Semiquantitative determination of quinonoid structures in isolated lignins by P-31 nuclear magnetic resonance

摘要

A semiquantitative analytical method has been developed for the determination of the total quinonoid content; (o-benzoquinones and p-benzoquinones) of soluble lignins. The method is based on detailed measurements and observations made with model o- and p-quinones, which in dry organic solvents were shown to form adducts with trimethyl phosphite in quantitative yield. These adducts gave P-31 NMR signals for o- and p-quinones at around -46 and -2 ppm, respectively. In the presence of moisture and lignin the adducts from o-quinones were shown to be hydrolyzed to the open ring product, dimethylphenyl phosphate (-2 ppm), at an overall yield of similar to 70%. Similarly, the hydrolysis yields of p-quinone adducts with trimethyl phosphite were similar to 70%. Consequently, a number of important issues in relation to the use of trimethyl phosphite toward the quantitative analysis of quinonoid groups in lignins have been investigated, which permitted the development of an experimental semiquantitative protocol recommended for spectral acquisition. [References: 26]