Physical Properties of Carboxylic Acids

NCERT Reference: Chapter 8 – Aldehydes, Ketones and Carboxylic and Carboxylic Acids – Page 207

Quick Notes

Carboxylic acids have high boiling points and are often solid at room temperature (especially those with dicarboxylic groups).
They form dimers via intermolecular hydrogen bonding, especially in non-polar solvents.
Solubility in water decreases with increasing alkyl chain length.
Boiling points are higher than alcohols, due to dimerisation and stronger hydrogen bonding.

Carboxylic acids have distinct physical properties due to their ability to form strong hydrogen bonds.

The boiling points of carboxylic acids are much higher than those of aldehydes, ketones, and alcohols of comparable molecular mass.

This is attributed to the formation of dimers via hydrogen bonding between two carboxylic acid molecules.

These dimers are stable even in the vapour phase, significantly increasing the boiling point.

Melting points also tend to be high, especially for dicarboxylic acids and aromatic acids, due to crystal lattice stability.

Shorter chain carboxylic acids (C₁–C₄) are highly soluble in water due to hydrogen bonding with water molecules.

As the alkyl chain length increases, the hydrophobic hydrocarbon portion dominates, leading to decreased solubility.

All carboxylic acids are soluble in alcohols and ethers, but less so in hydrocarbons.

Shorter chain aliphatic acids (e.g. methanoic acid and ethanoic acid) are sharp-smelling liquids.
Higher members (with more than 10 carbons) are waxy solids with a less pungent odour.
Aromatic acids like benzoic acid are crystalline solids at room temperature.

Carboxylic acids have very high boiling points due to strong hydrogen bonding and dimer formation.
Short-chain acids are water soluble; solubility decreases with longer alkyl chains.
Dicarboxylic and aromatic acids have high melting points due to lattice structure.
Low molecular mass acids are liquids with sharp odour, while higher members are solids.