Why is the pKa of 2-naphthol around 10 and not 16 like an alcohol?
The Correct Answer and Explanation is:
The pKa of 2-naphthol is around 10, which is significantly lower than that of typical alcohols (around 16). This difference is due to the structure and electronic effects of the aromatic ring in 2-naphthol.
In an alcohol, the hydroxyl group (-OH) is attached to a saturated carbon atom, and when it loses a proton (H+), the resulting negative charge is localized on the oxygen. This can be stabilized through hydrogen bonding or solvation, but there are no additional structural features to further stabilize the negative charge. As a result, alcohols are weak acids with a high pKa, typically around 16.
In contrast, 2-naphthol has a hydroxyl group attached to a benzene ring, and the conjugate base formed after deprotonation is stabilized by resonance. After the proton is removed from the hydroxyl group, the negative charge on the oxygen atom can be delocalized into the aromatic ring through resonance, forming a structure where the negative charge is spread over multiple positions on the ring. This resonance stabilization significantly lowers the pKa compared to alcohols, making 2-naphthol a stronger acid.
Moreover, the aromatic ring provides an additional resonance effect, where the negative charge on the oxygen can interact with the π-electrons of the ring, stabilizing the conjugate base even further. This stabilization makes the proton dissociation easier, thus lowering the pKa to around 10. In essence, the aromatic structure of 2-naphthol provides significant stabilization of the conjugate base, which is absent in simple alcohols, leading to a lower pKa.
