Effects of substituents on the acidity of phenols:
Existence of electron attracting group, (for example -NO2, -X, - NR+3, -CN, -CHO, -COOH) on the benzene ring increases the acidity of phenol since it enables the ring to draw much more electrons from the phenoxy oxygen and so releasing easily the proton. Additionally, the particular effect is more while the substituent is available on o- or p-position as compared to in m-position to the phenolic group.