See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Hückel's Rule for Aromaticity **Hückel's Rule**: A cyclic, planar, fully conjugated species is aromatic if it contains $(4n + 2)$ π electrons, where $n = 0, 1, 2, ...$ **Analysis of each species:** **(I) Cyclopentadienyl cation** ($C_5H_5^+$) - 5-membered ring with a positive charge - π electrons: 4 (from two double bonds) + 1 (from the p-orbital at the positive carbon) = **6 π electrons** - $6 = 4(1) + 2$ ✓ **AROMATIC** **(II) Cycloheptatrienyl cation** ($C_7H_7^+$) - 7-membered ring with a positive charge - π electrons: 6 (from three double bonds) + 1 (from p-orbital) = **8 π electrons** - $8 = 4(2) + 0$ ✗ Not aromatic (antiaromatic) **(III) Cyclobutadiene** ($C_4H_4$) - 4-membered ring with two double bonds - π electrons: **4 π electrons** - $4 = 4(1) + 0$ ✗ Antiaromatic (not aromatic) **(IV) Cyclopentadiene** (open 5-membered ring) - Not fully conjugated (sp³ CH₂ breaks conjugation) - ✗ Not aromatic **Answer: (A) I** Only the cyclopentadienyl cation satisfies Hückel's rule with 6 π electrons.