Which of the following is an ester?

• A. Ethanoic acid
• B. Ethene
• C. Ethyl ethanoate
• D. Ethyl alcohol

Answer: (C)

An ester has a specific functional group: a carbonyl carbon double-bonded to oxygen and single-bonded to an oxygen that is connected to another carbon chain (-COO-). Ethyl ethanoate contains this -COO- linkage, with the structure CH3-CO-O-CH2-CH3, which makes it an ester. It’s the ester formed from ethanoic acid and ethanol, commonly called ethyl acetate. The other possibilities are a carboxylic acid (ethanoic acid), an alkene (ethene), and an alcohol (ethyl alcohol), none of which have the ester -COO- group.