JEE Main 2026 Apr 8 Shift 2, Chemistry Q14: Important Concepts Of Chemical Bonding

Apply Hückel's rule ($4n+2$ $\pi$ electrons in a planar, fully conjugated, cyclic system) to each species.

1. p-Benzoquinone: the ring contains two C=O carbons that are $s{p}^2$ but the conjugation is cross-conjugated; the cyclic $\pi$ count is not a Hückel $(4n+2)$ system in the carbocyclic ring sense — not aromatic.

2. Tropylium cation (cycloheptatrienyl cation, $7$-membered, $6\pi$): aromatic.

3. Phenanthrene ($14\pi$, $n=3$): aromatic.

4. Naphthalene ($10\pi$, $n=2$): aromatic.

5. Cyclobutadienyl dication ($2\pi$, $n=0$): aromatic (Hückel).

6. Cyclohexadienyl anion as drawn (only one double bond plus carbanion in a 6-membered ring, not fully conjugated/planar around the ring): not aromatic.

Aromatic count: $4$ (tropylium, phenanthrene, naphthalene, cyclobutadienyl dication).