Question

Will cyclopenta-1,2-diene ion be aromatic?

• A. Yes
• B. No

Answer 1

Answer: (B)

No

Answer 2

Aromaticity requires a cyclic, planar structure with a fully conjugated $\pi$ system and $(4n+2)$ $\pi$ electrons. In cyclopenta-1,2-diene ion, the double bonds are at adjacent positions (1 and 2). For any ionic species (cation or anion) to be formed, at least two carbons in the ring will be $sp^3$ hybridized to satisfy valency. These $sp^3$ hybridized carbons break the planarity and prevent complete $\pi$ electron delocalization around the ring, thus disqualifying it from being aromatic.