STUDIES ON LIGNIN AND RELATED COMPOUNDS: XX. METHANOL LIGNIN AND ITS RELATION TO THE SO-CALLED "PRIMARY LIGNIN" OF FRIEDRICH AND DIWALD

Abstract

Pretreatment of spruce wood meal with cold 5% sodium hydroxide decreases the amount of lignin extractable with methanol and anhydrous hydrogen chloride (Hibbert-Brauns method). Such a prior treatment of the wood meal, either in an atmosphere of nitrogen or of air, apparently causes no structural change in the lignin as judged by the methoxyl content. The method of Friedrich and Diwald, which involves a preliminary treatment with 17% hydrochloric acid followed by addition of a hot alcohol, represents a much more drastic process than the Hibbert-Brauns method, as is indicated by the darker color, lower methoxyl content and behavior towards 8–10% alkali of the isolated lignin.Methanol lignin isolated front a resin-free spruce wood meal, previously subjected to treatment with 5% cold alkali in the presence or absence of air, shows no loss of methoxyl groups on treatment with 8–10% alkali; the same is true of the "primary lignin" prepared by the method of Friedrich and Diwald; in both cases the lignin is insoluble in sodium bicarbonate. These facts show that neither methanol lignin nor "native lignin" contains ester methoxyl groups, as assumed by Friedrich and Diwald. Prolonged treatment of methanol lignin with alkali in the presence of air, especially at higher temperatures, apparenthy brings about certain changes in its structure, including possibly the formation of carboxyl groups. The claim of Friedrich and Diwald that their product represents an "unchanged native lignin" is not in accordance with the facts, and their assumption of the presence of carboxyl groups in native lignin is incorrect.