Which of the following functional groups typically behaves as an acid?

• A. Carbonyl group (C=O)
• B. Amino group (NH2)
• C. Carboxyl group (COOH)
• D. Hydroxyl group (OH)

Answer: (C)

Verified step by step guidance

1. Understand that acidity in functional groups is related to their ability to donate a proton (H\textsuperscript{+}).
2. Recall that the carboxyl group (COOH) contains a hydrogen atom attached to an oxygen, which can be released as a proton, making it acidic.
3. Recognize that the carbonyl group (C=O) itself does not have a hydrogen to donate, so it typically does not behave as an acid.
4. Note that the amino group (NH2) tends to accept protons rather than donate them, so it behaves as a base rather than an acid.
5. Understand that the hydroxyl group (OH) can be weakly acidic, but it is generally less acidic than the carboxyl group due to the stability of the conjugate base formed after proton loss.