Which of the following is the most stable carbocation?

• A. $\mathrm{CH}{3}_1–\mathrm{CH}+$ (primary alkyl carbocation)
• B. $\mathrm{CH}{3}_2–\mathrm{CH}+$ (secondary alkyl carbocation)
• C. $C_6{H}_5–C+$ (benzylic carbocation)
• D. $\mathrm{CH}{3}_3–C+$ (tertiary alkyl carbocation)

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the types of carbocations given: primary alkyl, secondary alkyl, tertiary alkyl, and benzylic carbocation. Carbocation stability generally increases with the number of alkyl groups attached due to hyperconjugation and inductive effects.
2. Step 2: Recall that primary carbocations are the least stable because they have only one alkyl group donating electron density, while secondary and tertiary carbocations are more stable due to greater alkyl substitution providing more electron donation.
3. Step 3: Recognize that benzylic carbocations are especially stable because the positive charge is delocalized over the aromatic ring through resonance, which significantly stabilizes the carbocation beyond what alkyl substitution alone can provide.
4. Step 4: Compare the resonance stabilization of the benzylic carbocation to the hyperconjugation and inductive effects in tertiary carbocations. Resonance delocalization generally provides greater stability than alkyl substitution.
5. Step 5: Conclude that the benzylic carbocation is the most stable among the options because resonance stabilization outweighs the stability provided by tertiary alkyl substitution.