Bromination of phenol with bromine water gives product A while bromination of phenol with Br2-CS2 and at low temperature gives product B as a major product. Identify the products A and B in the reaction.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A is 2,4,6-tribromophenol while B is 4-bromophenol
The –OH group of phenol is strongly activating and ortho/para directing: its lone pair pushes electron density into the ring by resonance. So phenol undergoes bromination even without a Lewis acid, and the solvent decides how far the reaction goes:
Br2 in a non-polar solvent (CS2 or CHCl3) at low temperature (273 K): phenol hardly ionises, and the \( \ce{Br2} \) molecule is only weakly polarised. The reaction is mild and gives monobromophenols, mainly the para isomer (the ortho positions are more hindered).
\( \displaystyle \ce{C6H5OH} \xrightarrow[\text{273 K}]{\ce{Br2}\text{ in }\ce{CS2}} \text{p-bromophenol (major)} + \text{o-bromophenol (minor)} + \ce{HBr} \)
Bromine water: in water, phenol partly ionises to the phenoxide ion, which is even more strongly activated, and water polarises \( \ce{Br2} \). All three ortho and para positions are substituted at once, giving a white precipitate of 2,4,6-tribromophenol.
\( \displaystyle \ce{C6H5OH + 3Br2} \xrightarrow{\ce{H2O}} \ce{C6H2Br3OH v + 3HBr} \)
In this question the labels are the other way round: A is formed in bromine water and B in \( \ce{CS2} \). So A is 2,4,6-tribromophenol and B is 4-bromophenol (p-bromophenol).
Why the other options are incorrect
Option (1) A is 4-bromophenol, while B is 2,4,6-tribromophenol. — Incorrect. The two products are interchanged. Tribromination needs the strong activation available in water, not the milder conditions in \( \ce{CS2} \).
Option (2) A is 2,4-dibromophenol, while B is 2-bromophenol. — Incorrect. Tribromination, not dibromination, takes place in bromine water; and in \( \ce{CS2} \) the major product is the para isomer, not 2-bromophenol.
Option (4) A is 2-bromophenol and B is 2,4,6-tribromophenol. — Incorrect. In water all three positions are substituted, so A cannot be a monobromo compound; and tribromination does not occur in \( \ce{CS2} \) at low temperature.
NCERT (2026-27 edition):
Halogenation
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 20-05 S2 12-05 S2 | – | – | – | – |