In situ spectroscopic studies related to the mechanism of the Friedel–Crafts acetylation of benzene in ionic liquids using AlCl3 and FeCl3

Abstract

Several aspects of the mechanism of the Friedel–Crafts acetylation of benzene were studied by in situ spectroscopic methods in ionic liquids, prepared from MCl₃ (M = Al or Fe) and 1-butyl-3-methylimidazolium chloride ([bmim]Cl). Mössbauer measurements have revealed that the addition of FeCl₃ to [bmim]Cl leads to an equilibrium mixture that contains solid FeCl₃, [bmim][Fe₂Cl₇], and Fe₂Cl₆ and/or [bmim][FeCl₄], depending on the molar ratio of FeCl₃ and [bmim]Cl. The formation of [(CH₃CO)₂CHCO]⁺[MCl₄]⁻, a potential side product in the Friedel–Crafts acetylation of benzene, was shown to require the presence of both the acetylium ion [CH₃CO]⁺[MCl₄]⁻ and free acetyl chloride. We have confirmed that [(CH₃CO)₂CHCO]⁺[MCl₄]⁻ does not involve in the Friedel–Crafts acetylation of benzene. Experimental data and theoretical calculations indicate that the acetylium ion [CH₃CO]⁺[MCl₄]⁻ is the key intermediate in the Friedel–Crafts acetylation of benzene and the reaction proceeds through an ionic mechanism.