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.