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Dexter L. Criss, Thomas Elder, Thomas H. Fisher, Tor P. Schultz

Effect of the ?- and ?-Hydroxyls on the Alkaline Hydrolysis Rate of Nonphenolic ?-0-4 Lignin Diastereomers

Nonphenolic ?-0-4 erythro and threo lignin model diastereomers with various ?-groups (CH₃, CH₂-O-CH₃, and CH₂OH) and C_?-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 ?- or ?-hydroxyl, and subsequent displacement of the phenolate group to form an epoxide intermediate, were determined. The results suggest that the erythro ?-hydroxyl may participate in the hydrolysis to a significant extent, which results in a relatively high erythro/threo rate ratio for the ?,?-di-OH isomers. The influence of the erythro ?-hydroxyl on the hydrolysis rate may be due to the relatively favorable stability of the erythro ?-oxyanion. The electronic effect of the g-substituent appears to influence how fast the ?-hydroxyl displaces the phenoxyl. We had previously suggested that the ?-substituent sterically inhibits hydrolysis of the threo isomer, and computational modeling confirmed this.

Holzforschung, Walter de Gruyter

Print ISSN: 0018-3830
Volume: 56, 02/2002
Pages: 67 - 72

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