See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Solution: Indene + $CBr_3$ with t-BuOK Reaction ## Step 1: Identify the Substrate The starting material is **indene** (benzene fused to a cyclopentene ring). The cyclopentene ring contains an allylic C-H bond (at the methylene carbon adjacent to the double bond). ## Step 2: Recognize the Reagent System - $CBr_3$ (tribromomethyl) with **t-BuOK** (potassium tert-butoxide) generates a **:CBr carbene** (dibromocarbene after loss of one $Br^-$) - This is a **carbene addition reaction** under basic conditions ## Step 3: Predict Carbene Addition Dibromocarbenes add to C=C double bonds via **concerted cycloaddition**, forming a **cyclopropane ring** with the two bromines attached to the same carbon. For indene's double bond: $$\text{C=C} + :CBr_2 \rightarrow \text{Cyclopropane with } -CBr_2 \text{ group}$$ This gives a **fused cyclopropane** ring attached to the benzene-indene system. ## Step 4: Evaluate Options - **(A)**: Cyclopropane on five-membered ring only — incomplete - **(B)**: **Cyclopropane rings fused on both sides** with $Br_2$ substituents — matches double carbene addition - **(C)**: Naphthalene derivative — no cyclopropane formation - **(D)**: Simple indene bromide — no cyclopropane ## Answer: **(B)** The excess $CBr_3$ and strong base conditions lead to **double cyclopropanation** of both reactive positions, forming the bis-cyclopropane product with bromine substituents on each three-membered ring.