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

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.