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 [1]. The charged carbon atom in a carbocation is a "sextet", i.e. it 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 often unstable and very reactive, seeking to fill its octet of valence electrons as well as regain its neutral charge. The carbocation has sp2 hybridization with a trigonal planar molecular geometry.
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.
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 [2] 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 SN1 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.
Carbocations undergo rearrangement from less stable structures to equally stable or more stable ones with rate constants in excess of 1.0E9/sec. This fact complicates synthetic pathways to many compounds. 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. These cations react with chloride ion to produce about 1/3 3-chloropentane and 2/3 2-chloropentane.
Some carbocations such as the norbornyl cation exhibit more or less symmetrical three centre bonding. Cations of this sort have been referred to as non-classical ions. The energy difference between "classical" carbocations and "non-classical" isomers is often very small, and there is generally little, if any activation energy involved in the transition between "classical" and "non-classical" structures. The "non-classical" form of the 2-butyl carbocation is essentially 2-butene with a proton directly above the centre of what would be the carbon-carbon double bond. "Non-classical" carbocations were once the subject of great controversy. One of George Olah's greatest contributions to chemistry was resolving this controversy [3] .
