Physical properties of monocarboxylic acids

1. Physical state

The first three aliphatic acids are colorless liquids with pungent smell. The next six are oily liquids with an odor of rancid butter. The higher members are colorless, odorless, waxy solids. Benzoic acid is a crystalline solid.

2. Boiling points

Carboxylic acids have higher boiling point than aldehydes and ketones and even alcohols of comparable masses. This is due to more extensive association of carboxylic acid molecules through intermolecular hydrogen bonding.

intermolecular hydrogen bonding of carboxylic acid

The hydrogen bonds are not broken completely even in the vapor phase. Infact most carboxylic acids exists as dimers in the vapor phase and aprotic solvents.

The O-H bond in carboxylic acid is more polar than O-H bond in alcohols. This is due to electron withdrawing effect of carbonyl group in O-H. Hence H-bonds in carboxylic acids are relatively stronger than those in alcohols.

3. Solubility

Simple aliphatic carboxylic acids having upto four carbon atoms are miscible with water because they can form hydrogen bonds with water.

The solubility decreases with increasing number of carbon atoms. Higher carboxylic acids are practically insoluble in water due to increased influence of the hydrocarbon part.

Benzoic acid, the simplest aromatic carboxylic acid is nearly insoluble in cold water.

Carboxylic acids are also soluble in less polar organic solvents like benzene, ether, alcohol etc.

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