STUDIES ON LIGNIN AND RELATED COMPOUNDS.: VIII. A KINETIC STUDY OF THE ACTION OF SULPHUROUS ACID ON LIGNIN AND RELATED COMPOUNDS

Abstract

Electrical conductivity measurements have been made over the temperature range 18–145 °C. of reaction mixtures of sulphurous acid and a large number of type compounds, and data obtained on the stability of the resulting sulphonic acids and the mobility of the equilibria involved. The compounds investigated include saturated and unsaturated aldehydes; saturated, unsaturated and cyclic ketones; phenols; substances in which an ethylene linkage is the only reactive group; furane derivatives; and substances containing pyrone rings.The results are discussed from the standpoints of structure and reaction mechanism. The sulphonic acids of nuclear aldehydes, saturated ketones and cylic ketones are typically unstable. With compounds containing an ethylene linkage conjugated with a carbonyl group, addition normally takes place in the 1:4 positions and is followed by hydrogen migration. Where an ethylene linkage is the only reactive group present, addition proceeds only very slowly and at high temperatures. The difficulty of effecting a reaction between sulphurous acid and phenols in their tautomeric alicyclic forms is indicated by the non-reactivity of resorcinol, and the formation of only a small percentage of an extremely unstable derivative by phloroglucinol. Furfuryl alcohol and glucal are shown to form oxonium derivatives which serve as intermediates for the entry of the sulphonic acid group into the ring system.Conductivity-temperature curves for lignosulphonic acids from various sources indicate that the products are sulphonic acids of the —C = C—type. Other evidence points to an oxonium addition product with a ring oxygen as being a probable intermediate in the formation of the final and more stable lignosulphonic acid. The relation of such a mechanism to the experimental conditions employed in the commercial process of the manufacture of sulphite pulp is indicated.The formation of lignosulphonic acids through tautomerization of a phenolic nucleus is highly improbable. Also for such formation a carbonyl group does not necessarily have to be present. Strong additional evidence is presented for the presence of a heterocyclic ring containing oxygen and for an ethylene linkage in the lignin molecule. The latter is indicated to be in the same ring system as the oxygen atom.