An arenium ion in organic chemistry is a 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. The smallest arenium ion is the benzenium ion (C6H7+), which is protonated benzene.
Two hydrogen atoms bonded to one carbon lie in a plane perpendicular to the benzene ring. 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 via the pi system, as depicted on the following resonance structures:
A complexed electrophile can contribute to the stability of arenium
A 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
In this reaction sequence the R-Pd(II)-Br starting complex 1
List of publications in chemistry: Jerry March Advanced Organic Chemistry Aryl radical
^ Stable carbocations. CXVIII. General concept and structure of
carbocations based on differentiation of trivalent (classical)
carbenium ions from three-center bound penta- of tetracoordinated
(nonclassical) carbonium ions. Role of carbocations in electrophilic
George A. Olah