CLEAVAGE OF C-ALPHA-C-BETA BONDS OF LIGNIN MODEL COMPOUNDS BY NITRITEAND NITRIC-ACID

Nitrous acid treatment of kraft pulp has been proposed as a non-chlori ne bleaching method. The degradation mechanism of lignin by aqueous ni tric 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 gi ve veratraldehyde, whereas they were stable and converted to the corre sponding stereo isomers in the absence of nitrite. Nitration of aromat ic rings and oxidation of side chains were discussed using some monome ric model compounds. Veratrylglycerol-beta-guaiacyl ether labeled with deuterium at the C alpha position gave deuterium-labeled veratraldehy de, but did not give any dimeric alpha-carbonyl compounds. The mechani sm proposed previously for acidic oxygenation was discussed further co ncerning the acid-promoted conversion of nonphenolic lignin units into alpha, beta-unsaturated structures. followed by the oxidative cleavag e of the C alpha-C beta bonds.