Halogenation

Alkenes react readily with the halogens chlorine and bromine at room temperature. The halogen molecule adds across the C=C bond, with one halogen atom joining each carbon, to form a dihalogenoalkane. The two halogen atoms end up on neighbouring carbon atoms.CH₂=CH₂ + Br₂ → CH₂BrCH₂BrThe product is 1,2-dibromoethane. In the same way, propene and chlorine give 1,2-dichloropropane, CH₃CHClCH₂Cl.Test for unsaturation: shake the compound with bromine water. If a C=C bond is present, the orange bromine water is decolourised (turns colourless) quickly. Alkanes are saturated, so bromine water stays orange. In bromine water, water molecules compete with bromine for the alkene, so the main organic product is actually 2-bromoethanol, CH₂BrCH₂OH, but the decolourisation is still a positive result.Each C=C bond reacts with one Br₂, so the amount of bromine used tells you the number of double bonds.Worked example: 0.0100 mol of a hydrocarbon reacts completely with 3.20 g of Br₂ (M_r 159.8).
Amount of Br₂ = 3.20 ÷ 159.8 = 0.0200 mol.
Ratio Br₂ : hydrocarbon = 0.0200 : 0.0100 = 2 : 1, so each molecule contains two C=C bonds.