Question: 77. Which one of the following compounds does not decolourize bromine water?
Answer: Option (1)
Explanation:
Bromine water is used to detect unsaturation or compounds that can undergo electrophilic substitution with bromine.
Cyclohexane is a saturated hydrocarbon containing only single \mathrm{C-C} and \mathrm{C-H} bonds.
It does not undergo addition or substitution reactions with bromine water in the absence of light or a catalyst.
Therefore, cyclohexane does not decolourize bromine water.
Phenol reacts readily with bromine water due to the activating effect of the \mathrm{-OH} group on the benzene ring.
It undergoes electrophilic substitution to form tribromophenol, resulting in decolourization of bromine water.
Styrene contains a carbon–carbon double bond \mathrm{C=C}.
It reacts with bromine water by addition across the double bond, leading to decolourization.
Aniline has a strongly activating \mathrm{-NH_2} group.
It reacts rapidly with bromine water via electrophilic substitution to form tribromoaniline, causing decolourization.
Thus, among the given compounds, only cyclohexane does not decolourize bromine water.