(b) Phenols when treated with bromine water gives polyhalogen derivatives in which all the
hydrogen atoms present at ortho and para positions with respect to —OH group are replaced by bromine atoms.

However, in non-aqueous medium such as ${\mathrm{CS}}_2,{\mathrm{CCl}}_4,{\mathrm{CHCl}}_3$ monobromophenols are obtained.

In aqueous solution, phenol ionises to form phenoxide ion. This ion activates the benzene
ring to a very large extent and hence the substitution of halogen takes place at all three
positions.

On the other hand, in non-aqueous solution ionisation of phenol is greatly suppressed.
Therefore, ring is activated slightly and hence monosubstitution occur.

(c) Lewis acid is an electron deficient molecule. In bromination of benzene, Lewis acid is
used to polarise ${\mathrm{Br}}_2$ to form ${\mathrm{Br}}^{+}$ electrophile.

In case of phenol, oxygen atom of phenol itself polarises the bromine molecule to form ${\mathrm{Br}}^{+}$ ion (electrophile). So, Lewis acid is not required in the bromination of phenol.

Mechanism of bromination of phenol

Mechanism of bromination of benzene