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Among the following carbocations, which one would you expect to undergo a rearrangement (hydride or alkyl shift) to form a more stable cation? (Cations: a secondary carbocation that can shift to become tertiary, versus already-tertiary or already-most-stable cations.)

1. A secondary carbocation adjacent to a carbon bearing a methyl group, which can rearrange to a more stable tertiary cation
2. A tertiary carbocation that is already maximally stabilized
3. A symmetric cation where any shift gives an equally stable cation
4. A resonance-stabilized allylic/benzylic cation

Correct answer: A secondary carbocation adjacent to a carbon bearing a methyl group, which can rearrange to a more stable tertiary cation

Solution

Carbocation rearrangements (1,2-hydride or 1,2-alkyl shifts) occur when they convert a less stable cation into a more stable one. A secondary carbocation positioned next to a carbon bearing additional alkyl substitution (e.g. a methyl-substituted ring carbon) will undergo a 1,2-shift to generate a tertiary carbocation, which is more stable. An already-tertiary, symmetric, or resonance-stabilized cation has no driving force to rearrange. Hence the secondary-to-tertiary case is the one expected to rearrange.

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