nitroniumionit

The nitronium ion, with the chemical formula NO2+, is a diatomic cation composed of one nitrogen atom and one oxygen atom. It is a linear molecule and is isoelectronic with carbon dioxide. The nitronium ion is a highly reactive species and is primarily known as an electrophile in organic chemistry, particularly in electrophilic aromatic substitution reactions.

In organic synthesis, the nitronium ion is the active species responsible for nitration reactions, where a nitro group (NO2) is introduced onto an organic molecule. It is typically generated in situ by the reaction of nitric acid with a strong dehydrating agent, most commonly concentrated sulfuric acid. The sulfuric acid protonates the nitric acid, which then loses water to form the nitronium ion. The overall reaction is often referred to as "mixed acid nitration."

The nitronium ion's strong electrophilic character allows it to attack electron-rich species like aromatic rings. Once the nitronium ion attacks the aromatic ring, a sigma complex is formed, which then loses a proton to restore aromaticity and yield the nitrated product. The stability of the nitronium ion is relatively low, and it quickly reacts once formed. Its presence is transient, existing only for the duration of the nitration process. Beyond organic chemistry, the nitronium ion is also a component of some energetic materials and has been studied in atmospheric chemistry.