Friedel-Crafts Reaction: Theoretical Study of the Mechanism of Benzene Alkylation with Isopropyl Chloride Catalyzed by AlCl₃ and Al₂Cl₆

The classical alkylation reaction of benzene with isopropyl chloride catalyzed by the species AlCl₃ and Al₂Cl₆ was studied using reliable calculations at M06-2X/def2-TZVPP//X3LYP/def2/SVP level of theory and SMD model for solvent effect. We evaluated the formation of dimers, trimers, tetramers, and pentamers and showed that Al₂Cl₆ dimers exist in greater proportion, in agreement with experimental observations. The experimental solubility of Al₂Cl₆ in benzene was also included in the theoretical kinetics analysis. The reaction catalyzed by AlCl₃ species presents the highest barrier, in part due to unfavorable dissociation of the Al₂Cl₆ species. The mechanism via Al₂Cl₆ catalysis is more effective and even considering its low solubility, the calculated observed DG^‡ is only 20.6 kcal mol^-1, indicating a fast reaction rate. The mechanism involves the formation of the CH₃CHCH₃⁺…Al₂Cl₇^- ion pair, which reacts with benzene to form a Wheland intermediate and this carbon-carbon bond formation step corresponds to the rate-determining one.

Keywords:
electrophilic aromatic substitution; Lewis acid; free energy profile; microkinetic analysis; solvent effects; ion pairing