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

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). Mossbauer 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)(2)CHCO](+)[MC4](-), 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)(2)CHCO](+)[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.