In which case first has lower melting point than second ?

Options

A.
B.
C.
D.

Explanation

To determine in which case the first compound has a lower melting point than the second, we need to analyze the intermolecular forces, molecular weights, and molecular shapes of the compounds in the options provided.

### Step-by-Step Solution:

1. **Understanding Melting Points**:
- The melting point of a substance is influenced by the strength of the intermolecular forces present. Stronger forces lead to higher melting points, while weaker forces result in lower melting points.

2. **Analyzing Option A: Ether vs. Alcohol**:
- **Ether**: Contains dipole-dipole interactions.
- **Alcohol**: Contains hydrogen bonding (stronger than dipole-dipole).
- **Conclusion**: Alcohol has a higher melting point than ether. Therefore, this option does not satisfy the condition.

3. **Analyzing Option B: Butane vs. Propane**:
- **Butane**: C4H10 (4 carbon atoms).
- **Propane**: C3H8 (3 carbon atoms).
- Both have only Van der Waals forces.
- **Molecular Weight**: Butane has a higher molecular weight than propane.
- **Conclusion**: Butane has a higher melting point than propane. This option does not satisfy the condition.

4. **Analyzing Option C: Butane vs. Chloropropane**:
- **Butane**: C4H10 (Van der Waals forces).
- **Chloropropane**: C3H7Cl (dipole-dipole interactions due to the presence of chlorine).
- **Conclusion**: Chloropropane has stronger dipole-dipole interactions compared to the Van der Waals forces in butane. Therefore, chloropropane has a higher melting point than butane. This option does not satisfy the condition.

5. **Final Conclusion**:
- After analyzing the options, none of the options provided satisfy the condition where the first compound has a lower melting point than the second. However, if we consider the general trend:
- **Ethers have lower melting points than alcohols** due to the absence of hydrogen bonding in ethers.
- **Alkanes with fewer carbon atoms typically have lower melting points than those with more carbon atoms**.