Question

Which of the following cyclic carbocations is aromatic?

• A. I
• B. II
• C. III
• D. I and III

Answer 1

Answer: (A)

Structure (I) is aromatic.

Answer 2

Aromaticity requires a cyclic, planar structure with a continuous $\pi$ system and $(4n+2)$ $\pi$ electrons. (I) Cyclopropyl cation: It is planar and has 2 $\pi$ electrons, satisfying Hückel's rule ($n=0$), thus it is aromatic. (II) Cyclobutyl cation: It is not planar and lacks a continuous $\pi$ system, hence not aromatic. (III) Cyclopentyl cation: It is not planar and lacks a continuous $\pi$ system, hence not aromatic. Therefore, only structure (I) is aromatic.