nitroniumione

Nitronium ion, or nitronium cation, with the formula NO2+, is a reactive electrophile central to many nitration reactions in organic chemistry. It is formed in strongly acidic media and acts to introduce a nitro group (−NO2) into aromatic rings and other substrates.

Structurally, the nitronium ion is typically described as a linear O=N=O+ unit, with the positive charge delocalized over the two oxygens. It is isoelectronic with carbon dioxide and is best viewed as a highly electrophilic species that seeks electron density from substrates such as arenes. In solution, NO2+ is short-lived unless stabilized by counterions in salts or by nonaqueous media.

Generation and sources: In typical nitration processes, nitronium ion is generated in situ by the reaction of concentrated sulfuric acid with nitric acid: H2SO4 + HNO3 ⇌ NO2+ + HSO4− + H2O. In nonaqueous systems, nitronium ions are supplied by nitronium salts such as nitronium tetrafluoroborate (NO2BF4) or nitronium hexafluorophosphate (NO2PF6), which provide NO2+ without the need for a strongly dehydrating acid mixture.

Reactivity: The nitronium ion is the key electrophile in electrophilic aromatic substitution. It reacts with arenes to form nitroarene intermediates via an arenium ion (sigma complex) and subsequent deprotonation, restoring aromaticity. The nitro group can then be further transformed, for example reduced to an amine, or used as a synthetic handle in the preparation of dyes, pharmaceuticals, and high-energy materials.

Safety and scope: NO2+ is a nonisolable species in most contexts and is produced under harsh, corrosive conditions. Materials used to generate or stabilize it require careful handling in fume hoods with appropriate protective equipment.