Question

Why is acetic acid so soluble in water?

Answer 1

Like methanol and ethanol, to a first approximation, acetic acid is half a water molecule.

Explanation:

The acid functionality on acetic acid, `H_3C-C(=O)OH`, even without dissociation, is effectively solvated by water, and, like methanol, and ethanol, would be soluble in water in all proportions. The carboxyl group is capable of effective hydrogen bonding to the water molecule, and the alkyl chain attached is not so long as to make hydrophobic interaction too great.

I am pretty sure that acetic acid, like methanol, is insoluble in hexanes, and the same reasons apply. On the other hand, ethanol is soluble in hexanes. Can you account for this differential solubility?