Semiquantitative Determination of Quinonoid Structures in Isolated Lignins by 31P Nuclear Magnetic Resonance

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 oand p-quinones, which in dry organic solvents were shown to form adducts with trimethyl phosphite in quantitative yield. These adducts gave 31P NMR signals for oand 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 ∼70%. Similarly, the hydrolysis yields of p-quinone adducts with trimethyl phosphite were ∼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.