Which of the following is not an aromatic compound? 

CONCEPT:

Aromaticity and Hückel’s Rule

• A compound is considered aromatic if it satisfies the following conditions:
  • It must be cyclic.
  • It must be planar.
  • It must be fully conjugated (every atom in the ring has a p-orbital).
  • It must follow Hückel’s Rule: the molecule must contain (4n + 2) π-electrons, where n is an integer (0, 1, 2, ...).

EXPLANATION:

• Option 1 – Benzene:
  • 6 π-electrons, cyclic, planar, fully conjugated → Aromatic 
• Option 2 – Pyridine:
  • 6 π-electrons from the double bonds, nitrogen lone pair is in sp² orbital → Aromatic 
• Option 3 – Cyclopentadienyl :
  • Only 4 π-electrons in the ring → Does not satisfy Hückel’s rule (4n + 2)
  • also not in congugation
  • Hence, non-aromatic 
• Option 4 – Pyrrole:
  • Nitrogen contributes lone pair into conjugation, total 6 π-electrons → Aromatic 

Therefore, the compound in Option 3 is not aromatic.