Dissolution of Wood with Acetyl Bromide Solutions-Reactions of Lignin Model Compounds

Abstract

A Pinus radiata refiner mechanical pulp (RMP) sample and a range of lignin model compounds were treated with a solution of 25% w/w acetyl bromide (AcBr) in acetic acid to assess the reactions which take place during the dissolution of wood with the AcBr solution. The lignin model reactions were also carried out in solutions to which 4% w/w perchloric acid was added. The RMP sample gave a product which contained 9.2% bromine, and was severely degraded to low molecular weight products, the polysaccharides more so than the lignin fraction. The reactions of the lignin model compounds with AcBr included O-acetylation of phenolic and aliphatic hydroxyl groups, cleavage of β-ether bonds, C-acetylation of aromatic rings, particularly in positions para to methoxyl groups, demethylation of aromatic methoxyl groups and probable replacement of benzyl alcohol groups by bromine functions. The reactions were accelerated by perchloric acid, and the rates appeared to decrease in the order O-acetylation π bromine substitution >> β-ether cleavage π C-acetylation > demethylation. It is concluded that dissolution of wood in the AcBr solution is a consequence of depolymerisation of the polysaccharides and lignin, and acetylation to give products which would be soluble in the reaction medium.