Question

Which among the following is not an aromatic species:

• A. Benzene
• B. Cyclopentadienyl anion
• C. Cyclobutadiene
• D. Pyrrole

Hint:

For aromaticity:
- Count pi electrons: \(4n + 2\) (aromatic), \(4n\) (anti-aromatic).
- Ensure the molecule is cyclic, planar, and conjugated.
- Heterocycles like pyrrole contribute lone pairs to the pi system if conjugated.

Answer 1

The Correct Option is C

Aromaticity is determined by Hückel’s rules:
- Cyclic structure.
- Planar geometry.
- Fully conjugated pi electron system.
- \(4n + 2\) pi electrons (\(n = 0, 1, 2, \ldots\)) for aromaticity; \(4n\) for anti-aromaticity.
Step 1: Analyze each option
- (A) Benzene (\(\mathrm{C}_6\mathrm{H}_6\)): Cyclic, planar, 6 pi electrons (from 3 double bonds, \(4 \cdot 1 + 2 = 6\)). Aromatic (\(n = 1\)).
- (B) Cyclopentadienyl anion (\([\mathrm{C}_5\mathrm{H}_5]^-\)): Cyclic, planar, 6 pi electrons (4 from two double bonds, 2 from the lone pair on the negatively charged carbon, \(4 \cdot 1 + 2 = 6\)). Aromatic (\(n = 1\)).
- (C) Cyclobutadiene (\(\mathrm{C}_4\mathrm{H}_4\)): Cyclic, planar, 4 pi electrons (2 double bonds, \(4 \cdot 1 = 4\)). Anti-aromatic (\(4n, n = 1\)), not aromatic.
- (D) Pyrrole (\(\mathrm{C}_4\mathrm{H}_5\mathrm{N}\)): Cyclic, planar, 6 pi electrons (4 from two double bonds, 2 from nitrogen’s lone pair in the pi system, \(4 \cdot 1 + 2 = 6\)). Aromatic (\(n = 1\)).
Step 2: Conclusion
Cyclobutadiene is anti-aromatic (4 pi electrons), not aromatic, matching option (C).