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Carbocation
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==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 °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>
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