Question

Choose the correct statement(s) regarding the given compounds :

• A. (a) is aromatic.
• B. (b) is non-aromatic.
• C. (a) is anti-aromatic.
• D. (b) is aromatic.

Answer 1

Answer: (C), (D)

(b) is aromatic.

Compound (a) i.e. cyclobutadiene is cyclic and conjugated. There are two $\pi$ bonds and zero contributing lone pairs. Two $\pi$ bonds gives a total of 4 $\pi$ electrons, i.e. has $\left(4n\right)\pi e^{-}$. So it is anti-aromatic.

Structure (b)
Cyclobutadienyl dianion:
The cyclobutadiene dianion is cyclic, planar and conjugated. It has a single pi bond, and now two carbons bearing negative charge which can contribute to the pi system, giving us a total of six $\pi$ electrons. So, it follows Huckel rule and thus the Cyclobutadienyl dianion is aromatic.

Theory :
Special case of cyclobutadiene:
It is Anti aromatic in nature and unstable (more electrons in the antibonding molecular orbitals) so it dimerises at room temperature.
Thus it becomes non-aromatic.
(But whenever cyclobutadiene is mentioned in question we will count it as anti-aromatic only unless specified).