See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

**Aromaticity Criteria (Hückel's Rule):** A compound is aromatic if it: 1. Is cyclic 2. Is planar 3. Is fully conjugated 4. Has $(4n + 2)$ π electrons (where $n = 0, 1, 2, ...$) **Analysis:** **(A) Cyclopentadienyl cation** ($C_5H_5^+$) - Cyclic, planar, conjugated ✓ - π electrons: 5-membered ring with 4 π electrons from two C=C bonds + 1 from carbocation = **4 π electrons** - Check Hückel: $4n + 2 = 4 \Rightarrow n = 0.5$ ✗ - **Non-aromatic** ✓ **(B) Cyclopropene** ($C_3H_4^+$ cation) - Cyclic, conjugated, planar - π electrons: **2 π electrons** (one C=C double bond) - Check: $4(0) + 2 = 2$ ✓ - **Aromatic** **(C) Cyclopropenyl cation** ($C_3H_3^+$) - Cyclic, planar, conjugated - π electrons: **2 π electrons** (delocalized across three carbons) - Check: $4(0) + 2 = 2$ ✓ - **Aromatic** (antiaromatic in neutral form, but cation is aromatic) **(D) Naphthalene** ($C_{10}H_8$) - Two fused benzene rings - π electrons: **10 π electrons** - Check: $4(2) + 2 = 10$ ✓ - **Aromatic** **Answer: (A)** The cyclopentadienyl cation has 4 π electrons, which doesn't satisfy Hückel's rule ($4n + 2$), making it non-aromatic.