of kraft pulp has been proposed as a non-chlorine bleaching method. The degradation mechanism of lignin by aqueous nitric acid containing nitrite was studied using lignin model compounds. The Cα-Cβ bonds of nonphenolic β-O-4 type compounds were cleaved during the treatment with nitric acid containing nitrite to give veratraldehyde, whereas they were stable and converted to the corresponding stereo isomers in the absence of nitrite. Nitration of aromatic rings and oxidation of side chains were discussed using some monomeric model compounds. Veratrylglycerol-β-guaiacyl ether labeled with deuterium at the Cα position gave deuterium-labeled veratraldehyde, but did not give any dimeric α-carbonyl compounds. The mechanism proposed previously for acidic oxygenation was discussed further concerning the acid-promoted conversion of nonphenolic lignin units into α, β-unsaturated structures, followed by the oxidative cleavage of the Cα-Cβ bonds.