The alkylation mechanism catalyzed by an ionic liquid (as a Lewis acid) may be different from the traditional alkylation mechanism catalyzed by Br?nsted acid, especially as their initiation steps are still not clear. In this paper, an isotope exchange method is used to investigate the catalytic mechanism
of AlCl₃/butyl-methyl-imidazolium chloride ionic liquid in the alkylation of benzene with 1-dodecene. The proposed catalytic mechanism was confi rmed by analysis of ionic liquid before and after reaction and of the alkylation products of deuterated benzene (C₆D₆) with 1-dodecene. The proposed mechanism consists of the equilibrium reaction between [Al₂Cl₇]^?+H⁺ and [AlHCl₃]⁺+[AlCl₄]^?, in which the Br?nsted acid [AlHCl₃]⁺ is supplied by the reaction of 2-H on the imidazolium ring and [Al₂Cl₇]^-. The alkylation reaction is initiated by the Br?nsted acid [AlHCl₃]⁺ which reacts with 1-dodecene to form a carbonium ion, then the carbonium ion reacts with benzene to form an unstable σ complex, leading to the formation of 2-phenyldodecane.