Revisiting the mechanism of β-O-4 bond cleavage during acidolysis of lignin. Part 9: Comprehensive results for guaiacyl-type compounds and the difference in participation mode of bromide and chloride anions between C₆-C₃-type and C₆-C₂-type compounds

Abstract

A phenolic β-O-4-type lignin model compound with a C₆-C₃-type side-chain and two guaiacyl nuclei and its derivatives were individually acidolyzed in aqueous 82 vol% 1,4-dioxane containing 0.2 mol/L HBr, HCl, or H₂SO₄ at 85 °C in this study. The obtained results, combined with those in the series of this study in which phenolic and non-phenolic C₆-C₃- and C₆-C₂-type analogues were used, generated the following results and proposals. The H₂O addition reproducing the original compound is preponderant as a reaction mode of the benzyl cation intermediate (BC) over the other modes leading to the β-O-4 bond cleavage, especially in the acidolyses using H₂SO₄ and those of C₆-C₃-type compounds. A well-known phenomenon whether formaldehyde is liberated from C₆-C₃-type compounds is regulated by acid types appears less clearly in phenolic than in non-phenolic compounds. Br¯ as well as Cl¯ nucleophilically assists the H¯ transfer from the β- to α-carbon of BC, especially in C₆-C₃-types, which accelerates and regulates the acidolyses using HBr as well as HCl. Furthermore, Br¯ is suggested to nucleophilically attack the β-carbon of non-phenolic C₆-C₃-type BC to cleave the β-O-4 bond, which explains the characteristically peculiar observation in the acidolysis of the non-phenolic C₆-C₃-type compound using HBr described in our previous report.

Keywords:

• Biorefinery
• hydrobromic acid
• hydrochloric acid
• sulfuric acid
• woody biomass conversion

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No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Japan Society for the Promotion of Science [JSPS KAKENHI Grant Number JP 18J23098, Grant-in-Aid for JSPS Fellows (DC1)].