Electrophilic substitution of phenol

The -OH group attached to the benzene ring in phenol activates, it towards electrophilic substitution. It directs the incoming group to ortho and para positions in the ring as these positions become electron rich due to electronic effect (also mesomeric effect) caused by -OH group.

Common electrophilic aromatic substitution reactions taking place in phenol are as follow:

(i) Nitration - with dilute nitric acid at low temperature (298 K), phenol yields a mixture of ortho and para nitro phenols.

The ortho and para isomers can be separated by steam distillation O-Nitro phenol is steam volatile due to intra molecular hydrogen bonding while p-nitrophenol is less volatile due to intermolecular hydrogen bonding, which causes association of molecules.

With concentrated nitric acid, phenol is converted to 2,4,6 trinitrophenol. This reaction involves nitration accompanied by oxidation of Phenol.

Industrially, picric acid is prepared by treating phenol first with conc.H₂SO₄ which converts it to phenol 2,4 disulphonic acid. This yields picric acid on treatment with conc. HNO₃.