See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Solution: Most Acidic Hydrocarbon **Step 1: Identify the acidic hydrogens** Acidic hydrogens are those bonded to carbons with high electron-withdrawing character or those that form especially stable conjugate bases. **Step 2: Analyze each structure** - **(A)** Cyclopentadiene: CH hydrogens bridging the two double bonds - **(B)** Cyclopentene derivative: saturated CH hydrogens - **(C)** Cycloheptatriene: CH hydrogens in a 7-membered ring system - **(D)** Cyclooctatetraene: CH hydrogens in an 8-membered ring system **Step 3: Apply Hückel's rule and conjugation** Structure **(A)** is cyclopentadiene with **two conjugated double bonds** separated by one $sp^2$ hybridized carbon. The bridging CH hydrogens are **allylic hydrogens** positioned between two conjugated $\pi$ systems. **Step 4: Evaluate conjugate base stability** When the H is removed from **(A)**, the resulting carbanion is **cyclopentadienyl anion** ($C_5H_5^-$), which has: - **4π electrons** following Hückel's rule (4n+2, where n=1) - **Aromatic stability** — exceptional delocalization over all 5 carbons - Lowest pKa (~15) **Step 5: Compare other options** Options B, C, and D form non-aromatic or less-stabilized carbanions with poorer electron delocalization. **Answer: (A)** — Cyclopentadiene is the most acidic because its conjugate base achieves exceptional aromatic stability.