Physical Properties of Carboxylic Acids

Carboxylic acids form stable dimers through strong intermolecular hydrogen bonds between their polar carbonyl and hydroxyl groups, effectively doubling molecular weight. These dimers have equilibrium constants as large as 10^6–10^7 M^−1, much higher than alcohol dimers. This dimer formation, combined with dipole-dipole interactions, results in significantly higher boiling points compared to aldehydes, ketones, and alcohols.

Lower-molecular-weight carboxylic acids are liquids at room temperature, while saturated carboxylic acids with more than eight carbons are typically solids. This trend occurs because increased chain length enhances London forces and enables closer molecular packing. Unsaturated carboxylic acids have lower melting points than their saturated counterparts; for example, stearic acid melts at 70°C while linoleic acid melts at −5°C.

Lower-molecular-weight carboxylic acids are readily soluble in water due to hydrogen bonding interactions with water molecules. As the hydrophobic hydrocarbon chain increases, water solubility decreases because the nonpolar portion dominates. Dicarboxylic acids, with two additional hydrogen bonds, remain fairly water-soluble despite longer chains, demonstrating how hydrogen bonding strength influences solubility.

Long-chain carboxylic acids dissolve readily in alcoholic solvents due to London forces in addition to hydrogen bonding interactions. The nonpolar hydrocarbon chain interacts favorably with the alkyl portion of alcohols, while the carboxylic acid group forms hydrogen bonds with the hydroxyl group. This combination of intermolecular forces makes alcohols better solvents than water for these compounds.

In nonpolar solvents like carbon tetrachloride, carboxylic acids exist predominantly in their dimer form, making them highly soluble. The absence of competing polar interactions allows the stable dimers to persist and dissolve readily. This behavior contrasts sharply with polar solvents, where hydrogen bonding with the solvent can disrupt dimer formation.

Saturated carboxylic acids bearing long nonpolar hydrocarbon chains have higher melting points than unsaturated acids due to increased London forces and close packing of molecules. The rigid structure of saturated chains allows tighter molecular arrangement, whereas cis double bonds in unsaturated acids create kinks that disrupt packing. This structural difference significantly lowers melting points in unsaturated carboxylic acids.

Dicarboxylic acids have relatively higher melting points than monocarboxylic acids because they possess two carboxylic acid groups capable of forming additional hydrogen bonds. These extra hydrogen bonding interactions increase intermolecular attractions and stabilize the solid state. Some dicarboxylic acids remain fairly water-soluble due to their substantial hydrogen bonding interaction with water molecules.