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 increases from water (18) to methanol(32) to dimethyl ether(74)
B. The extent of H-bonding decreases from water to methanol while it is absent in ether
C. The extent of intramolecular H-bonding decreases from ether to methanol to water
D. The density of water is `1.00g*mL^(-1)`, methanol `0.7914g*mL^(-1)` and that of dimethyl ether is `0.7137g*mL^(-1)`

Answer

To explain the wide variations in boiling points of methanol, water, and dimethyl ether, we can analyze the structures and intermolecular forces present in each compound. Here’s a step-by-step solution:

### Step 1: Identify the boiling points
- Methanol: 65°C
- Water: 100°C
- Dimethyl ether: 34.5°C

### Step 2: Understand the role of hydrogen bonding
- Boiling point is significantly influenced by the strength and extent of intermolecular forces, particularly hydrogen bonding in this case.

### Step 3: Analyze the structures
- **Methanol (CH₃OH)**: Contains one hydroxyl (-OH) group, allowing it to form hydrogen bonds.
- **Water (H₂O)**: Contains two hydroxyl groups, enabling it to form extensive hydrogen bonding with other water molecules.
- **Dimethyl ether (CH₃OCH₃)**: Contains an ether functional group, which does not allow for hydrogen bonding in the same way as alcohols or water.

### Step 4: Compare the extent of hydrogen bonding
- Water has the highest boiling point (100°C) due to its ability to form a large number of hydrogen bonds.
- Methanol has a lower boiling point (65°C) than water because it can form fewer hydrogen bonds.
- Dimethyl ether has the lowest boiling point (34.5°C) because it lacks hydrogen bonding entirely.

### Step 5: Conclusion
The boiling points vary widely due to the differences in hydrogen bonding capabilities:
- Water > Methanol > Dimethyl Ether in terms of boiling points, correlating with the extent of hydrogen bonding.

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

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