CHEMICAL STRUCTURES OF SULFURIC-ACID LIGNIN .12. REACTION OF LIGNIN MODELS WITH CARBOHYDRATES IN 72-PERCENT H2SO4

Creosol and 5-methoxycreosol, two simple model compounds representing guaiacyl and syringyl nuclei in lignin, respectively, were reacted with monosaccharides in 72% sulfuric acid. Creosol gave two kinds of products: a dehydrative product formed from one molecule of creosol and one molecule of a monosaccharide, and also a compound formed by further addition of creosol to the initial product. 5-Methoxycreosol gave only one kind of dehydrative product, probably because of a steric hindrance of the methoxyl group on the aromatic ring. This was formed by the reaction with xylan. A condensed syringyl compound also reacted with xylose and xylan to give the adduct. The important difference in the behaviour between guaiacyl and syringyl nuclei lies in the fact that the latter gave only one kind of product and furthermore yielded the product 2.8 times more than in the initial reaction stage. This means that in the lignin determination of woody material with 72% sulfuric acid (Klason procedure), lignin reacts with carbohydrates in addition to self-condensation to form water soluble lignin-carbohydrate complexes, which must constitute the ''acid soluble lignin''.