Question

Why is water solubiilty of benzoic acid significantly less than the water solbilty of the benzoate ion? Briefly explain and relate to the partition coefficient (kD).

Answer 1

Benzoic acid is first and foremost a weak acid and thus dissociates partially in water:

C₆H₅COOH + H₂O <---> C₆H₅COO^- + H₃O⁺

From this equation and the structure of benzoic acid, we can tell three things:-

In a neutral solution, benzoic acid is slightly soluble in water. It is nearly insoluble in cold water but highly soluble in hot water. This is because even though water can form hydrogen bonds with the carboxylate functional group, the bulk of the benzoic acid is still the large benzene ring, which can only form Van der Waals forces of attraction with water (significantly weaker than hydrogen bonding). Also, even though benzoic acid can dissociate in water, as it is a weak acid, it can only dissociate partially, which is insufficient to dissolve benzoic acid. Thus, benzoic acid is insoluble in cold water. However, as temperature increases, more energy is present to overcome the hydrogen bonding between water molecules to form weaker Van der Waals forces of attraction between benzoic acid and water. Also, more energy is present to drive the dissociation of benzoic acid forward, as it is endothermic, thus benzoic acid becomes more soluble in hot water.