Addition-Elimination versus Direct Substitution Mechanisms for Arene Chlorination

Theoretical modelling of the chlorination of arenes (benzene, toluene and naphthalene) in a nonpolar solvent (CCl4) at the B3LYP-D3/6-311+G(2d,2p) level reveals two alternative reaction pathways, namely, direct substitution and addition-elimination, both of which lead to the same substitution products. The closeness of their barriers indicates competition between these alternative routes. Neither of these mechanistic pathways includes a sigma-complex intermediate, which is traditionally believed to be a key feature of electrophilic aromatic substitution (SEAr) mechanisms. Instead, the direct substitution pathway involves a single concerted transition state following the barrierless formation of a pi-complex between the reactants. The reported catalytic effect of HCl catalysis on the reaction product is modelled systematically and considerably decreases the transition-state energies. The computed reaction barriers are in excellent accord with the experimentally established reactivity trends for the arenes investigated.