Phenol
Aniline reacts with nitrous acid at 273- 278 K followed by treating with dilute acid gives a compound A. Compound A reacted with bromine water gives a Compound B as a white precipitate. Compound A reacts with Zinc dust to give compound C. Compound C reacts with Cl2 in the presence of Lewis acid to give compound D. Further, Compound A undergoes oxidation with chromic acid to give compound E.
What is the structure of compound B?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 2,4,6-tribromophenol
The passage describes a chain of reactions that starts from aniline. Each step, with its full equation:
A (phenol): aniline is diazotised with nitrous acid (from NaNO2 + HCl) at 273–278 K, and the diazonium salt is hydrolysed by warming with dilute acid.
\( \displaystyle \ce{C6H5NH2 + NaNO2 + 2HCl} \xrightarrow{\text{273-278 K}} \ce{C6H5N2^+Cl^- + NaCl + 2H2O} \)
\( \displaystyle \ce{C6H5N2^+Cl^- + H2O} \xrightarrow{\text{warm, dil. acid}} \ce{C6H5OH + N2 ^ + HCl} \)
B (2,4,6-tribromophenol): phenol with bromine water gives a white precipitate. In water, phenol partly ionises to the phenoxide ion, which strongly activates the ring, so all three ortho and para positions are brominated.
\( \displaystyle \ce{C6H5OH + 3Br2} \xrightarrow{\ce{H2O}} \ce{C6H2Br3OH v + 3HBr} \)
C (benzene): phenol heated with zinc dust loses its oxygen.
\( \displaystyle \ce{C6H5OH + Zn} \xrightarrow{\Delta} \ce{C6H6 + ZnO} \)
D (chlorobenzene): benzene with Cl2 and a Lewis acid undergoes electrophilic substitution.
\( \displaystyle \ce{C6H6 + Cl2} \xrightarrow{\text{anhyd. }\ce{FeCl3}} \ce{C6H5Cl + HCl} \)
E (benzoquinone): phenol is oxidised by chromic acid to the conjugated diketone.
\( \displaystyle \ce{C6H5OH + 2[O]} \xrightarrow{\ce{Na2Cr2O7}/\ce{H2SO4}} \underset{\text{benzoquinone}}{\ce{C6H4O2}} + \ce{H2O} \)
Why the other options are incorrect
Option (1) a mixture of 2-bromophenol and 4-bromophenol — Incorrect. This mixture forms when bromine is used in a non-polar solvent (\( \ce{CS2} \) or \( \ce{CHCl3} \)) at low temperature, not with bromine water.
Option (2) 3-bromophenol — Incorrect. The –OH group directs bromine to the ortho and para positions, not the meta position.
Option (4) 2,4-dibromophenol — Incorrect. In bromine water, the third position is also brominated, so the product is the tribromo compound, not the dibromo one.
NCERT (2026-27 edition):
Halogenation
