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Ion with a positively charged carbon atom

A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium CH3+, methanium CH5+ and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountered (e. g., ethylene dication ). Until the early 1970s, all carbocations were called carbonium ions. 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. 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 (CR5+, where R is H or alkyl), to the exclusion of non classical carbocations like the 2 norbornyl cation.

According to the IUPAC, a carbocation is any cation containing an even number of electrons in which a significant portion of the positive charge resides on a carbon atom. Prior to the observation of five coordinate carbocations by Olah and coworkers, carbocation and carbonium ion were used interchangeably. Olah proposed a redefinition of carbonium ion as a carbocation featuring any type of three center two electron bonding, while a carbenium ion was newly coined to refer to a carbocation containing only two center two electron bonds with a three coordinate positive carbon. Subsequently, others have used the term carbonium ion more narrowly to refer to species that are derived (at least formally) from electrophilic attack of H+ or R+ on an alkane, in analogy to other main group onium species, while a carbocation that contains any type of three centered bonding is referred to as a non classical carbocation. In this usage, 2 norbornyl cation is not a carbonium ion, because it is formally derived from protonation of an alkene (norbornene) rather than an alkane, although it is a non classical carbocation due to its bridged structure. The IUPAC acknowledges the three divergent definitions of carbonium ion and urges care in the usage of this term. For the remainder of this article, the term carbonium ion will be used in this latter restricted sense, while non classical carbocation will be used to refer to any carbocation with C–C and/or C–H σ bonds delocalized by bridging. Since the late 1990s, most textbooks have stopped using the term carbonium ion for the classical three coordinate carbocation. However, some university level textbooks continue to use the term carbocation as if it were synonymous with carbenium ion, or discuss carbocations with only a fleeting reference to the older terminology of carbonium ions or carbenium and carbonium ions. One textbook retains the older name of carbonium ion for carbenium ion to this day, and uses the phrase hypervalent carbonium ion for CH5+. A carbocation with a two coordinate positive carbon derived from formal removal of a hydride ion (H−) from an alkene is known as a vinyl cation. In the absence of geometric constraints, most substituted vinyl cations carry the formal positive charge on an sp hydridized carbon atom of linear geometry. A two coordinate approximately sp2 hybridized cation resulting from the formal removal of a hydride ion from an arene is termed an aryl cation. These carbocations are relatively unstable (aryl cations especially so) and are infrequently encountered. Hence, they are frequently omitted from introductory and intermediate level textbooks. The IUPAC definition stipulates that carbocations are even electron species; hence, radical cations like CH4•+ that are frequently encountered in mass spectrometry are not considered to be carbocations.

The history of carbocations dates back to 1891 when G. Merling reported that he added bromine to tropylidene (cycloheptatriene) and then heated the product to obtain a crystalline, water soluble material, He did not suggest a structure for it; however, Doering and Knox convincingly showed that it was tropylium (cycloheptatrienylium) bromide. This ion is predicted to be aromatic by Hückel's rule. In 1902, Norris and Kehrman independently discovered that colorless triphenylmethanol gives deep yellow solutions in concentrated sulfuric acid. Triphenylmethyl chloride similarly formed orange complexes with aluminium and tin chlorides. In 1902, Adolf von Baeyer recognized the salt like character of the compounds formed. The trityl carbocation (shown below) is a stable carbocationic system that has been used as homogeneous organocatalyst in organic synthesis, for example in the form of trityl hexafluorophosphate. He dubbed the relationship between color and salt formation halochromy, of which malachite green is a prime example. Carbocations are reactive intermediates in many organic reactions. This idea, first proposed by Julius Stieglitz in 1899, was further developed by Hans Meerwein in his 1922 study of the Wagner–Meerwein rearrangement. Carbocations were also found to be involved in the SN1 reaction, the E1 reaction, and in rearrangement reactions such as the Whitmore 1,2 shift. The chemical establishment was reluctant to accept the notion of a carbocation and for a long time the Journal of the American Chemical Society refused articles that mentioned them. The first NMR spectrum of a stable carbocation in solution was published by Doering et al. in 1958. It was the heptamethylbenzenium ion, made by treating hexamethylbenzene with methyl chloride and aluminium chloride. The stable 7 norbornadienyl cation was prepared by Story et al. in 1960 by reacting norbornadienyl chloride with silver tetrafluoroborate in sulfur dioxide at −80 °C. The NMR spectrum established that it was non classically bridged (the first stable non classical ion observed). In 1962, Olah directly observed the tert butyl carbocation by nuclear magnetic resonance as a stable species on dissolving tert butyl fluoride in magic acid. The NMR of the norbornyl cation was first reported by Schleyer et al. and it was shown to undergo proton scrambling over a barrier by Saunders et al.

Some carbocations such as the 2 norbornyl cation exhibit more or less symmetrical three center two electron bonding. Such structures, referred to as non classical carbocations, involve the delocalization of the bonds involved in the σ framework of the molecule, resulting in C–C and C–H bonds of fractional bond order. This delocalization results in additional stabilization of the cation. For instance, depicted as a classical carbenium ion, 2 norbornyl cation appears to be a secondary carbocation. However, it is more stable than a typical "secondary" carbocation, being roughly as stable as a tertiary carbocation like t butyl cation, according to hydride ion affinity. The existence of non classical carbocations was once the subject of great controversy. On opposing sides were Herbert C. Brown, who believed that what appeared to be a non classical carbocation represents the average of two rapidly equilibrating classical species (or possibly two structures exhibiting some degree of bridging or leaning but is nevertheless not symmetric) and that the true non classical structure is a transition state between the two potential energy minima, and Saul Winstein, who believed that a non classical structure that possessed a plane of symmetry was the sole potential energy minimum and that the classical structures merely two contributing resonance forms of this non classical species. George Olah's discovery of superacidic media to allow carbocations to be directly observed, together with a very sensitive NMR technique developed by Martin Saunders to distinguish between the two scenarios, played important roles in resolving this controversy. At least for the 2 norbornyl cation itself, the controversy has been settled overwhelmingly in Winstein's favor, with no sign of the putative interconverting classical species, even at temperatures as low as 6 K, and a 2013 crystal structure showing a distinctly non classical structure. A variety of carbocations (e. g., ethyl cation, see above) are now believed to adopt non classical structures. However, in many cases, the energy difference between the two possible "classical" structures and the "non classical" one is very small, and it may be difficult to distinguish between the two possibilities experimentally.

Pyramidal Carbocations Besides the classical and non classical carbocations, a third class can be distinguished: pyramidal carbocations. In these ions, a single carbon atom hovers over a four or five sided polygon, in effect forming a pyramid. The square pyramidal ion will carry a charge of +1, the Pentagonal pyramidal ion will carry +2. A stable hexagonal pyramidal configuration of tropylium trication, (C7H7)3+, has been also predicted. In this case, the coordination number of carbon reaches seven. The crystal structure of [C6(CH3)6][SbF6]2·HSO3F confirms the pentagonal pyramidal shape of the hexamethylbenzene dication. An example of the monovalent carbocation An example of the divalent carbocation