13.2: Physical Properties of Carboxylic Acids

Carboxylic acids with lower molecular weight exhibit a sharp and unpleasant odor. They also have higher boiling and melting points than analogous compounds, such as aldehydes, ketones, and alcohols.

In addition to the dipole–dipole interaction of the polar carbonyl and hydroxyl bonds, strong intermolecular hydrogen bonding between carboxylic acid molecules dictates several physical properties.

The strong interaction causes the carboxylic acid molecules to exist in stable dimers, increasing their molecular weight two-fold. Consequently, they have higher boiling points. These carboxylic acid dimers have much higher equilibrium constants (as large as 10⁶–10⁷ M⁻¹) than alcohol dimers (11 M⁻¹).

A similar trend is observed with the melting points of carboxylic acids. The melting point increases with the increase in hydrocarbon chain length. The saturated carboxylic acids with more than eight carbons are typically solids at room temperature. However, unsaturated carboxylic acids have lower melting points. For example, stearic acid (C₁₈H₃₆O₂), a saturated carboxylic acid, melts at 70 °C, while linoleic acid (C₁₈H₃₂O₂), bearing two cis double bonds, melts at −5 °C.

In terms of solubility, low molecular weight carboxylic acids are soluble in water, owing to their hydrogen bonding interaction with water molecules. In contrast, higher molecular weight carboxylic acids are more soluble in alcohols due to the higher van der Waals interaction between the molecules. Notably, carboxylic acids are highly soluble in non-polar solvents like CCl₄. Interestingly, some dicarboxylic acids are fairly water-soluble due to their substantial hydrogen bonding interaction.

Carboxylic acids have polar –C=O and –OH groups, ensuring dipole-dipole interactions between two molecules.

Further, the polar groups are involved in two strong intermolecular hydrogen bonds to form stable dimers, effectively doubling the molecular weights.

This results in a higher boiling point for carboxylic acids than for analogous compounds.

In general, lower-molecular-weight carboxylic acids are liquid, while higher-molecular-weight carboxylic acids are solid at room temperature.

Saturated carboxylic acids bearing long nonpolar hydrocarbon chains have higher melting points than unsaturated acids due to increased London forces and close packing.

Dicarboxylic acids, with two additional hydrogen bonds, have relatively higher melting points.

Carboxylic acids also form hydrogen bonds with water. Lower-molecular-weight carboxylic acids are readily soluble in water.

As the hydrophobic hydrocarbon chain increases, water solubility decreases. Such long-chain carboxylic acids can easily dissolve in alcoholic solvents due to the London forces in addition to hydrogen bonding.

If the solvent is nonpolar, acids exist in their dimer form, making them quite soluble.