Question

Give reason for the following:

Carboxylic acids have higher boiling point than alcohols of comparable molecular masses.

Hint:

Boiling point depends on the strength and extent of intermolecular hydrogen bonding.

Answer 1

Concept:
Boiling point depends on the strength and extent of intermolecular hydrogen bonding.

Step 1:
Carboxylic acid molecules form two hydrogen bonds with each other and exist as stable dimers (a cyclic pair held by two O-H...O hydrogen bonds), effectively doubling the molecular size in the liquid.

Step 2:
Alcohols also form hydrogen bonds, but each alcohol molecule forms fewer and the hydrogen bonding is weaker and less extensive than the dimeric association of acids.

Answer: Because carboxylic acids exist as hydrogen-bonded dimers, their intermolecular hydrogen bonding is stronger and more extensive than in alcohols, so more energy is needed to separate them and they have higher boiling points than alcohols of comparable molecular mass.