Interconversion of Gas-Phase Arenium Ions and π-Complexes

Arenium ions or σ-complexes are known to be produced as intermediates in electrophilic aromatic substitution (EAS), but the existence of π-complexes is less certain. Evidence is presented in this paper for the coexistence of σ- and π-complexes in the protonation of arenes in the gas phase. Protonation was accomplished in a high-pressure chemical ionization source of a tandem mass spectrometer to give the ethylbenzenium ion, the isomeric propyl- and butylbenzenium ions and five isomeric pentylbenzenium ions. It was established that identical ions are produced by protonation as are by alkylation with stable carbenium ions (i.e., by EAS). The existence of the π-complex is consistent with the observation that the alkyl moieties of secondary butyl- and pentylbenzenium ions isomerize prior to decomposition. The isomerization is directly detectable if it is nearly thermoneutral. Other more exothermic isomerizations (e.g., 1° alkyl —>2° alkyl —> 3° alkyl) are only inferred because the energy released causes the π-complex to decompose to benzene and a carbenium ion. The evidence for isomerization was obtained by using collisionally activated dissociation (CAD) and tandem mass spectrometry (MS/MS).