Organic Chemistry MCAT Practice Exam

Which functional group is recognized as the strongest acid?

1. Carboxylic acids

2. Phenols

3. Sulfonic acids

4. Alcohol and water

The correct answer is: Sulfonic acids

Sulfonic acids are recognized as the strongest acids among the functional groups listed. This is due to several key factors related to their chemical structure and the stability of their conjugate bases. Sulfonic acids contain a sulfonyl group (–SO₃H), where the sulfur atom is bonded to three oxygen atoms and one hydrogen atom. The strong acidity can be attributed to the resonance stabilization of the sulfonate ion (RSO₃⁻) formed when the acidic proton is lost. Once the proton is ionized, the negative charge is delocalized over the oxygen atoms, significantly stabilizing the conjugate base. This resonance contributes to the overall acidity, as more stable conjugate bases lead to stronger acids. Furthermore, the electronegativity of the oxygen atoms, combined with the ability of the sulfur atom to expand its valence shell, allows for efficient charge distribution, enhancing the acidity of sulfonic acids. This is in contrast to carboxylic acids and phenols, which, while acidic, have less stabilization of their conjugate bases compared to sulfonic acids. In summary, the combination of strong resonance stabilization and effective charge distribution makes sulfonic acids the strongest acids in the given options.