Question

The increasing order of boiling points of the following is

• A. I < III < II < IV
• B. III < I < II < IV
• C. I < IV < III < II
• D. III < I < IV < II

Hint:

General BP trend for comparable molar mass: Acid \textgreater Alcohol \textgreater Ketone \textgreater Aldehyde \textgreater Ether \textgreater Hydrocarbon.

Answer 1

The Correct Option is B

Step 1: Concept - Intermolecular Forces:

Boiling point depends on the strength of intermolecular forces:
1. Hydrogen Bonding: Strongest (Alcohols).
2. Dipole-Dipole Interaction: Moderate (Aldehydes, Ethers). Aldehydes generally have higher polarity than ethers.
3. Van der Waals (Dispersion) Forces: Weakest (Alkanes).

Step 2: Comparing Compounds:

- IV. Ethanol (C₂H₅OH): Has hydrogen bonding. Highest boiling point.
- II. Acetaldehyde (CH₃CHO): Polar molecule (dipole-dipole interaction). No hydrogen bonding.
- I. Dimethyl ether (CH₃OCH₃): Weakly polar (bent structure), but dipole moment is lower than aldehydes. No hydrogen bonding.
- III. Propane (C₃H₈): Non-polar. Only dispersion forces. Lowest boiling point.

Comparison between ether and aldehyde: Aldehydes typically have higher boiling points than isomeric ethers due to greater polarity of the C=O bond compared to the C–O–C bond.

Order: Propane < Ether < Aldehyde < Alcohol.
III < I < II < IV.

Step 3: Final Answer:
Option (B) III < I < II < IV.