Gas and Liquid Phase Reactions of Free Phenylium Cation with Methanol

A nuclear technique based on the spontaneous decay of multitritiated precursors that allows the generation of free carbonium ions of exactly the same nature in different environments was used for generating [p3H]phenylium ion (I) from [1,4-3H2]benzene and for investigating its reactivity and selectivity toward methanol both in the gas phase at various pressures (5-65 torr) and in the liq. phase. The differences between the reactivity pattern of phenylium cation under different exptl. conditions can be regarded as due to several factors, including the different efficiency of collisional stabilization of the excited reaction intermediates in the two media and the competition between the phenylium ion attack on the MeOH and the relaxation period of the MeOH dipole orientation, in both the gas and liq. phase at room temp. The mechanism of the major competitive processes involving I in the presence of methanol, i.e., MeOH phenylation vs. phenylium ion automerization, is discussed in light of theor. predictions and compared with related mass spectrometric and condensed-phase investigation. The results are in favor of the existence of a nucleogenic phenylium ion with a singlet-state electronic configuration, which tends to internally solvate its pos. charge by a relatively fast (k = 107 - 108 s-1) automerization process involving consecutive 1,2 H shifts.