Photodegradation of the lignin model α-guaiacoxyacetoveratrone, unusual effects of solvent, oxygen, and singlet state participation

Schmidt, John A.; Berinstain, Alain; Rege, F. De; Heitner, C.; Johnston, Linda; Scaiano, J. C.

doi:10.1139/v91-016

Cited by 45 publications

(20 citation statements)

References 10 publications

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“…If there is any triplet contribution, it must be sufficiently small ( 4 % ) to be undetectable under our experimental conditions. The result is in line with analogous observations in the case of similarly substituted a-phenoxyacetophenones (8)(9)(10). Typical aromatic ketones (e.g., benzophenone) undergo intersystem crossing with near-quantitative efficiency to yield a triplet state that is responsible for much of their photochemistry (19); in this sense methoxy-substituted acetophenones are unusual in that the singlet state plays a dominant role in their photochemistry.…”

Section: Discussionsupporting

confidence: 86%

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Kinetic study of the reactions of methoxy-substituted phenacyl radicals

Jovanović¹,

Renaud²,

Berinstain³

et al. 1995

Can. J. Chem.

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'The photochemistry of various mono-and dimethoxy-substituted a-bromoacetophenones has been investigated by laser flash photolysis in organic solvents. The short-lived excited singlet states cleave to yield bromine atoms and the corresponding methoxyphenacyl radicals with quantum yields ranging from 0.13 to 0.35. With the exception of 4-methoxy-a-bromoacetophenone (6), all other substrates yield readily detectable triplet states; these have .rr,.rre character and are very poor hydrogen abstractors. Triplet decay does not contribute to methoxyphenacyl radical formation. While methoxyphenacyl radicals may have a spin density as high as 0.3 at the carbonyl oxygen, which accounts for the alkoxy-like visible band in their absorption spectrum, their chemical reactivity is dominated by the radical character at the carbon site. Methoxyphenacyl radicals are modest hydrogen abstractors; for example, reaction of 6 with 1,4-cyclohexadiene occurs with a rate constant of 2.6 x 10-5 M-I s-I, while for addition to the double bond in I,]-diphenylethylene the rate constant is 9.4 x lo7 M-' s-'. Additions to other double bonds are likely to be slower (e.g., 12 + 1,3-cyclohexadiene, k 5 10' M-' s-I). In contrast, reaction with oxygen occurs with kc] = 2.5 x 10%~-' s-I. Under the conditions relevant for their participation in paper yellowing, the methoxyphenacyl radicals will be converted to peroxyl radicals. These probably play a key role in the oxidative photodegradation of lignin.Key rvords: methoxyphenacyl radicals, a-bromoacetophenones, laser flash photolysis. Rksum6: Faisant appel ii la photolyse laser Cclair, on a ttudit la photochimie de diverses mono-ou dimCthoxy-a-bromoacCtophCnones dans des solvants oganiques. Les &tats singulets excitCs de courte durCe de vie se brisent pour donner des atomes de brome et les radicaux mCthoxyphCnacyles correspondants, avec des rendements quantiques allant de 0,13 ii 0,35. A I'exception de la 4-mCthoxy-a-bromoacCtophCnone (6), tous les autres substrats fournissent des ttats triplets facilement dCtectables; ils posskdent un caractkre .rr,.rr:"t une trks mauvaise habilitC ii enlever des hydrogknes. La dCgCnCrescence de 1'Ctat triplet ne contribue pas la formation du radical mCthoxyphCnacyle. Alors que les radicaux mCthoxyphCnacyles peuvent possCder une densite de spin allant jusqu'ii 0.3 au niveau de I'oxygkne du carbonyle, ce qui explique la bande semblable ii celle des alcoxy dans leur spectre d'absorption, leur rCactivitC chimique est dominee par le caractere radicalaire au site carbonC. L'habilitC des radicaux mCthoxyphCnacyles ii enlever des hydrogknes est modeste; par exemple, la rCaction du composC 6 avec le cyclohexa-1,4-dikne se produit avec une constante de vitesse de 2,6 x 10' M-' s-', alors que, pour l'addition ii la double liaison du 1,l -diphCnylCthylkne, la constante de vitesse est de 9,4 x lo7 M-' s-I. 11 est probable que les additions sur d'autres doubles liaisons seront plus lentes (par exemple, pour 12 + cyclohexa-l,3-dikne, k 5 10-5 M-' s-I). Par ailleurs, la rC...

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Section: Discussionsupporting

confidence: 86%

“…These molecules tend to include some of the key features of lignin and, therefore, their behavior can be taken to represent a modest fraction of the reactions that may occur in native lignin. Among these model compounds, a-guaiacoxyacetoveratrone (1) has been the subject of numer-I ous studies (3,(5)(6)(7)(8)(9)(10).…”

Section: Introductionmentioning

confidence: 99%

Kinetic study of the reactions of methoxy-substituted phenacyl radicals

Jovanović¹,

Renaud²,

Berinstain³

et al. 1995

Can. J. Chem.

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“…The approximate twofold increase in the yield of the ketone 4 upon introduction of oxygen is consistent with a mechanism in which direct reaction with oxygen (reaction [8]) competes with the bimolecular disproportionation of 1' to give one equivalent of 1 and one of 4 (reaction [9]). In the presence of oxygen all of the ketyl radicals are trapped by reaction [8] and subsequently tranformed into 4. The slightly higher than twofold increase in the yield of 4 in the presence of oxygen may be indicative of a small amount of autoxidation under oxygen (presumably via the hydroperoxyl radical).…”

Section: Aah0xch3c(h)=ch2+ch3c(oh)=ch) =supporting

confidence: 72%

“…However, it has been pointed out4 that the concentration of oxygen in argon-saturated solutions may be has high as lo-' M. Since the average radical concentrations are on the order of lo-' M, it is likely that the disproportionation between two ketyl radicals will not occur and that the apparent disproportionation is mediated by oxygen (eqs. [8] and [lo]). Given this possibility and assuming klo = 5 x lo9 M-' s-', the upper limit for reaction [3] should be raised to k3 < 0.1k8[02] = 50 s-' at 60°C, which is more in line with the thermochemical prediction of 38 s-' at this temperature.…”

Section: Aah0xch3c(h)=ch2+ch3c(oh)=ch) =mentioning

confidence: 99%

“…They proposed that up to 70% of the photogenerated chromophores were generated via path C in which ketyl radicals, generated from P-guaiacoxybenzyl alcohols, underwent fragmentation to give the phenoxyl radical directly and a substituted acetophenone (via the initally formed enol). While the formation and reactivity of phenoxyl radicals in the lignin matrix is not well understood, a number of photochemical studies (5)(6)(7)(8)(9)(10)(11)(12) using model compounds in solution or adsorbed on solid supports have provided some insights into the factors influencing all of the proposed mechanisms. In one of these studies (7) it was suggested that the fragmentation of 1 -phenyl-2-phenoxyethanol-1 -yl radical (1') formed from the reaction of photogenerated tert-butoxyl radicals with 1-phenyl-2-phenoxyethanol (1, eqs. [I], [2]) undergoes rapid fragmentation to yield phenoxyl radical and acetophenone (2, eq.…”

Section: Introductionmentioning

confidence: 99%

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

Huang¹,

Pagé²,

Wayner³

1995

Can. J. Chem.

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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.

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The Reduction of α-X-Acetophenones (X = PhO, Br, Cl) in Hydrogen-Donating Solvents at Elevated Temperatures

et al. 1999

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The reduction of α‐X‐acetophenones (X = PhO, Br, Cl), as model compounds for lignin liquefaction studies, has been investigated in the presence of a hydrogen‐donating solvent such as 9,10‐dihydroanthracene (AnH2) or 2‐propanol, between 373 and 573 K. With α‐phenoxyacetophenone (PAP) in AnH2, acetophenone and phenol have been obtained with high selectivities. The mechanism involves the reverse radical disproportionation (RRD) with AnH2. Hydrodebromination of α‐bromoacetophenone (BrAP) is quantitative at 423 K using AnH2 as a reducing agent. Now, the hydrogen transfer proceeds by an uninhibited radical chain mechanism with anthracenyl radicals as the chain carriers. For the kinetic analysis, the C–X (X = Br, Cl) bond dissociation enthalpies (BDEs) have been determined by means of very low pressure pyrolysis to give BDE(C–Br) = 271 kJ mol–1 and, as a lower limit, BDE(C–Cl) ≈ 309 kJ mol–1, at 298 K. The BDEs are quite at variance with recently published insights derived from an electrochemical study. For comparison, density functional theory calculations (DFT) have been performed.

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Photodegradation of the lignin model α-guaiacoxyacetoveratrone, unusual effects of solvent, oxygen, and singlet state participation

Cited by 45 publications

References 10 publications

Kinetic study of the reactions of methoxy-substituted phenacyl radicals

Kinetic study of the reactions of methoxy-substituted phenacyl radicals

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

The Reduction of α-X-Acetophenones (X = PhO, Br, Cl) in Hydrogen-Donating Solvents at Elevated Temperatures

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