Question

Which one of the following is aromatic?

• A. Cyclopentadienyl cation
• B. Cyclooctatetraene
• C. Cycloheptatriene
• D. Cycloheptatrienyl cation

Answer 1

The correct option is D

Cycloheptatrienyl cation

Explanation for correct option

(D) Cycloheptatrienyl cation

Huckel rule of aromatic compounds

• According to the Huckel rule, an aromatic ring must be planar.
• Electrons in the ring are completely delocalized.
• The presence of $(4\mathrm{n}+2)$ electrons in the ring, where $\mathrm{n}$ is an integer.

Cycloheptatrienyl cation

• The pi system of the cycloheptatrienyl cation $\left({\mathrm{C}}_7{\mathrm{H}}_7^{+}\right)$ has six electrons.
• It has a conjugated structure, is planar, and cyclic.
• As a result, it is aromatic.

Explanation for incorrect options

(A) Cyclopentadienyl cation

• Cyclopentadienyl cation $\left({\mathrm{C}}_5{\mathrm{H}}_5^{+}\right)$ has a cyclic, planar structure, and the electrons can be delocalized.
• There are four electrons that do not meet Hückel's rule.

(B) Cyclooctatetraene

• Cyclooctatetraene $\left({\mathrm{C}}_8{\mathrm{H}}_8\right)$ has a cyclic yet non-planar structure.
• It also possesses eight electrons, which violates Hückel's criterion.

(C) Cycloheptatriene

• Cycloheptatriene $\left({\mathrm{C}}_7{\mathrm{H}}_8\right)$ is cyclic, planar, and possesses six electrons.
• Therefore it too follows Hückel's rule, however all of the -electrons are not conjugated to each other.
• As a result, this compound is also non-aromatic.

Therefore, the correct option is (D) Cycloheptatrienyl cation.