Which of the following carbocations would be most stable?

• A. $C_6{H}_5-C+H{\mathrm{CH}}_3{\mathrm{CH}}_3$ (a benzylic tertiary carbocation)
• B. $\mathrm{CH}{3}_{2}C+H$ (a secondary carbocation)
• C. $\mathrm{CH}{2}_{2}C+H$ (a primary carbocation)
• D. $\mathrm{CH}{3}_3-C+$ (a tertiary carbocation)

Answer: (A)

Verified step by step guidance

1. Identify the type of each carbocation based on its structure: primary, secondary, tertiary, or benzylic tertiary. Recall that tertiary carbocations have three alkyl groups attached to the positively charged carbon, secondary have two, and primary have one. Benzylic carbocations are those where the positive charge is adjacent to a benzene ring.
2. Understand that carbocation stability is influenced by several factors: alkyl substitution (hyperconjugation and inductive effects) and resonance stabilization. Tertiary carbocations are generally more stable than secondary, which are more stable than primary due to hyperconjugation and inductive effects.
3. Recognize that benzylic carbocations are especially stable because the positive charge can be delocalized into the aromatic ring through resonance, which greatly stabilizes the carbocation beyond what alkyl substitution alone can provide.
4. Compare the given carbocations: the benzylic tertiary carbocation benefits from both resonance stabilization and tertiary alkyl substitution, the tertiary carbocation benefits only from alkyl substitution, the secondary carbocation less so, and the primary carbocation the least.
5. Conclude that the benzylic tertiary carbocation is the most stable due to the combined effects of resonance stabilization from the benzene ring and the inductive/hyperconjugative effects of the tertiary alkyl groups.