What is the formula of the nitronium ion?

The nitronium ion (NO₂⁺) is one of the most important electrophiles in organic chemistry, particularly in electrophilic aromatic substitution (EAS) reactions such as the nitration of benzene. It carries a positive charge and is represented by the chemical formula NO₂⁺.

The nitronium ion is formed when a mixture of concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄) reacts. In this mixture, sulfuric acid acts as a stronger acid and protonates nitric acid to form water and the nitronium ion:

HNO3+2H2SO4→NO2++H3O++2HSO4−HNO₃ + 2H₂SO₄ \rightarrow NO₂⁺ + H₃O⁺ + 2HSO₄⁻HNO3+2H2SO4→NO2++H3O++2HSO4−

This reaction shows that the nitronium ion is generated in situ (within the reaction mixture) and is not stable in isolation. The ion has a linear structure, with nitrogen double-bonded to both oxygen atoms. Its molecular geometry can be represented as O=N=O, and due to the delocalization of electrons, it exhibits strong electrophilic character.

In the nitration of benzene, the nitronium ion acts as the attacking species that reacts with the π-electron cloud of benzene to form nitrobenzene (C₆H₅NO₂). This reaction is one of the most fundamental examples of electrophilic aromatic substitution, a key mechanism in organic synthesis.

The stability and reactivity of the nitronium ion make it useful for introducing a nitro group (–NO₂) into various aromatic and aliphatic compounds. Nitro groups are essential intermediates in the synthesis of aniline derivatives, dyes, explosives (like TNT), and pharmaceuticals.

Therefore, the correct formula of the nitronium ion is NO₂⁺, representing a highly reactive electrophilic species that plays a central role in aromatic nitration reactions.