Given the compound ${\mathrm{CH}}_3\mathrm{COOH}$ (acetic acid), identify the most acidic proton.

• A. The proton attached to the carboxyl group hydroxyl oxygen atom
• B. The proton attached to the carboxyl group carbon atom
• C. The proton attached to the methyl group carbon atom
• D. The proton attached to the carboxyl group oxygen atom

Answer: (A)

Verified step by step guidance

1. Identify the different types of protons present in acetic acid (CH\_3COOH): the protons on the methyl group (CH\_3), the proton on the hydroxyl oxygen (OH) of the carboxyl group, and any other hydrogens attached to the carboxyl carbon (which in this case, there are none).
2. Recall that acidity is related to the ability of a proton to be donated and the stability of the conjugate base formed after deprotonation. More acidic protons are easier to lose and result in a more stable conjugate base.
3. Consider the resonance stabilization of the conjugate base formed when the proton is lost. The carboxylate ion formed after losing the proton from the hydroxyl oxygen is resonance stabilized, which greatly increases acidity.
4. Compare the acidity of protons on the methyl group and the hydroxyl oxygen. Protons on alkyl groups (like methyl) are generally not acidic because their conjugate bases are not stabilized, whereas the hydroxyl proton in the carboxyl group is acidic due to resonance stabilization of the conjugate base.
5. Conclude that the most acidic proton in acetic acid is the one attached to the hydroxyl oxygen atom of the carboxyl group, because its removal leads to a resonance-stabilized carboxylate ion.