Question

Propanoic acid exists as dimer in which of the following:
(a) Vapour phase (b) Aqueous solvent (c) Aprotic solvent

• A. a, b
• B. a, b, c
• C. b, c
• D. a, c

Hint:

Carboxylic acids form dimers through intermolecular hydrogen bonding in vapour phase and non-polar or aprotic solvents, but not in water because water disrupts acid-acid hydrogen bonding.

Answer 1

The Correct Option is D

Step 1: Dimerization via hydrogen bonding.
Carboxylic acids like propanoic acid associate into dimers through two intermolecular hydrogen bonds.

Step 2: Behavior in vapor phase.
In vapor, without competing solvent interactions, propanoic acid readily forms dimers.

Step 3: Behavior in aqueous solution.
Water molecules compete for hydrogen bonds, disrupting acid–acid association; dimers do not persist.

Step 4: Behavior in aprotic solvents.
Aprotic solvents lack strong hydrogen-bond-donating ability, allowing the dimer to remain intact.

Step 5: Conclusion.
Propanoic acid exists as a dimer in vapor phase and in aprotic solvents.