Electrophilic Aromatic Nitration:  Understanding Its Mechanism and Substituent Effects

Theoretical calculations and gas-phase mass spectrometric studies were performed for the reaction of the naked (NO2 +) and monosolvated (CH3NO2·NO2 +) nitronium ion with several monosubstituted aromatic compounds. From these studies, we propose a general model for regioselectivity based on the single-electron transfer (SET) mechanism and an alternative mechanistic scheme for electrophilic aromatic nitration. This scheme considers the SET and the polar (Ingold−Hughes) mechanisms as extremes in a continuum pathway, the occurrence and extents of both mechanisms being governed mainly by the ability, or lack of ability, of the aromatic compound to transfer an electron to NO2 +.