Electron Transfer Reactions in Pulping Systems (II): Electrochemistry of Anthraquinone/Lignin Model Quinonemethides

Electron transfer reactions have been observed during the electrolyses of solutions containing anthraquinone and 8-aryl ether lignin model quinonemethides. In dry acetonitrile at a reduction potential of -0.9V (vs. Ag/AgCl) electrons are transferred from the electrode to anthraquinone (AQ) to form stable anthrahydroquinone radical anions (AHQ−). The lignin model quinonemethides are not reduced directly at the electrode at this potential but are reduced by AHQ− to give quinonemethide radical anions (QM−) and AQ. The QM− species rapidly fragment at their β-aryl ether bond to give phenolate ions and radicals; the latter further reduces to another phenolate ion. For example, the β-methyl lignin model QM 1 gives guaiacol and isoeugenol upon electrolysis at -0.9V in the presence of AQ. In wet acetonitrile, reduction of AQ at -0.9V leads to both anthrahydroquinone radical anion and dianion; the dianion is formed by direct electrolysis of the radical anion and by disproportlonation of the radical anion. Under all conditions and substrates examined, electron transfer reactions proceeded in preference to bond formation reactions which would generate “adducts.” The implication of these results Is that it should be possible to delignlfy wood by electron transfer reactions and that anthraquinone probably functions this way.