Editing Carbocation

{{Short description|Ion with a positively charged carbon atom}}
[[File:Tert-butyl cation resonance (cropped).svg|thumb|120px|The ''tert''-butyl cation is a relatively stable carbenium ion.<ref>{{cite journal |doi=10.1002/chem.201203260 |title=Superacidic or Not…︁? Synthesis, Characterisation, and Acidity of the Room-Temperature Ionic Liquid &#91;C(CH<sub>3</sub>)<sub>3</sub>&#93;<sup>+</sup> &#91;Al<sub>2</sub>Br<sub>7</sub>&#93;<sup>−</sup> |date=2013 |last1=Scholz |first1=Franziska |last2=Himmel |first2=Daniel |last3=Scherer |first3=Harald |last4=Krossing |first4=Ingo |journal=Chemistry – A European Journal |volume=19 |issue=1 |pages=109–116 |pmid=23180742 }}</ref>]]

'''Carbocation''' is a general term for [[ion]]s 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 ion]]s and five in the [[carbonium ion]]s. Among the simplest carbocations are the [[methenium]] {{chem|CH|3|+}} (a carbenium ion),  [[methanium]] {{chem|CH|5|+}}  (a carbonium ion), [[acylium ions]] {{chem2|RCO+}}, and [[Vinyl cation|vinyl]] {{chem|C|2|H|3|+}} cations.<ref>{{March6th|page=235}} </ref>

Until the early 1970s, carbocations were called ''carbonium ions''.<ref>{{cite book|author=Robert B. Grossman|title=The Art of Writing Reasonable Organic Reaction Mechanisms|url=https://archive.org/details/artofwritingreas00gros|url-access=registration|date=2007-07-31|publisher=Springer Science & Business Media|isbn=978-0-387-95468-4|pages=[https://archive.org/details/artofwritingreas00gros/page/105 105]}}</ref> This nomenclature was proposed by [[George Andrew Olah|G. A. Olah]].<ref name=OlahCXVIII>{{cite journal|first= George A.|last= Olah |date=1972|title=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=Journal of the American Chemical Society|volume=94|issue=3|pages=808–820 |doi=10.1021/ja00758a020}}</ref> Carbonium ions, as originally defined by Olah, are characterized by a [[Three-center two-electron bond|three-center two-electron]] delocalized bonding scheme and are essentially synonymous with so-called '[[non-classical carbocation]]s', 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 ({{chem|CR|5|+}}, where R is H or alkyl), to the exclusion of non-classical carbocations like the [[ Norbornyl cation |2-norbornyl cation]].<ref>{{Cite journal|last1=Sommer|first1=J.|last2=Jost|first2=R.|date=2000-01-01|title=Carbenium and carbonium ions in liquid- and solid-superacid-catalyzed activation of small alkanes|journal=Pure and Applied Chemistry|language=en|volume=72|issue=12|pages=2309–2318|doi=10.1351/pac200072122309|issn=1365-3075|doi-access=free}}</ref>

==Definitions==
According to the [[International Union of Pure and Applied Chemistry|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.<ref>{{Citation|publisher=International Union of Applied Chemistry|chapter-url=https://goldbook.iupac.org/html/C/C00817.html|language=en|doi=10.1351/goldbook.C00817|isbn=978-0967855097|access-date=2018-11-03|title=IUPAC Compendium of Chemical Terminology|year=2009|chapter=Carbocation|doi-access=free}}</ref>  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<sup>+</sup> or R<sup>+</sup> on an alkane, in analogy to other main group [[Onium compound|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.

<!--A carbocation with a two-coordinate positive carbon derived from formal removal of a hydride ion (H<sup>−</sup>) 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 sp<sup>2</sup>-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 {{chem|CH|4|•+}} that are frequently encountered in mass spectrometry are not considered to be carbocations.-->

==Structure and properties==
===Carbonium ions===
[[File:CSD CIF HIGNAOhires.png|thumb|Structure of the 2-norbornyl non-classical carbenium ion.  All other C-C bond lengths are normal (ca. 1.5 Å).<ref name=":0">{{Cite journal|last1=Scholz|first1=F.|last2=Himmel|first2=D.|last3=Heinemann|first3=F. W.|last4=Schleyer|first4=P. v R.|last5=Meyer|first5=K.|last6=Krossing|first6=I.|date=2013-07-05|title=Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation|journal=Science|language=en|volume=341|issue=6141|pages=62–64|doi=10.1126/science.1238849|issn=0036-8075|pmid=23828938|bibcode=2013Sci...341...62S|s2cid=206549219 }}</ref> ]]

Carbonium ions can be thought of as protonated or alkylated alkanes, bearing the general formula CR<sub>5</sub><sup>+</sup> (R = alkyl or H).  A typical example is the [[Methanium|methanium ion]], CH<sub>5</sub><sup>+</sup>, which is formed by protonation of methane using a [[superacid]].  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 with [[Fluxional molecule|fluxional 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 H<sub>2</sub> to give a carbenium ion.

===Carbenium ions===
{{Main|Carbenium ion}}
At least in a formal sense, carbenium ions (CR<sub>3</sub><sup>+</sup>) are derived from the protonation (addition of {{chem2|H+}}) or alkylation (addition of {{chem2|R+}}) of a [[carbene]] or [[alkene]].  They admit a [[Resonance (chemistry)|resonance]] depiction in which one carbon atom bears a formal positive charge and is surrounded by six [[valence electron]]s instead of the [[octet rule|usual octet]]. Therefore, carbenium ions (and carbocations in general) are often reactive, seeking to fill the valence octet and regain a neutral [[electric charge|charge]]. 

In accord with [[VSEPR theory|VSEPR]] and [[Bent's rule]], unless geometrically constrained to be pyramidal (e.g., 1-adamantyl cation), 3-coordinate carbon in carbenium ions are usually trigonal planar and [[orbital hybridization|{{math|''sp''<sup>2</sup>}} hybridized]]; the [[LUMO|lowest unoccupied molecular orbital]] is an empty pure ''p'' orbital pointing out-of-plane.  A prototypical example is the ''t-''butyl cation, {{chem2|CMe3+}}.  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 as [[hyperconjugation]], 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. <!--For the same reasons, carbocations that are 2-coordinate (vinyl cations) are generally linear in geometry, with CH/CC bonds formed from C(sp) orbitals.-->

==== Non-classical carbenium ions ====
Non-classical carbenium ions feature also σ delocalization (3c2e bonds) in their bonding but have the general formula CR<sub>3</sub><sup>+</sup> (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 the [[2-Norbornyl cation|2-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.

==History==
The history of carbocations dates back to 1891 when G. Merling<ref name="Merling1891">{{cite journal|last1=Merling|first1=G.|title=Ueber Tropin|journal=Berichte der Deutschen Chemischen Gesellschaft|volume=24|issue=2|year=1891|pages=3108–3126|issn=0365-9496|doi=10.1002/cber.189102402151}}</ref> reported that he added bromine to tropylidene ([[cycloheptatriene]]) and then heated the product to obtain a crystalline, water-soluble material, {{chem|C|7|H|7|Br}}. He did not suggest a structure for it; however, [[William von Eggers Doering|Doering]] and Knox<ref>{{cite journal|title=The Cycloheptatrienylium (Tropylium) Ion|first1=W. von E. |last1=Doering |first2=L. H. |last2=Knox |journal=[[Journal of the American Chemical Society]] |date=1954|volume=76|issue=12|pages=3203–3206 |doi=10.1021/ja01641a027}}</ref> convincingly showed that it was [[Tropylium cation|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 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 formation '''halochromy''', of which [[malachite green]] is a prime example. The [[Triphenylcarbenium|trityl carbocation]] (shown below) is indeed a stable carbocationic system, for example in the form of [[triphenylmethyl hexafluorophosphate|trityl hexafluorophosphate]].<ref name=Urch>{{cite journal |author=Urch, C. |title=Triphenylmethyl Hexafluorophosphate |journal=Encyclopedia of Reagents for Organic Synthesis |year=2001 |doi=10.1002/047084289X.rt363f|isbn=0471936235 }}</ref>

[[File:TriphenylmethanolCarbocationFormation.svg|center|reaction of triphenylmethanol with sulfuric acid]]

Carbocations are [[reactive intermediates]] in many organic reactions. This idea, first proposed by [[Julius Stieglitz]] in 1899,<ref>{{cite journal|title=On the Constitution of the Salts of Imido-Ethers and other Carbimide Derivatives|journal=American Chemical Journal|volume=21|page=101|issn=0096-4085}}</ref> was further developed by [[Hans Meerwein]] in his 1922 study<ref>{{cite journal|first1=H.|last1= Meerwein |first2=K. van |last2=Emster |title=About the equilibrium isomerism between bornyl chloride isobornyl chloride and camphene chlorohydrate|journal=Berichte|date= 1922 |volume=55 |page=2500}}</ref><ref>{{Cite journal |doi= 10.1021/ed800058c |title= Racemization of Isobornyl Chloride via Carbocations: A Nonclassical Look at a Classic Mechanism |year= 2010 |last1= Rzepa |first1= H. S. |last2= Allan |first2= C. S. M. |journal= Journal of Chemical Education |volume= 87 |issue= 2 |pages= 221 |bibcode= 2010JChEd..87..221R }}</ref> of the [[Wagner–Meerwein rearrangement]]. Carbocations were also found to be involved in the [[SN1 reaction|S<sub>N</sub>1 reaction]], the [[elimination reaction|E1 reaction]], and in [[rearrangement reaction]]s 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.

An [[NMR spectrum]] of a carbocation was first reported by Doering et al.<ref>{{cite journal|title=The 1,1,2,3,4,5,6-heptamethylbenzenonium ion|first1=W. von E. |last1=Doering |first2=M. |last2=Saunders |first3=H. G. |last3=Boyton |first4=H. W. |last4=Earhart |first5=E. F. |last5=Wadley |first6=W. R. |last6=Edwards|first7=G. |last7=Laber |journal=Tetrahedron |volume=4 |issue=1–2|date=1958|pages=178–185|doi=10.1016/0040-4020(58)88016-3}}</ref> in 1958. It was the heptamethyl[[Arenium ion|benzenium]] ion, made by treating [[hexamethylbenzene]] with [[methyl chloride]] and [[aluminium chloride]]. The stable 7-norbornadienyl cation was prepared by Story et al. in 1960<ref>{{cite journal|title=The 7-norbornadienyl carbonium ion |first1=Paul R. |last1=Story |first2=Martin |last2=Saunders |journal=[[Journal of the American Chemical Society]]|date=1960|volume=82|issue=23|page=6199|doi=10.1021/ja01508a058}}</ref> by reacting [[norbornadiene|norbornadienyl chloride]] with [[silver tetrafluoroborate]] in [[sulfur dioxide]] at −80&nbsp;°C. The NMR spectrum established that it was non-classically bridged (the first stable [[non-classical ion]] observed).

In 1962, [[George Andrew Olah|Olah]] directly observed the [[tert-butyl|''tert''-butyl]] carbocation by [[nuclear magnetic resonance]] as a stable species on dissolving ''tert''-butyl fluoride in [[magic acid]]. The NMR spectrum of the norbornyl cation was reported by Schleyer et al.<ref>{{cite journal|title=Stable Carbonium Ions. X.1 Direct Nuclear Magnetic Resonance Observation of the 2-Norbornyl Cation|first1=Paul von R. |last1=Schleyer|first2= William E. |last2=Watts |first3=Raymond C. |last3=Fort |first4=Melvin B. |last4=Comisarow|first5=George A. |last5=Olah |journal=[[Journal of the American Chemical Society]]|date=1964|volume=86|issue=24|pages=5679–5680|doi=10.1021/ja01078a056}}</ref> It was shown to rapidly undergo proton-scrambling .<ref>{{cite journal|title=Stable Carbonium Ions. XI.1 The Rate of Hydride Shifts in the 2-Norbornyl Cation |first1=Martin |last1=Saunders |first2=Paul von R. |last2=Schleyer|first3=George A. |last3=Olah |journal=[[Journal of the American Chemical Society]] |date=1964|volume=86|issue=24|pages=5680–5681|doi=10.1021/ja01078a057}}</ref>

==See also==
*[[Armilenium]]
*[[Carbanion]]
*[[Carbene]]
*[[Oxocarbenium]]

==References==
{{Reflist}}

==External links==
*{{Commons category-inline|Carbocations}}
*[http://nobelprize.org/nobel_prizes/chemistry/laureates/1994/press.html?print=1 Press Release] The 1994 Nobel Prize in Chemistry". Nobelprize.org. 9 Jun 2010

{{Authority control}}

[[Category:Carbocations| ]]
[[Category:Reactive intermediates]]