Bromine Water Test for Alkenes

The addition of bromine to a C=C gives a simple test for unsaturation. Shake the compound with bromine water, which is orange:

• An alkene (or any compound with a C=C) decolourises it: the colour changes from orange to colourless, because the Br₂ is used up as it adds across the double bond.
• An alkane is saturated, so it does not react and the bromine water stays orange.

This test tells cyclohexene (C₆H₁₀) apart from cyclohexane (C₆H₁₂). Both are colourless liquids that look alike, but only cyclohexene decolourises bromine water, forming 1,2-dibromocyclohexane.

Counting double bonds. One mole of Br₂ adds across each C=C. So measuring how much bromine a compound reacts with tells you how many double bonds each molecule has.

Worked example: 0.010 mol of a hydrocarbon reacts with exactly 0.020 mol of Br₂. The ratio is 0.020 ÷ 0.010 = 2 mol Br₂ per mole, so each molecule has two C=C bonds.

A diene therefore adds two Br₂ molecules. Buta-1,3-diene, CH₂=CHCH=CH₂, with excess bromine gives CH₂BrCHBrCHBrCH₂Br, 1,2,3,4-tetrabromobutane, with one Br on every carbon.

The same idea is used to compare how unsaturated fats and oils are: the more bromine a sample decolourises, the more C=C bonds it contains.