In situ spectroscopic studies related to the mechanism of the Friedel–Crafts acetylation of benzene in ionic liquids using AlCl3 and FeCl3Based on the presentation given at Dalton Discussion No. 4, 10–13th January 2002, Kloster Banz, Germany.

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.