The effect of substituents on phenols is not as straightforward as with aliphatic acids. Phenol itself is weakly acidic, with a pKa of 9.95.
Above is shown phenol (left), and the phenolate ion (right).
The strength of phenol as an acid can be increased by the addition of electron-withdrawing groups. For example, we can add a nitro group (a powerful electron withdrawer). This has both inductive (through σ-bonds) and mesomeric (through π-bonds) effects. The inductive effect falls off in the order of ortho, meta, para positions.
These positions are shown below:
i.e. the further away it is, the less of an effect it exerts.
However, mesomeric effects do not follow this pattern. There is no mesomeric effect in the meta position. This can be visualised by realising that the mesomeric effect is exerted through resonance, which can be depicted as follows:
This stabilises the anion by spreading out the charge, and hence makes the acid stronger. As we can see below, we can move the electrons onto the nitro group when it is either the ortho or para positions, but not when it is in the meta position:
Hence, the acid strengths follow this order:
Picric acid, as you can see from the diagram above has three nitro groups, and hence is a strong acid.