Ionic nucleophilic substitution in halobenzene-NH3 complexes: the entrance channel

Nucleophilic substitution reactions have been studied in halobenzene cations with ammonia. In ionic 1-1 complexes, three behaviours can be observed, depending on the nature of the halogen atom X (F, Cl, Br or I): (i) no reactivity, (ii) X elimination: XBz+NH3 → BzNH3+ + X, (iii) HX elimination: XBz+NH3 → BzNH2+ + HX. We propose a reactive potential energy surface in which the two eliminations proceed through the same entrance channel: the formation of an addition σ complex after a barrier crossing. This should be the key point for interpreting the ensemble of previous results. The reaction time for this barrier crossing has been estimated using the RRK formalism, giving a set of consistent barriers for σ complex formation in each halogenated compound studied from a comparison of modeling and experimental results.