Carbocation

*tert*-Butyl cation, demonstrating planar geometry and sp² hybridization

A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium CH⁺
₃, methanium CH⁺
₅ and vinyl C
₂H⁺
₃ cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountered (e.g., ethylene dication C
₂H²⁺
₄).^[1]

Until the early 1970s, all carbocations were called carbonium ions.^[2] In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further classified in two main categories according to the coordination number of the charged carbon: three in the carbenium ions and five in the carbonium ions. This nomenclature was proposed by G. A. Olah.^[3] Carbonium ions, as originally defined by Olah, are characterized by a three-center two-electron delocalized bonding scheme and are essentially synonymous with so-called 'non-classical carbocations', which are carbocations that contain bridging C–C or C–H σ-bonds. However, others have more narrowly defined the term 'carbonium ion' as formally protonated or alkylated alkanes (CR⁺
₅, where R is H or alkyl), to the exclusion of non-classical carbocations like the 2-norbornyl cation.^[4]

^ Grützmacher, Hansjörg; Marchand, Christina M. (1997). "Heteroatom stabilized carbenium ions". Coord. Chem. Rev. 163: 287–344. doi:10.1016/S0010-8545(97)00043-X.
^ Robert B. Grossman (2007-07-31). The Art of Writing Reasonable Organic Reaction Mechanisms. Springer Science & Business Media. pp. 105. ISBN 978-0-387-95468-4.
^ Olah, George A. (1972). "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". Journal of the American Chemical Society. 94 (3): 808–820. doi:10.1021/ja00758a020.
^ Sommer, J.; Jost, R. (2000-01-01). "Carbenium and carbonium ions in liquid- and solid-superacid-catalyzed activation of small alkanes". Pure and Applied Chemistry. 72 (12): 2309–2318. doi:10.1351/pac200072122309. ISSN 1365-3075.

[Gruetz-1] Grützmacher, Hansjörg; Marchand, Christina M. (1997). "Heteroatom stabilized carbenium ions". Coord. Chem. Rev. 163: 287–344. doi:10.1016/S0010-8545(97)00043-X.

[2] Robert B. Grossman (2007-07-31). The Art of Writing Reasonable Organic Reaction Mechanisms. Springer Science & Business Media. pp. 105. ISBN 978-0-387-95468-4.

[OlahCXVIII-3] Olah, George A. (1972). "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". Journal of the American Chemical Society. 94 (3): 808–820. doi:10.1021/ja00758a020.

[4] Sommer, J.; Jost, R. (2000-01-01). "Carbenium and carbonium ions in liquid- and solid-superacid-catalyzed activation of small alkanes". Pure and Applied Chemistry. 72 (12): 2309–2318. doi:10.1351/pac200072122309. ISSN 1365-3075.

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