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 Delta G* is only 20.6 kcal mol(-1), indicating a fast reaction rate. The mechanism involves the formation of the CH3CHCH3+center dot center dot center dot 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.

Automated summary

The classical alkylation reaction of benzene with isopropyl chloride catalyzed by AlCl3 and Al2Cl6 was studied using reliable calculations. The formation of Al2Cl6 dimers was found to be more dominant, with the mechanism involving the formation of CH3CHCH3+center dot center dot center dot Al2Cl7- ion pair being the rate-determining step. The study also considered the solubility of Al2Cl6 in the reaction.