Semi-quantitative method to evaluate the α-carbonyl content in lignin

A method to estimate the content of α-carbonyl structures in lignin was developed. This method consists of two successive treatments: NaBD4 treatment of pulp to reduce an α-carbonyl structure in lignin, and nitrobenzene oxidation. NaBD4 was used to convert an α-carbonyl structure to a deuterium-labeled hydroxymethine structure. The ratio of D-vanillin [(HO)(H3CO)C6H3CDO] to H-vanillin [(HO)(H3CO)C6H3CHO] or that of their syringyl analogues obtained by nitrobenzene oxidation was used as the measure of the content of α-carbonyl structure. Model experiments demonstrated that when sodium hydroxide was used as alkali for the nitrobenzene oxidation, the retention of deuterium at the side chain α-position was very low due to the displacement of deuterium with hydrogen by an unknown reaction mechanism. In order to depress this unexpected displacement, the reaction conditions of the nitrobenzene oxidation were modified. The modified nitrobenzene oxidation employs 0.5 mol/l of lithium hydroxide as a reaction medium instead of 2.0 mol/l sodium hydroxide. By this modification, this method could successfully trace the formation and the degradation of the α-carbonyl structure in milled wood lignins.