Effect of side groups on phenol acidity

Phenol itself is too weak an acid to react with sodium carbonate. However, it is possible that any attached side chains may affect the acidity of the phenolic group. Picric acid is the trivial name for 2,4,6-trinitrophenol. It was used as a high explosive. By looking at the K_a values we can see that it is quite acidic:

Acid	K_a (mol dm^-3)
Ethanoic acid	1.7 × 10^-5
Picric acid	4.0 × 10^-3
Nitric acid	40	Picric acid

Ethanoic acid will react with sodium carbonate solution, and picric acid is a stronger acid than ethanoic. This is because the nitro group has an electron-withdrawing effect as the nitrogen atom is quite positive in this group.

Groups which withdraw electrons from the ring make the phenol a stronger acid, because they spread out the negative charge on the anion, allowing the hydrogen ion to escape more easily. Groups which donate electrons to the ring (eg alkyl groups) will make the phenol a weaker acid.