The usual halogenation of benzene takes place in the
presence of a Lewis acid, such as FeBr3 (Unit 6, Class XII),
which polarises the halogen molecule In case of phenol, the
polarisation of bromine molecule takes place even in the
absence of Lewis acid It is due to the highly activating
effect of –OH group attached to the benzene ring (b) When 
phenol 
is 
treated 
with 
bromine 
water,
2,4,6-tribromophenol is formed as white precipitate