Which of the following compound is not aromatic?

Concept:

Aromaticity of a compound can be decided by Huckel’s rule.

In cyclopentadienyl cation (A), resonance takes place.

Hence,  is anti-aromatic.

Anti-aromaticity is a characteristic of a cyclic molecule with a π electron system that has higher energy due to the presence of 4n delocalised (π or lone pair) electrons in it.

Unlike aromatic compounds, which follow Hückel's rule ([4n + 2] π electrons) and are highly stable, anti-aromatic compounds are highly unstable and highly reactive.

Huckel’s rule as it has conjugated 4-electron system. Rest of the species are aromatic at each of them belongs to 6-electron [(4n + 2)π, n = 1] system.

Huckel's rule estimates whether a planar ring molecule will have aromatic properties.

The Möbius-Huckel’s concept, a cyclic ring molecule follows Huckel’s rule when the number of its π-electron equals 4n + 2 where n is a non-negative integer.

Compounds (B), (C), (D) are containing 6πe^- in complete conjugation and are aromatic

Compound (A) is anti-aromatic as it has 4πe in complete conjugation and doesn’t obey Huckel rule.