See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: Aromaticity requires a cyclic, planar, fully conjugated system with (4n+2) pi electrons (Huckel's rule), where n is a non-negative integer (0, 1, 2, ...). Step 2 - Analyze option (b): The cycloheptadienyl carbene shown is a 7-membered ring carbene. A singlet carbene carbon contributes a lone pair (2 electrons) to the pi system. The ring has 2 double bonds contributing 4 pi electrons, plus the lone pair on the carbene carbon contributing 2 pi electrons, giving a total of 6 pi electrons. With 6 pi electrons, 4n+2 = 6 gives n=1, satisfying Huckel's rule. The system is cyclic, planar, and fully conjugated (the lone pair on the carbene participates in conjugation), making it aromatic. Step 3 - Analyze option (a): The cycloheptatrienyl anion (7-membered ring, 3 double bonds + negative charge) has 8 pi electrons (6 from double bonds + 2 from lone pair). 8 = 4n gives n=2, so it is antiaromatic. Step 4 - Analyze option (c): The cyclopentadienyl cation (5-membered ring, 2 double bonds + positive charge) has 4 pi electrons (empty p orbital, no lone pair contribution). 4 = 4n gives n=1, so it is antiaromatic. Step 5 - Analyze option (d): The cyclobutadiene carbene (4-membered ring with carbene, 1 double bond) has 2 pi electrons from the double bond + 2 pi electrons from the carbene lone pair = 4 pi electrons total. 4 = 4n gives n=1, so it is antiaromatic. Step 6 - Conclusion: Only option (b), the cycloheptadienyl carbene with 6 pi electrons, satisfies Huckel's rule (4n+2, n=1) and is therefore aromatic. Therefore, the correct answer is B.