Carbocation - Properties

Properties

In organic chemistry, a carbocation is often the target of nucleophilic attack by nucleophiles like hydroxide (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 carbon to be ionized, as the double bond in such a system will stabilize the carbocation by resonance. 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 reactions from less stable structures to equally stable or more stable ones with rate constants in excess of 109/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.