Question

My teacher taught yesterday that though resonance happens in phenoxide ion, it is less stable because the negative charge gets distributed over the carbons in the ring. And hence, alcohols are more acidic than phenols.

His argument seemed convincing at the time but then I searched the Internet about it and everywhere it says that phenol is more acidic than alcohol.

So which is it?

Answer 1

Phenols are more acidic than alcohols due to resonance stabilization of the phenoxide ion, with phenols having a pKa around 10 and alcohols around 16. However, carboxylic acids are even more acidic than phenols, as the negative charge in their conjugate base is shared with a more electronegative oxygen.

Step-by-step explanation:

The confusion about the acidity of phenols and alcohols stems from the resonance stabilization present in phenols that is not found in alcohols. Phenols are indeed more acidic than alcohols because the negative charge on the phenoxide ion resulting from deprotonation is delocalized over the aromatic ring, which stabilizes the conjugate base. In contrast, in alcohols, the negative charge is localized on the oxygen atom, making it less stable compared to the phenoxide ion.

Resonance stabilization plays a critical role in increasing the acidity of phenols. The phenolic O-H bond is more willing to release a proton compared to the alcoholic O-H bond because the phenoxide ion is stabilized through resonance. This effect is quantifiable by comparing the pKa values; phenol has a pKa of around 10, while alcohols have higher pKa values, typically around 16, indicating that phenols are stronger acids.

However, compared to carboxylic acids, phenols are weaker acids because in carboxylic acids the negative charge is shared by a more electronegative oxygen, rather than less electronegative carbons, as in phenols. Carboxylic acids have pKa values around -5, much lower than those of phenols, reflecting their greater acidity.