Question

Which of the following compounds does not decolourise \(\text{Br}_2\) water?

• A. Ethene
• B. Ethyne
• C. Benzene
• D. Cyclohexene

Hint:

Compounds with C=C or C-triple-C add Br2, but benzene has delocalised pi electrons and resists addition.

Answer 1

The Correct Option is C

Step 1: Use the unsaturation test:
Decolourisation means bromine is used up in an addition reaction. Only compounds with localised pi bonds react this way at room temperature.

Step 2: Sort the four compounds:
Ethene, ethyne and cyclohexene have localised pi bonds. Benzene has a closed loop of 6 delocalised pi electrons that is very stable, and it needs a catalyst such as $\text{FeBr}_3$ for substitution.

Step 3: Pick:
Option C, benzene, is the compound that leaves bromine water unchanged.

Final Answer:
Only benzene lacks a localised C=C bond, so bromine water stays coloured. \[ \boxed{\text{(C) }\text{Benzene}} \]