Homogeneous benzoylation of cellulose in 1-allyl-3-methylimidazolium chloride: Hammett correlation, mechanism and regioselectivity

Abstract

Homogeneous benzoylation of cellulose with a series of substituted benzoyl chlorides, in which substituents varied from electron donating to electron withdrawing groups, was investigated in an ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl). The electronic effect of substituents had a considerable effect on the reaction. A plot of Hammett parameters of substituents vs. degree of substitution (DS) of resultant cellulose esters exhibited a V-shaped graph: a negative slope for negative Hammett parameters, and a positive slope for positive Hammett parameters. Such a Hammett plot indicated the mechanism of cellulose benzoylation: the reaction underwent a unimolecular ionization mechanism with a carbocationic intermediate when benzoyl chlorides were with electron-donating substituents, and a bimolecular addition–elimination mechanism with a tetrahedral intermediate when benzoyl chlorides were with electron-withdrawing substituents. In addition, ¹³C-NMR analysis showed that most of the substituted benzoyl chlorides exclusively preferred the primary hydroxyl at the C-6 position. This unusually high regioselectivity was attributed to the synergistic effect of appropriate reaction rate, moderate steric effect and reaction mechanism of benzoylation.