Cleavage of C alpha-C beta bonds of lignin model compounds by nitrite and nitric acid

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 C alpha-C beta bonds of nonphenolic beta-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-beta-guaiacyl ether labeled with deuterium at the C alpha position gave deuterium-labeled veratraldehyde, but did not give any dimeric alpha-carbonyl compounds. The mechanism proposed previously for acidic oxygenation was discussed further concerning the acid-promoted conversion of nonphenolic lignin units into alpha, beta-unsaturated structures. followed by the oxidative cleavage of the C alpha-C beta bonds.