The Big Picture: Radicals and Carbocations prefer a greater degree of alkyl substitution. While, Carbanions are the opposite: Carbanions prefer. A carbanion is an anion in which carbon is trivalent (forms three bonds) and bears a formal .. See also. Carbocation · Enolates · Nitrile anion. A carbocation is an ion with a positively charged carbon atom. Among the simplest examples .. See also. Armilenium · Carbanion · Carbene · Carbo- mer · Oxocarbenium · Nonclassical Ion.
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Occasionally, carbocations that bear more than one positively charged carbon atom are also encountered e. Until the early s, all carbocations were called carbonium ions. This nomenclature was proposed by G. However, others have more narrowly defined the term ‘carbonium ion’ as formally protonated or alkylated alkanes i.
According to the IUPACa carbocation is any cation containing carbocxtion even number of electrons in which a significant portion of the positive charge resides on catbanion carbon atom. 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. In this usage, 2-norbornyl cation is not a carbonium ion, because it is formally derived from protonation of an alkene carbocaton 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. Since the late s, most textbooks have carbnion using the term carbonium ion for carbocatikn 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.
A carbocation with an two-coordinate sp-hybridized positive carbon is known as a vinyl cationwhile a two-coordinate approximately sp 2 -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 history of carbocations dates back to when G. Merling  reported that he added bromine to tropylidene cycloheptatriene and then heated the product to obtain a crystalline, water-soluble material, C cabrocation H 7 Carbanio. He did not suggest a structure for it; however, Doering and Knox  convincingly showed that it was tropylium cycloheptatrienylium bromide.
InNorris 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.
InAdolf von Baeyer recognized the salt-like character of the compounds formed.
Reaction Intermediates: Radical, Carbocation, Carbanion | Organic Chemistry Help
He dubbed the relationship between color and salt formation halochromyof which malachite green is a prime example. Carbocations are reactive intermediates in many organic reactions. This idea, first proposed by Julius Stieglitz in was further developed by Hans Meerwein in his study   of the Wagner—Meerwein rearrangement.
Carbocations were also found to be involved in the S N 1 reactionthe E1 reactionand in rearrangement reactions such as the Whitmore 1,2 shift.
Reaction Intermediates: Radical, Carbocation, Carbanion
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. It was the heptamethyl benzenium ion, made by treating hexamethylbenzene with methyl chloride and aluminium chloride. The stable 7-norbornadienyl cation was prepared by Story et al.
The NMR spectrum established that it was non-classically bridged the first stable non-classical ion observed. InOlah 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. Carbonium ions can be thought of as protonated alkanes. Although alkanes are usually considered inert, under superacid conditions e. This results in a species that contains a 3c-2e bond between a carbon and two hydrogen atoms, a type of bonding common in boron chemistry, though relatively uncommon for carbon.
As an alternative view point, the 3c-2e bond of carbonium ions could be considered as a molecule of H 2 coordinated to a carbenium ion. Indeed, carbonium ions frequently decompose by loss of molecular hydrogen to form the corresponding carbenium ion. However, the various possible structures of the ion are close in energy and separated by shallow barriers. Hence, the structure of the ion is often described as fluxional. Although there appear to be five bonds to carbon in carbonium ions, they are not hypervalentas the electron count around the central carbon is only eight, on account of the 3c-2e bond.
The charged carbon atom in a carbenium ion is a “sextet”, i. Therefore, carbocations are often reactive, seeking to fill the octet of valence electrons as well as regain a neutral charge. In most, if not all cases, the cxrbanion state of alleged primary carbocations consist of bridged structures in which positive charge is shared by two or more carbon atoms and are better described as side-protonated alkenes, edge-protonated cyclopropanes, or corner-protonated cyclopropanes rather than true primary cations.
The same is true for higher homologues like n -propyl cation.
The stabilization by alkyl groups is explained by hyperconjugation. Hence, vinyl cations are relatively uncommon intermediates.
They can be generated by the ionization of a vinyl electrophile, provided the leaving group is sufficiently good e.
They have been implicated as intermediates in some vinyl substitution reactions designated as S N 1 vinyl and as intermediates in the electrophilic addition reactions of arylalkynes.
In terms of reactivity, carbocations are susceptible to attack by nucleophileslike water, alcohols, carboxylates, azide, and halide ions, to form the addition product. Strongly basic nucleophiles, especially hindered ones, favor elimination over addition. Because even weak nucleophiles will react with carbocations, most can only be directly observed or isolated in non-nucleophilic media like superacids.
Carbocations typically undergo rearrangement reactions from less stable structures to equally stable or more stable ones by migration of an alkyl group or hydrogen to the cationic center to form a new carbocationic center.
Typically, carbocations will rearrange to give a tertiary isomer. This fact often complicates synthetic pathways. For example, when 3-pentanol is heated with aqueous HCl, the initially formed 3-pentyl carbocation rearranges to a statistical mixture of the 3-pentyl and 2-pentyl.
The Friedel-Crafts alkylation suffers from this limitation; for this reason, the acylation followed by Wolff-Kishner or Clemmensen reduction to give the alkylated product is more frequently applied.
A carbocation may be stabilized by resonance by a carbon-carbon double bond next to the ionized carbon. Molecules that can form allyl or benzyl carbocations are especially reactive. The doubly- and triply-benzylic carbocations, diphenylcarbenium and triphenylcarbenium trityl cation, are particularly stable. For the same reasons, the partial p character of strained C—C bonds in cyclopropyl groups also allows for donation of electron density and stabilizes the cyclopropylmethyl cyclopropylcarbinyl cation.
The effect of hyperconjugation is strongly stabilizing for carbocations: Although conjugation to unsaturated groups results in significant stabilization by the mesomeric effect resonancethe benefit is partially offset by the presence of a more electronegative sp 2 or sp carbon next to the carbocationic center. Thus, as reflected by hydride ion affinities, a secondary carbocation is more stabilized than the allyl cation, while a tertiary carbocation is more stabilized than the benzyl cation, results that may seem counterintuitive on first glance.
Oxocarbenium and iminium ions have important secondary canonical forms resonance structures in which carbon bears a positive charge.
As such, they are carbocations according to the IUPAC definition although some chemists do not regard them to be “true” carbocations, as their most important resonance contributors carry the formal positive charge on an oxygen or nitrogen atom, respectively. Some carbocations such as the 2-norbornyl cation exhibit more or less symmetrical three-center two-electron bonding. As circumstantial evidence of their unusual bonding, the 2-norbornyl cation is also more stable than a typical “secondary” carbocation, being roughly as stable as 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 Brownwho believed that the what appeared to be a non-classical carbocation represents the average of two rapidly equilibrating classical species and that the true non-classical structure is a transition state between the two potential energy minima, and Winsteinwho believed that the non-classical carbocation 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. 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. Cyclopropylcarbinyl cations can be studied by NMR: In the NMR spectrum of a dimethyl derivative, two nonequivalent signals are found for the two methyl groups, indicating that the molecular conformation of this cation is not perpendicular as in Awhich possesses a mirror plane, but is bisected as in B with the empty p-orbital parallel to the cyclopropyl ring system:.
In terms of bent bond theory, this preference is explained by assuming favorable orbital overlap between the filled cyclopropane bent bonds and the empty p-orbital. From Wikipedia, the free encyclopedia. General concept and structure of carbocations based on differentiation of trivalent classical carbenium ions from three-center bound penta- of tetracoordinated nonclassical carbonium ions.
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