See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Solution **Step 1: Identify the isomers** For $C_2H_2Br_2$, there are three possible isomers: - **(A) trans-1,2-dibromethene:** Br atoms on opposite sides (trans) - **(B) cis-1,2-dibromethene:** Br atoms on same side (cis) - **(C) 1,1-dibromethene:** Both Br atoms on same carbon **Step 2: Analyze dipole moments** - **(A) trans:** Br dipoles cancel → $\mu = 0$ (nonpolar) - **(B) cis:** Br dipoles add → $\mu > 0$ (polar, highest) - **(C) geminal:** Both Br on one C → $\mu > 0$ (polar) **Highest dipole moment: (B) cis isomer** **Step 3: Analyze boiling points** **(B) cis-isomer** has the highest dipole moment → strongest intermolecular dipole-dipole interactions → **highest boiling point** ✓ **Step 4: Analyze melting points** **(C) geminal isomer** has: - Symmetric structure (both Br on same carbon) - High molecular symmetry and crystalline packing efficiency - **Lowest melting point** despite lower dipole moment The cis-isomer's lower symmetry leads to poor crystal packing, giving an intermediate melting point. **Answer: (C)** combines the highest dipole moment of the cis-isomer's boiling point behavior with the geminal isomer's poor crystalline packing (lowest melting point).