The Reactions of Lignin during Sulphate Cooking. Part VIII. The Mechanism of Splitting of beta-Arylether Bonds in Phenolic Units by White Liquor.

Gierer, Josef; Smedman, Leif-Åke; Cederberg, G.; Jensen, R. B.; Pederson, C. Th; Larsen, Erik

doi:10.3891/acta.chem.scand.19-1103

Cited by 30 publications

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“…The formation and the opening of the threemembered ring are reminiscent of the corresponding reaction steps in other eliminations of P-aroxy substituents involving neighbouring group participation such as the cleavage of P-aryl ether linkages by neighbouring mercaptide groups (12). The function of a spiro cyclohexadienone as a transient intermediate has been previously (13) proposed in order to interpret a related reaction, i.e.…”

Section: Resultsmentioning

confidence: 99%

Studies on the condensation of lignins in alkaline media. Part II. The formation of stilbene and arylcoumaran structures through neighbouring group participation reactions

Gierer¹,

Pettersson²

1977

Can. J. Chem.

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JOSEF GIERER and INGEGERD PETTERSSON. Can. J. Chem. 55, 593 (1977). The alkaline condensation of p-hydroxyarylglycerol-a-aryl ether units in lignin with other phenolic structures and the subsequent reactions of the condensation products have been elucidated using appropriate model compounds. It is shown that the primary condensation of simple phenols (2,6-xylenol or resorcinol) into the a-position of phenolic compounds of the f3-aryl ether type is followed by an aryl participation (A,-3) reaction resulting in the cleavage of the a-aryl ether bond and formation of the corresponding stilbene derivative. When resorcinol is used as reaction partner, the condensation is also followed by the cleavage of the P-aryl ether linkage through another type of neighbouring group participation reaction involving the phenolate anion ortho to the site of condensation and giving rise to the corresponding arylcoumaran derivative.The consequences of these findings for the course of alkaline delignification processes are briefly discussed. Chem. 55, 593 (1977). On a ClucidC, utilisant des composes modiles approprits, le mecanisme de la condensation alcaline d'unites d'ether p-hydroxyarylglycCro1 a-aryle de la lignine avec d'autres structures phCnoliques et les rCactions subsCquentes des produits de condensation. On a montr6 que la condensation primaire de phenols simples (xylenol-2,6 ou rCsorcino1) en position a des composes phCnoliques du type ether a-aryle est suivie par une reaction de participation du groupe aryle (A1-3) qui conduit au bris du lien ether P-aryle et a la formation du derive stilbene correspondant. Quand on utilise le resorcinol comme partenaire de la rCaction, la condensation est aussi suivie par une coupure du lien ether a-aryle par un autre type de reaction de participation de groupe voisin impliquant I'anion phenolate ortho par rapport au site de condensation et donnant lieu au dCrivC arylcoumarane correspondant.On discute des consCquences de ces resultats pour les processus de delignification alcaline. [Traduit par le journal]Numerous studies on the alkaline condensation of lignins have shown that the benzylic position in phenolic units constitutes the main site of reaction in this complex process (1-6). In most of these studies, model compounds of the phydroxybenzyl alcohol type were reacted with simple phenols and the resulting diarylmethanes were isolated. The condensation reaction was formulated as a nucleophilic addition of an ortho-or para-cyclohexadienone carbanion to the appropriate para-quinonemethide, followed by proton or side-chain elimination with rearomatization (7). Results from studies using model compounds of the y-hydroxyaryl coumaran (8) and P-arylglycerol (9) types indicated that this conjugate addition reaction is not prevented by the presence of an aryi or an aroxy group in the adjacent P-position (cf. also condensations in acidic media (10)). Thus, carbanions formed from phenolic lignin structures compete, in fact, with the other nucleophiles present in the pulping liquor (SH-, OH-ions) ...

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Section: Resultsmentioning

confidence: 99%

Studies on the condensation of lignins in alkaline media. Part II. The formation of stilbene and arylcoumaran structures through neighbouring group participation reactions

Gierer¹,

Pettersson²

1977

Can. J. Chem.

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“…Structures 23 and 24 then react further into 4-hydroxy-3-metoxy-propiophenone 16 and guaiacyl acetone 7, whereas structure 2Q may react further, e.g. forming dimeric structures (Gierer and Smedman 1965;Brunow and Miksche 1969). In the case of enol ether the reaction may proceed similarly as for vinylguaiacol and isoeugenol, forming an ot-carbonyl which subsequently fragments into acetovanillone and guaiacol, Figure 7, as has been suggested in the literature (Gierer and Ljunggren 1979;Hise et al 1987).…”

Section: General Remarksmentioning

confidence: 96%

Influence of Polysulphide on the Rate of Degradation of Six p-OH Styrene Structures at Two OH¯ Concentrations

Berthold¹,

Gellerstedt²

1998

Holzforschung

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Keywords Modified kraft pulping Polysulphide Vinylguaiacoi Isoeugenol Coniferyl alcohol Enol ether Diguaiacyl stilbene Coniferaldehyde Sulfenylium cation carrier SummarySix lignin model structures containing a conjugated double bond in the side chain -vinylguaiacol 1, isoeugenol 2, coniferyl alcohol 3, diguaiacyl stilbene 4, vinylguaiacol-ß-guaiacyl ether (enolether) 5 and coniferaldehyde 6 -were treated with either 0.2M HS~ or 0.2M polysulphide in 0.32M OH~ or 0.03 M OH~ at 125 °C. In all cases, with the exception of coniferaldehyde and diguaiacyl stilbene, the degradation rates were greater in the presence of polysulphide ions than in the presence of HS". When polysulphide was applied, a lowering of the OH" concentration increased the degradation rate further. In 0.03 M OH", the rate increased between 1.6 and 175 times when HS" was replaced by polysulphide. These results demonstrate that the presence of polysulphide during alkaline pulping can influence the degradation of the dissolved lignin and probably also the lignin matrix and can therefore be beneficial for lignin removal during pulping.

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“…The strength of the S-C bond (bond energy (Me-SR) «313 kJ/mol) is higher than that of the SS-C bond (bond energy (Me-SSR) « 241 kJ/mol) (Chatgilialoglu and Asmus 1990), suggesting that the bond in the polysulphidequinone methide adduct 4 is weaker than the HS'-quinone methide adduct, leading to a higher reverse reaction rate (k_ 3 ). Under very alkaline conditions, the HS'-quinone methide is known to degrade rapidly (Gierer and Smedman 1965) suggevSting a low energy barrier in step 4, making the reverse reaction (k_ 3 ) less important. Hence, if step 3 is regarded as being practically irreversible (low energy barrier in step 4), the observed decrease in the formation of enol ether must be due to the fact that the polysulphide ion is a stronger nucleophile than HS".…”

Section: Potential Energy Diagrammentioning

confidence: 99%

Degradation of Guaiacylglycerol-β-Guaiacyl Ether in the Presence of HSor Polysulphide at Various Alkalinities. Part I. Degradation Rate and the Formation of Enol Ether

Berthold¹,

Lindfors²,

Gellerstedt³

1998

Holzforschung

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The degradation of guaiacylglycerol-ß-guaiacyl ether by HS" and polysulphide was studied at various alkalinities between 0.025 M and 1.1 M OH-at 130°C. Polysulphide strongly suppressed the reverse aldol pathway and thereby the formation of enol ether over the whole [OH~] interval investigated. We therefore conclude that polysulphide increases the fragmentation step and probably also acts as a stronger nucleophile than HS" towards intermediate quinone methide structures during the alkaline treatment of guaiacylglycerol-ß-guaiacyl ether. At alkalinities above 0.2 M OH", polysulphide tended to degrade the tested ß-model faster than HS".

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The Reactions of Lignin during Sulphate Cooking. Part VIII. The Mechanism of Splitting of beta-Arylether Bonds in Phenolic Units by White Liquor.

Cited by 30 publications

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Studies on the condensation of lignins in alkaline media. Part II. The formation of stilbene and arylcoumaran structures through neighbouring group participation reactions

Studies on the condensation of lignins in alkaline media. Part II. The formation of stilbene and arylcoumaran structures through neighbouring group participation reactions

Influence of Polysulphide on the Rate of Degradation of Six p-OH Styrene Structures at Two OH¯ Concentrations

Degradation of Guaiacylglycerol-β-Guaiacyl Ether in the Presence of HSor Polysulphide at Various Alkalinities. Part I. Degradation Rate and the Formation of Enol Ether

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The Reactions of Lignin during Sulphate Cooking. Part VIII. The Mechanism of Splitting of beta-Arylether Bonds in Phenolic Units by White Liquor.

Cited by 30 publications

References 0 publications

Studies on the condensation of lignins in alkaline media. Part II. The formation of stilbene and arylcoumaran structures through neighbouring group participation reactions

Studies on the condensation of lignins in alkaline media. Part II. The formation of stilbene and arylcoumaran structures through neighbouring group participation reactions

Influence of Polysulphide on the Rate of Degradation of Six p-OH Styrene Structures at Two OH¯ Concentrations

Degradation of Guaiacylglycerol-β-Guaiacyl Ether in the Presence of HS­or Polysulphide at Various Alkalinities. Part I. Degradation Rate and the Formation of Enol Ether

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Degradation of Guaiacylglycerol-β-Guaiacyl Ether in the Presence of HSor Polysulphide at Various Alkalinities. Part I. Degradation Rate and the Formation of Enol Ether