Substrate and Positional Selectivity of Free Phenylium Ions Toward Arenes In the Gas and Liquid-phases

A nuclear technique, based on the spontaneous decay of tritiated precursors that allows the generation of free carbenium ion of exactly the same nature in different environments, has been exploited in a comparative study of arom. phenylation by free phenylium ions, both in the gas phase at various pressures and in the liq. phase. The differences between the reactivity pattern of the phenylium ion in the two environments can essentially be reduced to significant ion-neutral electrostatic interaction in the gas phase and, to the much greater efficiency of collisional stabilization, in the condensed phase, allowing a larger fraction of the excited ionic intermediates, from the highly exothermic attack of phenylium ion on the competing arenes, to survive dissocn. and isomerization. The mechanism of the major competitive processes promoted by phenylium-ion attack on arenes, i.e. phenyldehydrogenation and phenyldephenylation, are discussed, together with the substrate and positional selectivity displayed by the nuclear-decay-formed phenylium ion toward the selected arenes. The kinetic and mechanistic features of arom. phenylation, deduced from the present decay expts., are compared with those of related phenylation reactions carried out in the dil. gas state and in soln.