See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Identifying the Aromatic Compound **Analysis of each option:** **(1) Square (Cyclobutadiene)** - 4π electrons in a 4-membered ring - According to Hückel's rule: $4n + 2$ where $n = 0, 1, 2, ...$ - For 4π electrons: $4 = 4n + 2$ gives $n = 0.5$ (not an integer) - **Non-aromatic** (actually antiaromatic) **(2) Cyclopentadienyl anion (shown with ⊕ charge indicator)** - 5-membered ring with negative charge (5 electrons in π system) - Follows Hückel's rule: $5 = 4n + 2$ where $n = 1$ ✓ - **Aromatic** — this is the cyclopentadienyl anion, $C_5H_5^-$ **(3) Naphthalene (two fused benzene rings)** - 10π electrons in the fused ring system - Follows Hückel's rule: $10 = 4n + 2$ where $n = 2$ ✓ - **Also aromatic**, but option (2) is the fundamental example **(4) Cyclopropene** - 3-membered ring with 2π electrons from double bond - Does not follow Hückel's rule; lacks aromatic character - **Non-aromatic** **The answer is (2)** because the cyclopentadienyl anion satisfies Hückel's rule with $n = 1$, providing $(4 \times 1 + 2) = 6$ π electrons worth of aromaticity in a 5-membered ring system.