The role of the halogen in the gas phase ionic halomethylation of benzene

The gas phase electrophilic attack of radiolytically formed CH2X+ (X = F, Cl, Br) ions on C6H6 in homogeneous systems contg. a large excess of CH3X and low concns. of C6H6 in the presence of a thermal radical scavenger (O2), and of an added base, when required, gives forth methylation and halomethylation of the substrate. The stability of the arenium ions originating from the latter process is affected by the nature of the halogen atom involved. A fast unimol. dissocn. is obsd. when the electrophile is CH2F+; the adducts from the attack of CH2Cl+ or CH2Br+ on C6H6 are remarkably stable. The fluorinated intermediate has a strong H bond-like interaction, which promotes its fast dissocn.