Which of the following compound is not aromatic?

Step 1: Check Each Compound

(a) Cyclopentadienyl cation $\ce{C5H5^+}$: 4 pi electrons ($4n$, $n=1$). Antiaromatic. NOT aromatic.

(b) Pyrrole ($\ce{C4H4NH}$): N lone pair in p orbital contributes 2 pi e; 2 double bonds contribute 4 pi e. Total = 6 pi electrons ($4n+2$, $n=1$). Cyclic, planar. Aromatic.

(c) Pyridine ($\ce{C5H5N}$): 6 pi electrons from 3 double bonds in ring (N lone pair in sp2 orbital, not part of pi system). Total pi e = 6 ($4n+2$). Aromatic.

(d) Furan ($\ce{C4H4O}$) or similar from image: O lone pair + 2 double bonds = 6 pi electrons. Aromatic.

The compound that is NOT aromatic = (a) cyclopentadienyl cation (antiaromatic, 4 pi e).

Key Points:

• Cyclopentadienyl cation: 4 pi e = antiaromatic (very unstable)
• Cyclopentadienyl anion: 6 pi e = aromatic (very stable)
• Pyrrole and pyridine: both aromatic despite having N in ring
• Furan: aromatic (O lone pair contributes to 6 pi e system)