Role of the spectator ring in the gas-phase alkylation of 1,2-diphenylethane by (CH3)2X+ (X = F, Cl) and CF3+ ions.

The gas-phase alkylation of C6D5CH2CH2C6H5 by Me2F+, Me2Cl+, and CF3+ ions has been studied at 720 Torr, in the temperature range 47-120-degrees-C with the radiolytic technique. An interannular hydron-exchange process between the two aromatic rings of (beta-phenylethyl)arenium ions has been observed to occur on the nanosecond time scale and to obey thermal kinetics, with the normal positive dependence on temperature. When the efficiency of collisional deactivation is decreased (at 200 torr) or a highly exothermic reaction (alkylation by CF3+ ion) is carried out, an ''anomalously'' large extent of interannular hydron exchange takes place. The higher methylation rate (6.0:1) of 1,2-diphenylethane than of toluene and the marked positional selectivity (2 para/ortho ratio of 5.0) shown by Me2X+ ions have pointed out the fundamental role of persistent ion-neutral complexes between reactant pairs, as already suggested in previous radiolytic and mass spectrometric studies.