What is the functional group characteristic of carboxylic acids?

• A. The carbonyl group C=O
• B. The amino group -NH2
• C. The hydroxyl group -OH
• D. The carboxyl group -COOH

Answer: (D)

Carboxylic acids are defined by the carboxyl group -COOH, a carbon atom double-bonded to an oxygen (C=O) and singly bonded to a hydroxyl group (-OH) on the same carbon. This combination is why they behave as acids: the hydrogen on -OH can be released to form a carboxylate anion, and resonance between the two oxygens stabilizes the negative charge. This distinctive -COOH unit sets carboxylic acids apart from other groups that have just a carbonyl (C=O) or just an -OH, or an amino group.