Which of the following carbocations is most likely to undergo a rearrangement to form a more stable carbocation?

• A. ${\mathrm{CH}}_3-C_{+}HH$ (a primary carbocation with no adjacent more substituted carbon)
• B. ${\mathrm{CH}}_2={\mathrm{CH}}_{+}{\mathrm{CH}}_3$ (an allylic carbocation)
• C. ${\mathrm{CH}}_3-C_{+}H{\mathrm{CH}}_3$ (a secondary carbocation adjacent to a tertiary carbon)
• D. ${\mathrm{CH}}_3-C_{+}{\mathrm{CH}}_3{\mathrm{CH}}_3$ (a tertiary carbocation)

Answer: (C)

Verified step by step guidance

1. Step 1: Understand carbocation stability and rearrangement tendencies. Carbocations tend to rearrange if they can form a more stable carbocation, typically moving from primary to secondary or tertiary, or forming resonance-stabilized carbocations.
2. Step 2: Analyze each carbocation given. Identify the degree of substitution (primary, secondary, tertiary) and check for resonance stabilization (such as allylic carbocations).
3. Step 3: For the primary carbocation with no adjacent more substituted carbon, note that it is unlikely to rearrange because there is no more stable carbocation nearby to form.
4. Step 4: For the allylic carbocation, recognize that it is already resonance stabilized, so it is less likely to rearrange since it is relatively stable.
5. Step 5: For the secondary carbocation adjacent to a tertiary carbon, consider possible hydride or alkyl shifts that can rearrange it into a more stable tertiary carbocation, making this carbocation the most likely to undergo rearrangement.