See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: For a compound to be aromatic, it must satisfy Huckel's rule: (1) it must be cyclic, (2) it must be planar, (3) it must be fully conjugated (every atom in the ring must have a p-orbital participating in the pi system), and (4) it must contain (4n+2) pi electrons where n is a non-negative integer. Step 2 - Evaluate option (a) Pyridine: It is a 6-membered planar ring with nitrogen, fully conjugated, and has 6 pi electrons (n=1, 4(1)+2=6). It is aromatic. Step 3 - Evaluate option (c) Pyrrole: It is a 5-membered planar ring with NH. The nitrogen lone pair participates in the pi system giving 6 pi electrons total (n=1). It is aromatic. Step 4 - Evaluate option (d) Naphthalene: Two fused benzene rings, planar, fully conjugated, 10 pi electrons (n=2, 4(2)+2=10). It is aromatic. Step 5 - Evaluate option (b) Norbornadiene (bicyclo[2.2.1]hepta-2,5-diene): This is a bicyclic compound with a CH2 bridge that forces the molecule into a non-planar geometry. The two double bonds present are not part of a continuous cyclic conjugated pi system because the sp3 bridgehead carbons and the CH2 bridge prevent planarity and full cyclic conjugation. Since it is not planar and not fully conjugated in a cyclic manner, it does not satisfy the criteria for aromaticity. It behaves as a non-aromatic (olefinic) compound. Step 6 - Conclusion: Norbornadiene (option b) fails the planarity and cyclic conjugation requirements for aromaticity, so it would not be considered aromatic in its behaviour. Therefore, the correct answer is B.