Carbocation

A carbocation (/ˌkɑːrboʊˈkætaɪən/) is an ion with a positively charged carbon atom. Among the simplest examples are the methenium CH+3, methanium CH+5 and vinyl C2H+3 cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountered (e.g., ethylene dication C2H2+4).The crystal structure of 2•HSO3F confirms the pentagonal-pyramidal shape of the hexamethylbenzene dication. A carbocation (/ˌkɑːrboʊˈkætaɪən/) is an ion with a positively charged carbon atom. Among the simplest examples are the methenium CH+3, methanium CH+5 and vinyl C2H+3 cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountered (e.g., ethylene dication C2H2+4). Until the early 1970s, all carbocations were called carbonium ions. In present-day chemistry, a carbocation is any ion with a positively charged carbon atom, 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 'nonclassical 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 (i.e., CR5+, where R is hydrogen or alkyl), to the exclusion of nonclassical 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 nonclassical 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 nonclassical 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 nonclassical 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 CH+5. A carbocation with an two-coordinate sp-hybridized positive carbon is known as a vinyl cation, while 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 very 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, C7H7Br. 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. 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.