The Reactions of Chromophores of the Stilbene Type with Hydroxyl Radical (HO·) and Superoxide Radical (O₂·̄/HO₂·). Part I. The Cleavage of the Conjugated Double Bond

Abstract

The reactions of three hydroxy-and/or rnethoxy-substituted stilbenes with hydroxyl radicals (HO·) or with hydroxyl radicals and Superoxide radicals (ΗΟ·+θ2 τ /ΗΟ 2 ·) under alkaline and acidic conditions have been studied. In this part, the course of cleavage of the conjugated double bond in the stilbenes is described. The preparation of the starting stilbenes and of a suggested intermediate using previously published methods with some modifications is also given.

Characterization of active oxygen species under oxygen-alkali bleaching conditions

“…The above findings that a particular OX n species plays a role in the early stage of the reaction and that correlations between the degradation of VG and MGP are linear during this period strongly support this conclusion. Another possible species responsible for degradation of the model compounds in the early stage of the reaction, the TMPh peroxyl radical, could play a minimal role, which has already been reported in the literature (Gierer et al 1992(Gierer et al , 1996aSugimoto et at. 2000).…”

“…Based on this consideration, the hydroxyl radical is likely the most predominant species. Another possible predominant species is the TMPh peroxyl radical, formation of which could be explained by the reaction between a TMPh phenoxyl radical and molecular oxygen, although it has been reported that the phenoxyl radical is unreactive to molecular oxygen under the conditions employed in the literature (Gierer et al 1992(Gierer et al , 1996aSugimoto et al 2000). Chain-type reactions could not significantly contribute to degradation of the model compounds in this period due to the rapid production of OX n species, which originate from the reaction between TMPh and molecular oxygen, and the interruption of chain-type reactions by TMPh.…”

“…Delignification has to be terminated before the kappa number reaches approximately 50% of that for the original unbleached pulp due to severe damage to carbohydrates. Although extensive research has been carried out with the aim of suppressing the degradation of carbohydrates and understanding the chemistry in oxygen bleaching (Ericsson et al 1971;Gierer and Imsgard 1977;Gierer 1986Gierer , 1990aGierer ,b, 1997Ljunggren 1986Ljunggren , 1990Ek et al 1989;Ljunggren and Johansson 1990a,b;Sultanov and Wallis 1991;Gierer et al 1992Gierer et al , 1993Gierer et al , 1996aGierer et al ,b, 2001Gierer and Nilvebrant 1994;Johansson and Ljunggren 1994;Yasumoto et al 1996;Yokoyama et al 1996Yokoyama et al , 1998Yokoyama et al , 1999a, this drawback has not been solved yet.…”

“…Furthermore, chain-type reactions initiated by the species also operate in the system and result in the degradation of lignin and carbohydrates. Gierer et al (1992Gierer et al ( , 1996a have studied the reactivity of the hydroxyl radical in detail. In these studies, lignin and carbohydrate model compounds were reacted with hydroxyl radicals in situ generated by pulse-or gradiolysis of water.…”

The early oxidative biodegradation steps of residual kraft lignin models with laccase

“…Stilbenoids, which are formed during pulping, constitute a signi®cative structural feature of the residual kraft lignin. 8 Stilbene units are known to be fairly oxygen sensitive, 16,17 and under our reaction conditions they reacted even in the absence of laccase. It is noteworthy that the nature of the products obtained from laccase incubation and direct oxygen degradation are the same.…”