In situ spectroscopic studies related to the mechanism of the Friedel-Crafts acetylation of benzene in ionic liquids using AlCl_3 and FeCl_3

摘要

Several aspects of the mechanism of the Friedel-Crafts acetylation of benzene were studied by in situ spectroscopic methods in ionic liquids, prepared from MCl_3 (M=Al or Fe) and 1-butyl-3-methylimidazolium chloride ([bmim]Cl). Mossbauer measurements have revealed that the addition of FeCl_3 to [bmim]Cl leads to an equilibrium mixture that contains solid FeCl_3, [bmim][Fe_2Cl_7], and Fe_2Cl_6 and/or [bmim][FeCl_4], dempending on the molar ratio of FeCl_3 and [bmim]Cl. The formation of [(CH_3CO)_2CHCO]~+[MCl_4]~-, a potential side product in the Friedel-Crafts acetylation of benzene, was shown to require the presence of both the acetylium ion [CH_3CO]~+[MCl_4]~- and free acetyl chloride. We have confirmed that [(CH_3CO)_2CHCO]~+[MCl_4]~-, does not involve in the Friedel-Crafts acetylation of benzene. Experimental data and theoretical calculations indicate that the acetylium ion [CH_3CO]~+[MCl_4]~- is the key intermediate in the Friedel-Crafts acetylation of benzene and the reaction proceeds through an ionic mechanism.