Cellulose – Hemicelluloses – Lignin – Wood Extractives
Editor-in-Chief: Faix, Oskar / Salmén, Lennart
Editorial Board: Daniel, Geoffrey / Militz, Holger / Rosenau, Thomas / Sixta, Herbert / Vuorinen, Tapani / Argyropoulos, Dimitris S. / Balakshin, Yu / Barnett, J. R. / Burgert, Ingo / Rio, Jose C. / Evans, Robert / Evtuguin, Dmitry V. / Frazier, Charles E. / Fukushima, Kazuhiko / Gindl-Altmutter, Wolfgang / Glasser, W. G. / Holmbom, Bjarne / Isogai, Akira / Kadla, John F. / Koch, Gerald / Lachenal, Dominique / Laine, Christiane / Mansfield, Shawn D. / Morrell, J.J. / Niemz, Peter / Potthast, Antje / Ragauskas, Arthur J. / Ralph, John / Rice, Robert W. / Salin, Jarl-Gunnar / Schmitt, Uwe / Schultz, Tor P. / Sipilä, Jussi / Takano, Toshiyuki / Tamminen, Tarja / Theliander, Hans / Welling, Johannes / Willför, Stefan / Yoshihara, Hiroshi
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Effect of the α- and γ-Hydroxyls on the Alkaline Hydrolysis Rate of Nonphenolic β-0-4 Lignin Diastereomers
Nonphenolic β-0-4 erythro and threo lignin model diastereomers with various γ-groups (CH3, CH2-O-CH3, and CH2OH) and Cα-substituents (OH, OCH3) were synthesized, and the alkaline hydrolysis rates and activation parameters determined. In addition, two of the diastereomer pairs were computationally modeled and the thermodynamic values for the ionization of the α- or γ-hydroxyl, and subsequent displacement of the phenolate group to form an epoxide intermediate, were determined. The results suggest that the erythro γ-hydroxyl may participate in the hydrolysis to a significant extent, which results in a relatively high erythro/threo rate ratio for the α,γ-di-OH isomers. The influence of the erythro γ-hydroxyl on the hydrolysis rate may be due to the relatively favorable stability of the erythro γ-oxyanion. The electronic effect of the g-substituent appears to influence how fast the α-hydroxyl displaces the phenoxyl. We had previously suggested that the γ-substituent sterically inhibits hydrolysis of the threo isomer, and computational modeling confirmed this.
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