Concept: Dimerization of carboxylic acids in non-polar solvents occurs due to strong intermolecular hydrogen bonding.

Why (B) is correct:

Carboxylic acids, like acetic acid ($C{H}_{3}COOH$), have both a carbonyl group ($C=O$) and a hydroxyl group ($-OH$). In non-polar solvents such as benzene, they do not ionize. Instead, two acetic acid molecules can form strong intermolecular hydrogen bonds between the carbonyl oxygen of one molecule and the hydroxyl hydrogen of another, and vice-versa. This results in the formation of a stable cyclic dimer structure.

The structure of the acetic acid dimer is:

$C{H}_3-C(=O)-OH\cdots O=C(C{H}_3)-OH$

This dimerization effectively doubles the molecular weight of acetic acid in benzene, which can be observed through colligative properties like elevation in boiling point or depression in freezing point.

Option Analysis:

• A) Condensation reaction: A condensation reaction involves the joining of two molecules with the elimination of a small molecule (like water). Dimerization via hydrogen bonding does not involve the elimination of any molecule.
• C) Presence of carboxyl group: While the carboxyl group is essential for hydrogen bonding, simply its presence doesn't explain dimerization. It's the specific interaction (hydrogen bonding) between two carboxyl groups that leads to dimerization.
• D) None of the above: Since hydrogen bonding is the correct reason, this option is incorrect.

Correct Answer: (B)