Why is 2-benzoic acid (pKa 4.5) a stronger acid than 2-naphthol
The Correct Answer and Explanation is:
2-Benzoic acid is a stronger acid than 2-naphthol due to the different structures and resonance effects involved in the dissociation of each compound.
In the case of 2-benzoic acid, the carboxyl group (-COOH) can donate a proton (H+) to form the conjugate base (the carboxylate anion, -COO-). The key to its strength as an acid lies in the ability of the conjugate base to stabilize the negative charge on the oxygen atom through resonance. In 2-benzoic acid, the negative charge on the oxygen can delocalize into the aromatic ring, forming a resonance structure that distributes the charge over the ring. This stabilization makes the conjugate base more stable, and as a result, 2-benzoic acid dissociates more readily, making it a stronger acid.
On the other hand, 2-naphthol, which has a hydroxyl group (-OH) attached to a naphthalene ring, is less acidic. While the hydroxyl group can also donate a proton to form the phenoxide anion (the conjugate base), the conjugate base of 2-naphthol is less stabilized than that of 2-benzoic acid. The negative charge on the oxygen in the phenoxide anion is localized on the oxygen atom, and there is no resonance with the aromatic ring to delocalize the charge effectively. As a result, the conjugate base is less stable, making 2-naphthol a weaker acid.
Additionally, the pKa value of 2-benzoic acid (4.5) is lower than that of 2-naphthol (pKa ~9.5), further confirming that 2-benzoic acid is a stronger acid. A lower pKa indicates a stronger acid because it means the compound is more likely to lose a proton.
