What is the structure of the butanoate ion?

The butanoate ion, also known as butyrate, is derived from butanoic acid by deprotonating the carboxyl group. The correct structure portrays a four-carbon chain with a carboxylate functionality.

In the correct answer, the structure has a chain of three carbon atoms followed by a carbonyl group (C=O) bonded to an oxygen atom that carries a negative charge (indicating it has lost a proton). This results in the formation of the anionic form of butanoic acid, which is the butanoate ion. The presence of the carbonyl group and the negatively charged oxygen is essential in representing the carboxylate ion formation properly.

The other structures do not represent the butanoate ion accurately because they either represent alternate forms (like protonated carboxylic acids or different carbon chain lengths) or lack the proper functional group arrangement needed for the carboxylate structure.