Isopropylation of halobenzenes by sec-C3H7+ cations. Evidence for a participation of the substituent in a gas-phase alkylation

lsopropylation of the halobenzenes (PhF, PhCI, and PhBr) by isopropyl cations, generated in the diluted gaseous phase from the radiolysis of propane, leads to formation of the corresponding halocumenes. The regioselectivity of the substitution changes from apparent kinetic control, i.e., ortho >SO% at 760 Torr, to thermodynamic control Le., meta, as the overall pressure is lowered to 20 Torr. The results of competitive alkylation experiments with toluene show the reactivity trend PhF > PhCl N PhBr. Appreciable yields of anisole, increasing in the order PhF > PhCl N PhBr, are measured when the system is radiolyzed in the presence of CH30H. The formation of PhOCH3 is traced to the attack of methanol on the aromatic ring, activated toward nucleophilic displacement by a specific interaction of sec-C3H7+ ions with the halogenated substrates. The same interaction helps also to explain the exceptionally high reactivity of the ring positions ortho to a n-donor substituent.