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Carbocation is a general term forions with a positively chargedcarbonatom. 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 thecoordination number of the charged carbon: three in thecarbenium ions and five in thecarbonium ions. Among the simplest carbocations are themetheniumCH+
3 (a carbenium ion),methaniumCH+
5 (a carbonium ion),acylium ionsRCO+, andvinylC
2H+
3 cations.[2]
Until the early 1970s, carbocations were calledcarbonium ions.[3] This nomenclature was proposed byG. A. Olah.[4] Carbonium ions, as originally defined by Olah, are characterized by athree-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 the2-norbornyl cation.[5]
According to theIUPAC, acarbocation is any cation containing an even number of electrons in which a significant portion of the positive charge resides on a carbon atom.[6] Prior to the observation of five-coordinate carbocations by Olah and coworkers,carbocation andcarbonium ion were used interchangeably. Olah proposed a redefinition ofcarbonium ion as a carbocation featuring any type of three-center two-electron bonding, while acarbenium 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 termcarbonium 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 grouponium species, while a carbocation that contains any type of three-centered bonding is referred to as anon-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 termcarbonium ion will be used in this latter restricted sense, whilenon-classical carbocation will be used to refer to any carbocation with C–C and/or C–H σ-bonds delocalized by bridging.
Carbonium ions can be thought of as protonated or alkylated alkanes, bearing the general formula CR5+ (R = alkyl or H). A typical example is themethanium ion, CH5+, which is formed by protonation of methane using asuperacid. By necessity of having five bonds on carbon but only four valence electron pairs available for bonding, they feature delocalized 3c-2e σ bonding and are thus regarded as type of non-classical carbocation. Like carbenium ions, carbonium ions are often invoked as intermediates in the upgrading of hydrocarbons in refineries. They are generally fleeting intermediates withfluxional structures that are challenging to observe and interpret spectroscopically. They can undergo decomposition by expulsion of a proton or alkyl group, or by loss of H2 to give a carbenium ion.
At least in a formal sense,carbenium ions (CR3+) are derived from the protonation (addition ofH+) or alkylation (addition ofR+) of acarbene oralkene. They admit aresonance depiction in which one carbon atom bears a formal positive charge and is surrounded by sixvalence electrons instead of theusual octet. Therefore, carbenium ions (and carbocations in general) are often reactive, seeking to fill the valence octet and regain a neutralcharge.
In accord withVSEPR andBent's rule, unless geometrically constrained to be pyramidal (e.g., 1-adamantyl cation), 3-coordinate carbon in carbenium ions are usually trigonal planar andsp2 hybridized; thelowest unoccupied molecular orbital is an empty purep orbital pointing out-of-plane. A prototypical example is thet-butyl cation,CMe+3. Although classical carbenium ions have a structure that corresponds to a non-bridging Lewis structure, it is important to note that donation of electron density from neighboring C–H or C–C bonds into the "empty" p orbital, known ashyperconjugation, is still an important stabilizing factor, and these bonds have a tendency to "lean" towards the carbocationic center to improve orbital overlap.
There is, in fact, an entire spectrum of bonding scenarios between a slight lean due to hyperconjugation to a fully symmetric bridging structure featuring 3c2e bonding. Consequently, there is no firm dividing line between "classical" and the so-called "non-classical" structures.
Non-classical carbenium ions feature also σ delocalization (3c2e bonds) in their bonding but have the general formula CR3+ (R = alkyl or H). Thus, in principle, one can propose non-bridged, classical structures for these cations, as well as a bridged non-classical structure. Because of the subtle differences in the expected behavior of a non-classical carbenium ions compared to the alternative hypothesis of two rapidly equilibrating classical structures, a lively and often acrimonious debate took place over several decades regarding the merits of each model. For a detailed history of this dispute, see the article on the2-norbornyl cation. Currently, there is overwhelming evidence that, at least in some cases (notably the extremely well-studied 2-norbornyl cation), the equilibrium structure of a carbenium ion is non-classical, although even minor changes in structure could result in a classical structure being favored.
The history of carbocations dates back to 1891 when G. Merling[8] reported that he added bromine to tropylidene (cycloheptatriene) and then heated the product to obtain a crystalline, water-soluble material,C
7H
7Br. He did not suggest a structure for it; however,Doering and Knox[9] convincingly showed that it wastropylium (cycloheptatrienylium) bromide. This ion is predicted to bearomatic byHückel's rule.
In 1902, Norris and Kehrman independently discovered that colorlesstriphenylmethanol gives deep-yellow solutions in concentratedsulfuric acid.Triphenylmethyl chloride similarly formed orange complexes when treated 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 formationhalochromy, of whichmalachite green is a prime example. Thetrityl carbocation (shown below) is indeed a stable carbocationic system, for example in the form oftrityl hexafluorophosphate.[10]
Carbocations arereactive intermediates in many organic reactions. This idea, first proposed byJulius Stieglitz in 1899,[11] was further developed byHans Meerwein in his 1922 study[12][13] of theWagner–Meerwein rearrangement. Carbocations were also found to be involved in theSN1 reaction, theE1 reaction, and inrearrangement reactions such as theWhitmore 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.
AnNMR spectrum of a carbocation was first reported by Doering et al.[14] in 1958. It was the heptamethylbenzenium ion, made by treatinghexamethylbenzene withmethyl chloride andaluminium chloride. The stable 7-norbornadienyl cation was prepared by Story et al. in 1960[15] by reactingnorbornadienyl chloride withsilver tetrafluoroborate insulfur dioxide at −80 °C. The NMR spectrum established that it was non-classically bridged (the first stablenon-classical ion observed).
In 1962,Olah directly observed thetert-butyl carbocation bynuclear magnetic resonance as a stable species on dissolvingtert-butyl fluoride inmagic acid. The NMR spectrum of the norbornyl cation was reported by Schleyer et al.[16] It was shown to rapidly undergo proton-scrambling.[17]
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