Question

Why do propanoic and butanoic acid and water mix but hexanoic acid and water do not?

Answer 1

Propanoic acid contains the carbonyl group and hydroxyl group with a smaller number of carbon atoms in the hydrocarbon chain. These polar group forms hydrogen bonds with water molecules. Due to this hydrogen bonding, the propanoic acid is soluble in water.

With increasing carbon atoms in molecules, solubility decreases. Hence pentanoic acid and butanoic acid are most soluble in water.

Whereas in case of hexanoic acid, the hydrocarbon chain contains six carbon atoms, which increases the non-polar characteristics in hexanoic acid. Therefore, hexanoic acids become less soluble in water.

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