31P NMR Spectroscopy in Wood Chemistry Part V. Qualitative Analysis of Lignin Functional Groups

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Abstract
Dioxane lignin isolated from a hardwood (birch, betula verrucosa) and milled wood lignin isolated from a softwood (black spruce, picea mariana) were subjected to a variety of selective reactions to modify some of their functional groups. All the lignins were then treated with 1, 3, 2-dioxaphospholanyl chloride (I) and the 31P NMR spectra of the derivatives were recorded. Most of the 31P NMR signals were assigned from the chemical shifts previously obtained from model compounds. The signals arising from derivatizing me labile protons in carbohydrates, erythro and threo forms in β-O-4 structures, primary hydroxyls in a variety of lignin structures, syringyl and guaiacyl phenolic hydroxyls and those of carboxylic acids were assigned. The alpha benzylic hydroxyls in β-O-4 structures within birch lignin were less reactive toward alkylation than those in spruce lignin, consistent with the findings of previous workers; a difference attributable to the aryl part of the β-O-4 ether. The 31P NMR spectroscopy of lignins derivatized with 1, 3, 2 dioxaphospholanyl chloride is thus shown to have considerable potential for the structural elucidation of lignins. This simple method is recommended for the rapid analysis of soluble lignins.