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 (\( \text{C}_2\text{H}_5\text{OH} \)): Has Hydrogen bonding. Highest BP. - II. Acetaldehyde (\( \text{CH}_3\text{CHO} \)): Polar molecule (Dipole-Dipole). No H-bonding. - I. Dimethyl ether (\( \text{CH}_3\text{OCH}_3 \)): Weakly polar (bent structure), but dipole moment is generally lower than aldehydes/ketones. No H-bonding. - III. Propane (\( \text{C}_3\text{H}_8 \)): Non-polar. Only dispersion forces. Lowest BP. 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 arrangement. Order: Propane < Ether < Aldehyde < Alcohol. III < I < II < IV.
Step 3: Final Answer:

Option (B) III \textless I \textless II \textless IV.