The boiling points of methanol, water and dimethyl ether are respectively `65^@ C, 100^@ C` and `34.5^@ C`. Which of the following best explains these wide variations in b.p. ?

Options

A. The molecular mass increase from water (18) to methanol(32) to diethyl ether(74)
B. The extent of H-bonding decrease from water too methanol while it is absent in ether
C. The extent of intramolecular H-bondin decrease from ether to methanol to water
D. The density of water is `1.00g*mL^(-1)`, methanol `0.7914g*mL^(-1)` and that of diethyl ether is `0.7137g*mL^(-1)`

Answer

To explain the wide variations in boiling points of methanol (65°C), water (100°C), and dimethyl ether (34.5°C), we can analyze the molecular structure and the types of intermolecular forces present in each compound.

### Step-by-Step Solution:

1. **Identify the Compounds and Their Structures**:
- **Methanol (CH₃OH)**: Contains a hydroxyl (-OH) group, allowing for hydrogen bonding.
- **Water (H₂O)**: Also contains two hydroxyl groups, allowing for strong hydrogen bonding.
- **Dimethyl Ether (CH₃OCH₃)**: Contains an ether functional group, but lacks a hydrogen atom bonded to oxygen, thus does not form hydrogen bonds.

2. **Analyze the Types of Intermolecular Forces**:
- **Hydrogen Bonding**:
- Water exhibits strong hydrogen bonding due to its ability to form multiple hydrogen bonds (each water molecule can form up to four hydrogen bonds).
- Methanol also forms hydrogen bonds, but not as extensively as water because it has fewer hydrogen bonding sites.
- Dimethyl ether does not participate in hydrogen bonding because it lacks an -OH group that can donate a hydrogen atom for bonding.

3. **Compare the Boiling Points**:
- **Water (100°C)**: Highest boiling point due to strong hydrogen bonding.
- **Methanol (65°C)**: Lower boiling point than water due to fewer hydrogen bonds.
- **Dimethyl Ether (34.5°C)**: Lowest boiling point as it does not engage in hydrogen bonding.

4. **Conclusion**:
- The wide variations in boiling points can be attributed primarily to the presence and strength of hydrogen bonding in these molecules. Water has the highest boiling point due to strong hydrogen bonding, followed by methanol, which has weaker hydrogen bonding, and dimethyl ether, which has no hydrogen bonding.

### Final Answer:
The best explanation for the wide variations in boiling points of methanol, water, and dimethyl ether is that the extent of hydrogen bonding decreases from water to methanol while it is absent in dimethyl ether.