why does more stable carbocations react more rapidly than less stable....?shouldn't it be the other

Asked by  | 5th Apr, 2008, 10:42: PM

Expert Answer:

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 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.

Answered by  | 12th May, 2008, 09:16: AM

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