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.
• 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.