An arenium ion 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.

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.

See also

References

1. 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 reactions George A. Olah; J. Am. Chem. Soc.; 1972; 94(3); 808-820. Abstract
2. A Quantum Mechanical Investigation of the Orientation of Substituents in Aromatic Molecules G. W. Wheland; J. Am. Chem. Soc.; 1942; 64(4); 900-908. Abstract
3. Isolating Benzenium Ion Salts Christopher A. Reed, Kee-Chan Kim, Evgenii S. Stoyanov, Daniel Stasko, Fook S. Tham, Leonard J. Mueller, and Peter D. W. Boyd J. Am. Chem. Soc.; 2003; 125(7) pp 1796 - 1804; Abstract

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