Arenium ion

An arenium ion in organic chemistry is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution.

Two hydrogen atoms bonded to one carbon lie in a plane perpendicular to the benzene ring.

[4] The arenium ion is no longer an aromatic species; however it is relatively stable due to delocalization: the positive charge is delocalized over 3 carbon atoms by the pi system, as depicted on the following resonance structures: A complexed electrophile can contribute to the stability of arenium ions.

Bond lengths deduced from X-ray crystallography are consistent with a cyclohexadienyl cation structure.

Electrophilic attack of methyl triflate forms methylene arenium ion 3 with (based on X-ray crystallography) positive charge located in aromatic para position and with the methylene group 6° out of the plane of the ring.