Aromaticity is determined by Hückel’s rules:
- Cyclic structure.
- Planar geometry.
- Fully conjugated pi electron system.
- pi electrons ( ) for aromaticity; for anti-aromaticity.
Step 1: Analyze each option
- (A) Benzene ( ): Cyclic, planar, 6 pi electrons (from 3 double bonds, ). Aromatic ( ).
- (B) Cyclopentadienyl anion ( ): Cyclic, planar, 6 pi electrons (4 from two double bonds, 2 from the lone pair on the negatively charged carbon, ). Aromatic ( ).
- (C) Cyclobutadiene ( ): Cyclic, planar, 4 pi electrons (2 double bonds, ). Anti-aromatic ( ), not aromatic.
- (D) Pyrrole ( ): Cyclic, planar, 6 pi electrons (4 from two double bonds, 2 from nitrogen’s lone pair in the pi system, ). Aromatic ( ).
Step 2: Conclusion
Cyclobutadiene is anti-aromatic (4 pi electrons), not aromatic, matching option (C).