Mechanism of alkylation of benzene with ethylene catalyzed by [bmim] Cl/FeCl3 ionic liquid

The acidic properties of the 2-position H of 1-n-butyl-3-methylimidazolium chloride ([bmim]Cl) were measured by titration, FT-IR, and H-1 NMR analysis. The 2-H signal at a chemical shift of 9.5 disappeared completely and KCl was produced after titration with KOH solution, implying the significant Bronsted acidic property of the 2-H. When FeCl3 was added into [bmim] Cl to form an ionic liquid I the chemical shift of 2-H shifted downfield, which indicated that 2-H was deshielded by FeCl3 and was easy to disengage from imidazole. The alkylation of deuterated benzene with ethylene catalyzed by the dehydrated [ bmim] Cl/FeCl3 ionic liquid was investigated by GGMS and H-1 NMR. The intensity of NMR signal of 2-H was reduced by 23% after reaction. This suggested that the H+ for initiating the reaction was supplied partly by the 2-H of imidazole. On the basis of these results, a typical carbocation mechanism for the alkylation of benzene with ethylene catalyzed by the [bmim]Cl/FeCl3 ionic liquid was proposed.