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 σ‐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 π‐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. The present comprehensive DFT computational exploration of aromatic chlorination predicts that the barriers for the addition of Cl2 to benzene, toluene and naphthalene catalyzed by HCl in the gas phase and in simulated CCl4 solution can be even lower than the barriers for direct substitution. Moreover, substitution products also can arise from stepwise Cl2 addition/HCl elimination routes.