See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Finding Alkynes with Molecular Formula $C_9H_{8}$ **Step 1: Determine the degree of unsaturation** Using the formula: $\text{DBE} = \frac{2C + 2 - H}{2} = \frac{2(9) + 2 - 8}{2} = \frac{12}{2} = 6$ This means 6 degrees of unsaturation total. **Step 2: Account for triple bond requirement** For alkynes, we need at least one $C \equiv C$ triple bond, which accounts for **2 degrees of unsaturation**, leaving **4 degrees** remaining. These 4 remaining degrees must come from rings and/or additional double bonds/triple bonds. **Step 3: Identify possible structural isomers** The three possible alkynes with $C_9H_8$ are: 1. **Phenylacetylene (ethynylbenzene)**: Benzene ring (4 DBE) + terminal alkyne (2 DBE) = 6 DBE ✓ 2. **Methylphenylacetylene**: Benzene ring (4 DBE) + internal alkyne (2 DBE) = 6 DBE ✓ 3. **Indene (with a triple bond isomer)**: A bicyclic structure with one triple bond satisfying the formula ✓ **Conclusion:** There are exactly **3 possible structural isomers** of alkynes with molecular formula $C_9H_8$. **Answer: (B) 3**