CBSE Class 12-science Answered

A carbocation (pronounced /ˌkɑrboʊˈkætaɪɒn/) is an ion with a positively-charged carbon atom. 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 carbocations are often reactive, seeking to fill the octet of valence electrons as well as regain a neutral charge. One could reasonably assume a carbocation to have sp³ hybridization with an empty sp³ orbital giving positive charge. However, the reactivity of a carbocation more closely resembles sp² hybridization with a trigonal planar molecular geometry.

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.