Which of the following carbocations would you expect to...

Which of the following carbocations would you expect to rearrange? SOLUTION A is a secondary carbocation. It does not rearrange because a 1,2 -hydride shift would convert it to a different secondary carbocation, so there is no energetic advantage to the rearrangement. B is a secondary carbocation. It rearranges because a 1,2 -hydride shift converts it to a more stable tertiary carbocation. C is a tertiary carbocation. It does not rearrange because its stability is not improved by rearrangement. D is a tertiary carbocation. It does not rearrange because its stability is not improved by rearrangement. E is a secondary carbocation. It does rearrange because a 1,2 -hydride shift converts it to a more stable tertiary carbocation. F is a secondary carbocation. It does not rearrange because rearrangement would form a carbocation with the same stability - that is, another secondary carbocation.

Which of the following carbocations would you expect to rearrange?
SOLUTION
A is a secondary carbocation. It does not rearrange because a 1,2 -hydride shift would convert it to a different secondary carbocation, so there is no energetic advantage to the rearrangement.
B is a secondary carbocation. It rearranges because a 1,2 -hydride shift converts it to a more stable tertiary carbocation.
C is a tertiary carbocation. It does not rearrange because its stability is not improved by rearrangement.
D is a tertiary carbocation. It does not rearrange because its stability is not improved by rearrangement.
E is a secondary carbocation. It does rearrange because a 1,2 -hydride shift converts it to a more stable tertiary carbocation.
F is a secondary carbocation. It does not rearrange because rearrangement would form a carbocation with the same stability - that is, another secondary carbocation.

Key Concepts

Carbocation Stability

Carbocation stability refers to how the positive charge of a carbocation is stabilized by its surrounding substituents. More substituted carbocations, such as tertiary carbocations, are generally more stable due to hyperconjugation and inductive effects, which help distribute and delocalize the positive charge.

Carbocation Rearrangement

Carbocation rearrangement is a process in which a carbocation undergoes a structural shift, such as a hydride or alkyl migration, to form a more stable carbocation. This rearrangement often occurs when the new carbocation has a higher degree of substitution or benefits from the stabilization afforded by adjacent groups.

1,2-Hydride Shift

A 1,2-hydride shift is a specific type of carbocation rearrangement where a hydrogen atom with its bonding electrons moves from an adjacent carbon to the carbocation center. This shift can convert a less stable carbocation into a more stable one, typically by increasing the degree of substitution at the positively charged carbon.

Hyperconjugation

Hyperconjugation is a stabilizing interaction where electrons from neighboring sigma bonds are delocalized into the empty p-orbital of a carbocation. This phenomenon increases the stability of the carbocation, with a greater effect observed in more substituted carbocations, such as tertiary over secondary carbocations.

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