Alkanoic acids have higher boiling points than alkanols because

Answer Details

The boiling point of a substance depends on the strength of the intermolecular forces that must be overcome to convert it from liquid to gas. Stronger intermolecular forces mean higher boiling points.

Alkanoic acids (carboxylic acids, R-COOH) have higher boiling points than alkanols (alcohols, R-OH) of comparable molecular mass because alkanoic acids form stronger hydrogen bonds.

The -COOH group in a carboxylic acid can form two hydrogen bonds simultaneously with another carboxylic acid molecule. This is because the -COOH group has both a highly polar O-H bond and a C=O group that can act as a hydrogen bond acceptor. Carboxylic acids commonly exist as dimers, with two molecules linked by a pair of strong hydrogen bonds:

\[\text{R-COOH} \cdots \text{HOOC-R}\]

In contrast, the -OH group in an alkanol can only form one hydrogen bond per molecule. The hydrogen bonding in alcohols is therefore less extensive than in carboxylic acids.

The other options are incorrect:

• Saying alkanoic acids are more volatile would imply they have lower boiling points, which contradicts the question.
• Alkanols are not more polar than alkanoic acids; carboxylic acids are more polar.
• For molecules of comparable chain length, the size difference is not significant enough to explain the boiling point gap.