Explain the mechanism of Bromination of Phenol

Bromination phenol

Phenol on treatment with chlorine or bromine water gives an immediate precipitate of 2,4,6−trihalogen derivative, when treated with bromine water it gives bromination of phenol. Phenol in aqueous medium is partially ionised and the phenoxide ion thus obtained is much more reactive than phenol itself towards electrophilic attack. Moreover, halogen reacts with water forming halogen acid and hypohalous acid. The proton attacks at the OH group of hypohalous acid to give H₂O⊕−X, which acts as a stronger electrophile. Therefore, halogenation takes place at all the ortho and para positions. Check out Chemistry Formulas and NCERT Solutions for class 11 Chemistry prepared by Physics Wallah. 

Br₂  + H₂O  H−Br  + HO−Br  HO−Br 

If the halogenation is carried out in non-aqueous medium like CS₂ or CCl₄, only mono−substitution takes place. This is because the ionisation of phenol does not occur in non-aqueous medium. The benzene ring of phenol is less activated than that of phenoxide ion as well as the bromine in Br₂ molecule is not as electrophilic as in 

o−Bromophenol is also prepared by protecting one ortho and the para positions by sulphonation.

Mechanism of Bromination

So strong is the activation of benzene ring in aqueous medium that derivatives of phenol containing −COOH group or −SO₃H group either at the ortho or at the para position are displaced by Br in bromination reaction. This is an example of brominative decarboxylation. For example, salicylic acid when treated with bromine water gives 2,4,6−trisubstituted phenol.

In case when SO₃H group is present at ortho and para positions, desulphonation takes place liberating SO₃ gas.

Trisubstitution of benzene is also observed when aniline is treated with bromine because the reactivity of aniline is same as that of phenoxide ion towards electrophilic attack.