Identify the most stable carbocation among: ${\text{C}}_6{\text{H}}_5{\text{CH}}_2^{+}$, ${\text{CH}}_2=\text{CH}-{\text{CH}}_2^{+}$, $({\text{CH}}_3{)}_3{\text{C}}^{+}$, and ${\text{CH}}_3{\text{CH}}_2^{+}$.

Solution

1. Stability of carbocations is determined by resonance, hyperconjugation, and inductive effects.
2. ${\text{C}}_6{\text{H}}_5{\text{CH}}_2^{+}$ is stabilized by resonance with the entire aromatic ring.
3. ${\text{CH}}_2=\text{CH}-{\text{CH}}_2^{+}$ is stabilized by resonance with one double bond.
4. $({\text{CH}}_3{)}_3{\text{C}}^{+}$ is stabilized by hyperconjugation with nine alpha-hydrogens.
5. Resonance with an aromatic ring provides greater stabilization than simple hyperconjugation or isolated resonance.
6. Hence the answer is (A).