Carbocation
Carbocation is a general term for ions with a positively charged carbon atom. 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. Among the simplest carbocations are the methenium CH+
3 (a carbenium ion), methanium CH+
5 (a carbonium ion), acylium ions RCO+, and vinyl C
2H+
3 cations.
Until the early 1970s, carbocations were called carbonium ions. This nomenclature was proposed by G. A. Olah. 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.