The Mechanism of Friedel-Crafts Chlorination Historically Reconsidered

This short review proposes a revision to the currently accepted textbook mechanism for the direct Friedel-Crafts chlorination of benzene. After briefly reviewing the history of Friedel-Crafts chemistry and the origins of the current textbook mechanism for the direct chlorination of benzene, it is pointed out how this mechanism is at variance with the predictions of both generalized Lewis acid-base chemistry and simple frontier orbital theory, which require that dichlorine should function as a sigma antibonding electron-pair acceptor (σ*-EPA) or electrophile (Lewis acid) rather than as a pi antibonding electron-pair donor (π*-EPD) or nucleophile (Lewis base). A modification of the currently accepted mechanism consistent with these concepts is then proposed. After reviewing the debates over the structure of the active catalyst, previous proposals for the modification of the current mechanism, and other examples in which the dihalogens function as σ*-EPA electrophiles, the revised mechanism is evaluated using DFT calculations.