Effect of solvent type on the formation rate of benzyl cation intermediate in acidolysis of lignin

Masaki Hirata; Tomoya Yokoyama

doi:10.1515/hf-2021-0193

Article

Effect of solvent type on the formation rate of benzyl cation intermediate in acidolysis of lignin

Masaki Hirata and Tomoya Yokoyama

Published/Copyright: December 24, 2021

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From the journal Holzforschung Volume 76 Issue 3

Abstract

The purpose of this study was to examine how the type of solvent among aqueous 1,4-dioxane, tetrahydrofuran, ethanol, iso-propyl alcohol, or ethylene glycol and its content (mol%) affect the formation rate of benzyl cation intermediate (BC) in the acidolysis of lignin, using a simple model compound, 1,2-dimethoxy-4-methoxymethylbenzene. Because the BC forms from the model compound via two steps, i.e., protonation of the benzyl methoxymethyl group as the pre-equilibrium step and liberation of the methanol as the rate-determining step, the observed variation of the formation rate with type of solvent and solvent content originates from the effects on both steps undistinguishably. The formation rate of BC decreased with increasing mol% of any of the organic solvents for a range of relatively low mol%, but increased with it for relatively high mol%. The formation rate varied more in the ether than in the alcohol systems. These results seem to be regulated by the effect of changing the mol% on the pre-equilibrium step, i.e., on the proton activity, rather than on the rate-determining step. Two reaction products, 4-alkoxymethyl-1,2-dimethoxybenzene and 4-hydroxymethyl-1,2-dimethoxybenzene, exclusively formed in the aqueous alcohol systems. The former compound was confirmed to be thermodynamically more stable and kinetically the more favorable product.

Keywords: acidolysis; benzyl cation; delignification; trifluoromethanesulfonic acid

Corresponding author: Tomoya Yokoyama, Laboratory of Wood Chemistry, Department of Biomaterial Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan, E-mail: yokoyama@woodchem.fp.a.u-tokyo.ac.jp

Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: None declared.
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Supplementary Material

The online version of this article offers supplementary material (https://doi.org/10.1515/hf-2021-0193).

Received: 2021-09-21

Accepted: 2021-11-11

Published Online: 2021-12-24

Published in Print: 2022-03-28

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Supplementary Material

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Keywords for this article

acidolysis; benzyl cation; delignification; trifluoromethanesulfonic acid