I was asked which of the two following isomeric compounds, a carboxylic acid and an ester, was more soluble in water.

Butanoic acid and ethyl acetate

I believe both molecules should be somewhat soluble in water as they have polar C=O and C–O bonds, which should allow them to form dipole–dipole interactions with water. However, I'm not sure how the difference in the structures would affect the solubility.


Try to solve the question by analogy. Imagine that you perform a couple of experiments on different glasses of water:

  1. Add vinegar to one glass with water. Does it mix? Yes, it does!
  2. Add olive oil to another glass with water? Does it mix? No, it does not.

Vinegar is a carboxylic acid, whereas olive oil is made of esters. This should suggest to you that carboxylic acids in general are more soluble than esters.

Carboxylic acids, $\ce{R-COOH}$, are acids, that partly dissociate in water

$$\ce{R-COOH + H2O -> H3O+ + R-COO-}$$

to form ions. Supposing that the alkyl chain isn't extremely long, even the undissociated part of the acid added is solvated in water, a polar protic solvent, by hydrogen bonding interactions. In other words, the carboxylic acid is highly soluble in water.

The solubility of ethyl acetate in water is 8.3 g/100 ml (Wikipedia), whereas butanoic acid is completely miscible in water. That is to say, butanoic acid completely dissolves in water, regardless of the proportion of acid to water.


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