Arenium ion



An arenium ion in organic chemistry is the cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution. For historic reasons this complex is also called a Wheland intermediate or a sigma complex or σ-complex.


 * [[Image:AreniumIon.png|400px|benzenium ion]]

In the ion the positive charge is delocalized over 5 carbon atoms as a result of resonance:
 * [[Image:AreniumIonResonance.png|500px|Arenium Ion Resonance structures]]

The smallest arenium ion is protonated benzene C6H7+. The benzenium ion can be isolated as a stable compound when benzene is protonated by the carborane superacid H(CB11H(CH3)5Br6). The benzenium salt is crystalline with thermal stability up to 150°C. Bond lengths deduced from X-ray crystallography are consistent with a cyclohexadienyl cation structure.

More methods exist for the generation of stable arenium ions. In one study a methylene arenium ion is stabilized by metal complexation:


 * [[Image:Methyleneareniumion.png|400px|Methylene arenium ion]]