STUDIES ON LIGNIN AND RELATED COMPOUNDS: XVIII. LIGNIN SULPHONIC ACID—A PRELIMINARY INVESTIGATION ON ITS ISOLATION AND STRUCTURE

Abstract

Several methods for the isolation of a pure lignin sulphonic acid have been studied. A satisfactory method, employing dialysis in the presence of an excess of sulphurous acid, followed by precipitation with quinoline and subsequent decomposition with sodium hydroxide, has been developed. The free acid is unstable, becoming insoluble on standing or when heated above 50 °C., particularly in the presence of mineral acids. Treatment with pyridine and acetic anhydride also yields an insoluble material. The salts of lignin sulphonic acid are stable towards heat and methylation reagents at room temperature, and therefore are more suitable for investigations on its structure. The methyl and benzyl esters have been prepared but in low yield only,—a result, presumably, of the well-known abnormally reactive character of esters of sulphonic acids. The permutoid character of its salts makes it difficult to prepare the neutral salts in a pure state. The pure sodium salt, however, was prepared by the use of electrometric titration to indicate the neutral point. The salts (Na, Tl, Ag) are soluble in alcohol containing a trace of water, but are precipitated by inorganic salts such as sodium or thallium acetate. The acid on methylation with methyl alcohol-hydrogen chloride yields a product which is soluble in organic solvents and in water, and which contains an additional methoxyl group. Methylation of the free acid with diazomethane gives a water-insoluble product (OCH3, 18.4%), while a similar methylation of the sodium salt yields a water-soluble product (OCH3, 19.4%, ash-free basis). The acid therefore contains at least one free phenolic or enolic hydroxyl group. It can be methylated at room temperature with dimethyl sulphate and sodium hydroxide (5% excess) to a maximum methoxyl content of 27.7% (ash-free basis), no structural change, other than the loss of some loosely bound sulphurous acid, apparently taking place. The analytical data provide additional support for the empirical formula for lignin proposed by Brauns and Hibbert.