Which one of the following compounds does not decolourize bromine water?

CONCEPT:

Reaction of Bromine Water with Compounds

• Bromine water is commonly used to test for unsaturation in organic compounds. Unsaturated compounds like alkenes and alkynes decolorize bromine water by reacting with it to form dibromides or addition products.
• However, compounds that are either saturated or have functional groups that do not undergo addition reactions with bromine will not decolorize bromine water.
• In this question, we are looking for a compound that does not react with bromine water and hence does not decolorize it.

EXPLANATION:

• Option 1: Cyclohexane (C6H12): Cyclohexane is a saturated hydrocarbon (alkane). Alkanes do not undergo addition reactions with bromine water because they do not have double or triple bonds. Therefore, it will not decolorize bromine water.
• Option 2: Phenol (C6H5OH): Phenol has a hydroxyl group attached to an aromatic ring. While phenol does not have double bonds like alkenes or alkynes, the hydroxyl group can act as an electron-donating group, but phenol can still decolorize bromine water by reacting with it, especially under certain conditions (e.g., electrophilic substitution).
  
• Option 3: Styrene (C6H5CH=CH2): Styrene contains a C=C double bond (alkene), which can easily react with bromine water, decolorizing it by undergoing an addition reaction to form a dibromide.
  
• Option 4: Aniline (C6H5NH2): Aniline has an amine group (-NH2), which is electron-donating, and can participate in reactions with bromine, leading to the decolorization of bromine water by forming substitution products like brominated aniline.
  

Therefore, the compound that does not decolorize bromine water is Cyclohexane (Option 1), as it is a saturated alkane and does not have any reactive sites like double bonds or groups that could react with bromine.

Thus, the correct answer is Option (1): Cyclohexane.