Radical-induced degradation of a lignin model compound. Decomposition of 1-phenyl-2-phenoxyethanol

Abstract

Reaction of 1-phenyl-2-phenoxyethanol (1) with thermally or photochemically generated tert-butoxyl radicals leads, via the intermediate ketyl radical, to the formation of the corresponding ketone, α-phenoxyacetophenone (4), as the only product at low conversion under an inert atmosphere. An approximately twofold increase in the product yield is observed when the reactions are carried out under oxygen. Under the photochemical conditions it is shown that 4 is the primary product and that acetophenone and phenol are formed as a result of secondary photolysis of 4. These data suggest that the rate constant for fragmentation of the ketyl radical derived from 1 is on the order of 10 s−1 at 298 K and contradict a report in the literature that suggests a rate constant of >106 s−1. The relevance of this study to the photodegradation of lignin and consequent photoyellowing is discussed and a revised mechanism for the photoyellowing of pulp is proposed. Keywords: ketyl radical, photochemical degradation, thermal degradation, lignin.