Carbocation Stability Tertiary carbons form the most stable carbocations due to a combination of factors. The three
alkyl groups on the tertiary carbon contribute to a strong
inductive effect. This is because each alkyl group will share its
electron density with the central carbocation to stabilize it. Additionally, the surrounding sp3 hybridized carbons can stabilize the carbocation through
hyperconjugation. This occurs when adjacent sp3 orbitals have a weak overlap with the vacant p orbital; since there are 3 surrounding carbons with sp3
hybridization, there are more opportunities for overlap, which contributes to increasing carbocation stability.
Reaction Mechanisms A tertiary carbocation will maximize the rate of reaction for an
SN1 reaction by producing a stable carbocation. This happens because the rate determining step of a SN1 reaction is the formation of the carbocation. The rate of the reaction is therefore reliant on the stability of the carbocation because it means that the
transition state has a lower energy level which makes the activation energy lower. Tertiary carbons are similarly preferred in E1 for the same reasons as it has a carbocation intermediate. E1 and E2 reactions follow Zaitsev's rule which states that the most substituted product in an elimination reactions is going to be the major product because it will be favored for its stability. This leads to tertiary carbons being preferred for their stability in elimination reactions. In general, SN2 reactions do not occur with tertiary carbons because of the steric hindrance produced by the substituted groups. However, recent research has shown there are exceptions to this rule; for the first time, a bimolecular nucleophilic substitution, aka
SN2 reaction, can happen to a tertiary carbon. == References ==