Which of the following compounds would react rapidly with bromine?

Step 1: Analyze the question We need the compound that reacts rapidly with bromine (usually bromine water or bromine in an inert solvent like CCl₄) under ordinary conditions.

Step 2: The concept Bromine reacts quickly with unsaturated compounds (those with C=C double bonds or C≡C triple bonds) by an addition reaction. The reddish-brown colour of bromine disappears almost instantly. This is the standard laboratory test for unsaturation.

Step 3: Examine each option

• A. Benzene (C₆H₆): It is unsaturated on paper, but its electrons are delocalised, which makes it very stable. It does not decolourise bromine water. It only reacts with bromine by substitution, and that needs a catalyst such as FeBr₃.
• B. Hexane (C₆H₁₄): A saturated alkane. It reacts with bromine only slowly, and only in the presence of UV light (substitution). It does not react rapidly in the dark.
• C. Hexene (C₆H₁₂): An alkene with a C=C double bond. It undergoes rapid addition with bromine:

C₆H₁₂ + Br₂ → C₆H₁₂Br₂ (1,2-dibromohexane)

The brown colour is decolourised immediately.

• D. Hexanol (C₆H₁₃OH): A saturated alcohol. It has no C=C bond, so it does not react rapidly with bromine.

Step 4: Answer Hexene reacts rapidly with bromine because of its reactive double bond.

Exam tip: "Decolourises bromine water quickly" means an alkene or alkyne (unsaturated). Remember that benzene does not behave like a typical alkene, even though it looks unsaturated, and this trap appears often in WASSCE and JAMB.