Assertion (A): Phenol reacts with bromine water to give a white precipitate of 2,4,6-tribromophenol without the use of a Lewis acid such as FeBr3.
Reason (R): The –OH group attached to the benzene ring strongly activates the ring towards electrophilic substitution.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
The halogenation of benzene needs a Lewis acid such as FeBr3, which polarises the bromine molecule into an electrophile. Phenol is far more reactive than benzene.
Assertion (A): Phenol reacts with bromine water to give a white precipitate of 2,4,6-tribromophenol without the use of a Lewis acid such as FeBr3. — True. With bromine water, all three ortho and para positions are substituted at once:
\( \displaystyle \ce{C6H5OH + 3Br2 -> C6H2Br3OH v + 3HBr} \)
Reason (R): The –OH group attached to the benzene ring strongly activates the ring towards electrophilic substitution. — True. By its resonance effect the –OH group makes the ring, mainly the ortho and para positions, rich in electrons.
Why R explains A?
Because the –OH group activates the ring so strongly, the bromine molecule is polarised even in the absence of a Lewis acid, and the electron-rich ortho and para positions are all brominated.
So R gives the cause of A.
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “1. Electrophilic aromatic substitution … The –OH group attached to the benzene ring activates it towards electrophilic substitution. Also, it directs the incoming group to ortho and para positions in the ring as these positions become electron rich due to the resonance effect caused by –OH group.”
“…The usual halogenation of benzene takes place in the presence of a Lewis acid, such as FeBr3 …, 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.”