Aromatic substitution in the gas phase. Mechanism of the dehalogenation reactions of halobenzenes and dihalobenzenes promoted by gaseous Broensted acids

The attack of CH5+ and C2H5+ ions, obtained in the dil. gas state from the γ radiolysis of CH4, on halo- and dihalobenzenes causes extensive dehalogenation via 2 distinct channels, leading resp. to protodehalogenated and methyldehalogenated products, whose relative rate depends primarily on the nature of the leaving halogen. Kinetic and mass spectrometric evidence suggest that direct attack of the Broensted acid to the halogen substituent leads to formation of the correspondent arylium cation, via hydrogen halide elimination, at a rate which decreases in the order F » Cl ≥ Br. The subsequent electrophilic attack of the arylium cations on CH4 yields methylated arenium ions, and eventually the obsd. methyldehalogenated products. On the other hand, attack of the Broensted acid to the arom. ring of halobenzenes promotes protodehalogenation, at a rate increasing in the order F « Cl