ISOPROPYLATION OF HALOBENZENES BY SEC-C3H7+ CATIONS - EVIDENCE FOR A PARTICIPATION OF THE SUBSTITUENT IN A GAS-PHASE ALKYLATION

Isopropylation of the halobenzenes (PhF, PhCl, 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 >80% at 760 Torr, to thermodynamic control i.e., meta, as the overall pressure is lowered to 20 Torr. The results of competitive alkylation experiments with toluene show the reactivity trend PhF > PhCl PhBr. Appreciable yields of anisole, increasing in the order PhF > PhCl PhBr, are measured when the system is radiolyzed in the presence of CH3OH. 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. © 1979, American Chemical Society. All rights reserved.