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: Understanding the Concept:
Bromine water is decolourised when bromine adds across a carbon-carbon double or triple bond. This is the standard test for unsaturation.

Step 2: Check each compound:
Ethene has a C=C and adds \(\text{Br}_2\) readily. Ethyne has a triple bond and adds \(\text{Br}_2\). Cyclohexene has a C=C in the ring and adds \(\text{Br}_2\).

Step 3: Check benzene:
Benzene has delocalised pi electrons that make it unusually stable (aromatic stabilisation). It does not undergo addition with bromine water under ordinary conditions, so the orange-brown colour remains.

Step 4: Conclusion:
Benzene does not decolourise bromine water.

Final Answer:
Benzene does not decolourise bromine water. \[ \boxed{\text{(C) }\text{Benzene}} \]