A molecular orbital study of the mechanism of chlorination reaction of benzene catalyzed by Lewis acid

Abstract

In order to explore the mechanism of electrophilic aromatic substitution reaction, the potential energy surface of the chlorination of benzene with aluminium chloride catalyst was examined with ab initio molecular orbital method. The reaction among C 6H 6, Cl 2 and AlCl 3 experiences the initial π-complex, the first transition state TS1, two σ-complex intermediates, the second transition state TS2, and the final product complex along the reaction coordinate without forming explicit electrophile Cl +. The aluminium chloride always stays with the reacting species and its role as a Lewis acid catalyst is shown to be very important throughout the overall reaction course. The TS1 corresponds to the rate-determining step of the electrophilic aromatic substitution reaction, which leads to the ion-pair complex between C 6H 6Cl + and AlCl 4 −. The structure at TS1 indicates that the formation of C–Cl and Cl–Al bonds and the breaking of Cl–Cl bonds occur synchronously. The σ-complex intermediate is extremely stabilized by the Coulombic interaction between C 6H 6Cl + and AlCl 4 −. The structure of TS2 demonstrates that both the cleavage of C–H bonds and the formation of HCl occur in a concerted manner with the assistance of AlCl 4 −. The free energy curve along the reaction coordinate well reproduces the generally suggested potential energy curve.