Which of the following compounds would NOT typically be considered an ester?

Ester compounds are formed from the reaction between an alcohol and a carboxylic acid, resulting in the functional group -COO-. In this context, propanoic acid is a carboxylic acid and does not possess the ester functional group. Instead, it is the starting material used in the formation of esters.

The other compounds listed—ethyl butanoate, methyl methacrylate, and butyl acetate—are all esters. They contain the ester functional group derived from the appropriate alcohol and acid. In summary, propanoic acid is not an ester itself but a precursor to making esters, which is why it is the correct answer in this case.