See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Anti-aromatic Species Analysis **Apply Hückel's Rule:** A cyclic, planar, fully conjugated system is anti-aromatic if it contains $(4n+2)$ π electrons where $n = 0, 1, 2, ...$ is NOT satisfied. Instead, anti-aromatic species have $4n$ π electrons (where $n \geq 1$). **Evaluate each option:** **(A) Benzene:** 6 π electrons = $4(1) + 2$ → **Aromatic** ✗ **(B) Thiophene:** 6 π electrons (S donates 2 lone pair electrons into conjugation) = $4(1) + 2$ → **Aromatic** ✗ **(C) Cyclooctatetraene:** 8 π electrons = $4(2)$ → **Anti-aromatic if planar**, but it adopts a non-planar "tub" conformation to avoid anti-aromaticity → **Not anti-aromatic in practice** ✗ **(D) Cyclopentadienyl Cation:** - Cyclopentadiene loses H⁺ from a CH₂ group - Remaining 5-membered ring has **4 π electrons** = $4(1)$ - Planar, cyclic, fully conjugated structure with $4n$ electrons → **Anti-aromatic** ✓ **Answer: (D)** is anti-aromatic because the cyclopentadienyl cation has exactly 4 π electrons in a planar, conjugated 5-membered ring, satisfying the $4n$ condition for anti-aromaticity.