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 tertbutoxyl radicals leads, via the intermediate ketyl radical, to the formation of the corresponding ketone, aphenoxyacetophenone (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-I at 298 K and contradict a report in the literature that suggests a rate constant of >lo6 sf'. 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.Key words: ketyl radical, photochemical degradation, thermal degradation, lignin.ResumC : Lorsqu'on effectue la reaction du 1-phCnyl-2-phCnoxyCthanol (1) avec des radicaus tert-butoxyles gCnCrCs thermiquement ou photochimiquement B de faibles taux de conversion, sous une atmosphere inerte, le seul produit 2. se former, par le biais du radical cCtyle intermediaire, est la cCtone correspondante, a-phCnoxyacCtophtnone (4). Lorsqu'on effectue la rCaction sous une atmosphere d'oxygbne, le rendement en produit est approximativement double. Dans des conditions photochimiques, il a Ct C dCmontrC que 4 est le produit primaire et que de llacCtophtnone et du phCnol se forment par le biais d'une photolyse secondaire de 4. Ces donnCes suggbrent que la constante de vitesse de fragmentation du radical cCtyle dCrivC de 1 est de l'ordre de 10 s-', B 298 K, et elles sont en contradiction avec des donnCes de la 1ittCrature qui suggbrent que la constante de vitesse est >lo6 S-I. On discute de IYintCr&t de cette Ctude pour la photodCgradation de la lignine et jaunissement photochimique qui en dCcoule et on propose un mCcanisme rCvisC pour jaunissement de la pulpe.

Decomposition of β-O-4 Linked Lignin Model Compound in Anhydrous Ethanol without any Added Catalyst

“…Moreover, the selectivity of acetophenone was much lower than that of phenol with a SP/SA of 1.8, which could have been due to the fact there were more transformation pathways available for acetophenone, such as hydrogen abstraction and recoupling reactions, leading to the presence of some black floccules in the products (Route Ia-2, Fig. S8 in SI) (Fouassier and Lougnot 1987;Netto-Ferreira et al 1990;Huang et al 1995;Harwood et al 2008). Also, there were more generation pathways available for phenol via oxidation in Route III.…”

“…Essentially, the active hydrogen species generated from the solvents (illustrated in Scheme 1) participated in the proceedings and advanced the followup reactions (Guthrie 1975). The active hydrogen species (bonded to the surface of the steel wall, or as Metal-H) first attacked the oxygen atoms in α-carbonyl of the 2-PAP molecule and formed POPE radicals (Huang et al 1995;Dorrestijn et al 1999). The POPE radicals subsequently reacted along three routes, Routes Ia, Ib, or Ic, which led to further cleavage, rearrangement, or over-hydrogenation, respectively.…”

“…The POPE radicals subsequently reacted along three routes, Routes Ia, Ib, or Ic, which led to further cleavage, rearrangement, or over-hydrogenation, respectively. According to the products' distribution of the decomposition of 2-PAP in AE offered in Tables 2 and S4, the POPE radicals split into phenoxy radicals and phenylethyl alcohol along Route Ia (Huang et al 1995;Andersen et al 1996;Lanzalunga and Bietti 2000). Then, the resultant 1-phenylethenol (an enol) further tautomerized to a more stable ketone form of acetophenone (AP in Scheme 2) (Huang et al 1995), while the phenoxy radicals were terminated with the formation of phenol (P in Scheme 2) by abstracting hydrogen from acetophenone or the solvent (Netto-Ferreira et al 1990).…”

“…According to the products' distribution of the decomposition of 2-PAP in AE offered in Tables 2 and S4, the POPE radicals split into phenoxy radicals and phenylethyl alcohol along Route Ia (Huang et al 1995;Andersen et al 1996;Lanzalunga and Bietti 2000). Then, the resultant 1-phenylethenol (an enol) further tautomerized to a more stable ketone form of acetophenone (AP in Scheme 2) (Huang et al 1995), while the phenoxy radicals were terminated with the formation of phenol (P in Scheme 2) by abstracting hydrogen from acetophenone or the solvent (Netto-Ferreira et al 1990). POPE radicals underwent isomerization along Route Ib and further cleaved to benzaldehyde along Route Ib-1 (BAD in Scheme 2) (Dorrestijn et al 1999).…”

“…POPE radicals also hydrogenated directly into POPE along Route Ic (Scheme 2). Ethyl benzoate (EBA in Scheme 2) was also detected in some cases, which was likely generated along Route III (Huang et al 1995;Hanson et al 2010). The detection of EBA was caused by a small amount of air remaining in the autoclave and the redox catalysis characteristics of the components in the stainless steel wall (Table S1) (Hanson et al 2010).…”