Which one of the following compounds does not decolourize bromine water?
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Answer
(3) [figure]
Solution
As per NCERT, decolourisation of bromine water is a test for unsaturation; activated arenes (phenol, aniline) also decolourise it by electrophilic substitution.
(1) Styrene: Br2 adds across the C=C bond of the side chain, so the colour disappears.
(2) Aniline: the strongly activating -NH2 group gives 2,4,6-tribromoaniline (white precipitate).
(3) Cyclohexane: a saturated hydrocarbon; it neither adds nor substitutes bromine under these conditions, so the colour stays.
(4) Phenol: gives 2,4,6-tribromophenol (white precipitate).
Answer: (3)