REACTIONS OF CHLORINE DIOXIDE WITH LIGNINS IN UNBLEACHED PULPS .1.

The oxidative reactions of chlorine dioxide with model compounds, representing phenolic and non-phenolic structural elements in residual lignins, have been studied. A general mechanism comprising initial electrophilic attack by the bleaching reagent followed by nucleophilic processes and rearrangements, is proposed to account for the formation of quinone, muconic acid, oxirane and oxidized side chain structures. In these reactions, chlorine dioxide is reduced to give hypochlorous and chlorous acid. The latter may undergo bimolecular disproportionation with formation of hypochlorous and chloric acid. The equilibrium partner of hypochorous acid, elemental chlorine, acts as chlorinating reagent giving rise to a variety of chlorine-substituted compounds. The significance of the oxidative cleavage of carbon-carbon and aryl ether bonds for the fragmentation of residual lignins during chlorine dioxide bleaching is demonstrated.