Synthesis and reactions of hydroxylated stilbenes and their possible occurrence as chromophore precursor structures in lignin

Chromophores which contribute to the color of kraft lignins and the generation of dihydroxystilbene structures as chromophore precursors are discussed briefly. The various o,p- and p,p-dihydroxystilbenes and 4,4-dihydroxy-3,3-dimethoxy-5-(4-hydroxy-3-methoxystyryl)stilbene, obtained viaWittig syntheses, are colorless substances which become more or less red colored when exposed to air. By reductive acetylation (zinc—acetic anhydride) stilbenoquinones, o- and p-quinonoid, diphenoquinoic, quinone methide, and stable oxygen radical structures are converted to colorless compounds, stabilized by simultaneous blocking of the formed phenolic hydroxyl groups. Application of that reaction to technical kraft lignin fractions let assume that a low number of chromophore structures (max. 5%) are responsible for the intensive color of the material.