See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Hydrocarbon Acidity Analysis **Key Principle:** Hydrocarbon acidity increases with the stability of the conjugate anion formed after H⁺ removal. **Analyzing each structure:** **(a) Indane (benzene + saturated 5-membered ring)** - H removal from the saturated ring gives a carbanion with minimal stabilization - No extended conjugation **(b) Indene (benzene + unsaturated 5-membered ring)** - H removal from the CH₂ group gives a **allylic/benzylic carbanion** - The resulting anion is stabilized by: - Resonance with the benzene ring (aromatic stabilization) - Resonance with the double bond in the 5-membered ring - **Extended conjugation** across the entire system **(c) & (d) Tetralin variants** - Both have saturated 5-membered rings (no internal double bond) - Limited conjugation compared to (b) **Why (b) is most acidic:** The conjugate anion of indene achieves significant **aromatic character and resonance stabilization** because: $$\text{Indene-H} \rightarrow \text{Indene}^- + H^+$$ The resulting carbanion resonates between multiple forms involving both the benzene ring and the five-membered ring, making it exceptionally stable. **Answer: (b)** — Indene is most acidic due to maximum resonance stabilization of its conjugate anion.