Some mechanistic aspects of the light-induced yellowing of lignin: A further study of the photochemical reactions of α-guaiacoxy-β-hydroxy-proprio-veratrone, and some substituted methoxybenzenes

Abstract

The primary photoreduction of a model lignin system α-guaiacoxy-β-hydroxypropioveratrone occurs via a triplet mechanism leading to polarized ketyl radicals which undergo β-ether cleavage similarly to the photochemical reactions of α-guaiacoxyacetoveratrone. Comparison of the photoyellowing rates of solutions of the two lignin models showed a slightly greater rate for the derivative with the βhydroxymethyl function, with some subtle differences in behavior between the two compounds probably due to the intramolecular hydrogen bonding in the former. In addition it was found that photooxidation of several methoxybenzenes was considerably slower than that of corresponding methoxyphenols. This is reassuring that the ubiquitous presence of methoxybenzene groups in lignin matrices is unlikely to contribute significantly on the timescale relevant to the photoyellowing of CTMP pulps.