Carbocation

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A carbocation (pronounced "carbo-cat-ion") is an ion with a positively-charged carbon atom. A carbocation was previously often called a carbonium ion but questions arose on the exact meaning [link]. The charged carbon atom in a carbocation has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability (octet rule). Therefore the carbon cation is unstable and very reactive, seeking to fill its octet of valence electrons as well as regain its neutral charge.

The history of the science of carbocations dates back to 1902 when chemists Norris and Kehrman independently discovered that colorless triphenylmethyl alcohol gave deep yellow solutions in concentrated sulfuric acid. Triphenylmethyl chloride similarly formed orange complexes with aluminum and tin chlorides. Adolf von Baeyer recognized in 1902 the salt like character of the compounds formed.

Ph₃C-OH + H₂SO₄ → Ph₃C⁺HSO₄^- + H₂O (Ph stands for a phenyl substituent)

He dubbed the relationship between color and salt formation halochromy of which malachite green is a prime example.

Order of stability of examples of
tertiary ( III ), secondary ( II ), and primary ( I ) alkyl carbocations

A carbocation in a chemical reaction is a reactive intermediate. This idea was first developed by Hans Meerwein in his 1922 study of the Wagner-Meerwein rearrangement. Carbocations were also found to be involved in the SN1 reaction and 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 it. In 1962 Olah directly observed the tert-butyl carbocation by Nuclear magnetic resonance as a stable species by dissolving tert-butyl fluoride in a superacid.

In organic chemistry,a carbocation is often the target of nucleophilic attack by nucleophiles like OH^- ions or halogen ions.

Carbocations are classified as primary, secondary, or tertiary depending on the number of carbon atoms bonded to the ionized carbon. Primary carbocations have one or zero carbons attached to the ionized carbon, secondary carbocations have two carbons attached to the ionized carbon, and tertiary carbocations have three carbons attached to the ionized carbon.

Stability of the carbocation increases with the number of alkyl groups bonded to the charge-bearing carbon. Tertiary carbocations are more stable (and form more readily) than secondary carbocations; primary carbocations are highly unstable because, while ionized higher-order carbons are stabilized by Hyperconjugation, unsubstituted (primary) carbons are not. Therefore, reactions such as the S_N1 reaction and the E1 elimination reaction normally do not occur if a primary carbocation would be formed. An exception to this occurs when there is a carbon-carbon double bond next to the ionized carbon. Such cations as allyl cation CH₂=CH-CH₂⁺ and benzyl cation C₆H₅-CH₂⁺ are more stable than most other carbocations. Molecules which can form allyl or benzyl carbocations are especially reactive.

A carbocation can undergo a rearrangement from a less stable form to a more stable form in a chemical reaction.

References

http://nobelprize.org/chemistry/laureates/1994/olah-lecture.html
H. Meerwein and K. van Emster, Berichte, 1922, 55, 2500.

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