13.2 : カルボン酸の物理的特性

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.