Question

Which among the following has lowest boiling point?

• A. Pentanal
• B. Propanal
• C. Methanal
• D. Ethanal

Hint:

Boiling point $\propto$ Molar Mass (for similar functional groups).

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
In a homologous series of aldehydes, the boiling point increases with the increase in molar mass.
Step 2: Detailed Explanation:
As the size of the carbon chain increases, the magnitude of Van der Waals forces (dispersion forces) increases.
- Methanal (\( HCHO \)): 1 Carbon atom (smallest).
- Ethanal (\( CH_{3}CHO \)): 2 Carbon atoms.
- Propanal (\( CH_{3}CH_{2}CHO \)): 3 Carbon atoms.
- Pentanal (\( CH_{3}(CH_{2})_{3}CHO \)): 5 Carbon atoms.
Methanal is actually a gas at room temperature, while the others are liquids. It has the lowest molecular weight and the weakest intermolecular attractions.
Step 3: Final Answer:
Methanal has the lowest boiling point.