The Distribution of Phenolic Hydroxyl Groups in Hardwood Lignins

Periodate oxidation, because of its high selectivity in the degradation of free-phenolic units, has been used along with the combined phenyl nucleus exchange and nitrobenzene oxidation techniques to estimate the distribution of phenolic hydroxyl groups in hardwood lignins. Analyses of the aspen and white birch wood lignin in situ indicate that the percentage of uncon-densed structures having a phenolic hydroxyl group was considerably higher for guaiacyl (∼20%) than for syringyl (-5%) units. Thus, the total phenolic hydroxyl groups of hardwood lignins are largely associated with the guaiacyl propane units.