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

Article abstract of DOI:10.1039/b109303g

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). Moessbauer 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.