Qualitative Tests for Alkenes

Alkenes contain a carbon−carbon double bond, C=C. The test for this unsaturation is bromine water.Add a few drops of bromine water to the sample and shake.• Positive result: the orange bromine water decolourises (turns colourless).
• Negative result: the bromine water stays orange, e.g. with an alkane.The colour change happens because bromine undergoes electrophilic addition across the C=C bond. The coloured Br₂ is used up, forming a colourless dibromo compound. With ethene:CH₂=CH₂ + Br₂ → CH₂BrCH₂BrThe product is 1,2-dibromoethane: one bromine atom has added to each carbon of the former double bond.Worked example: a student tests cyclohexane and cyclohexene with bromine water. Only cyclohexene decolourises it, because only cyclohexene has a C=C bond. The product is 1,2-dibromocyclohexane.Take care interpreting the test: phenol also decolourises bromine water, but it gives a white precipitate at the same time (substitution on the ring), so the two can be told apart. Benzene does not decolourise bromine water, because its delocalised electrons make it much less reactive than an alkene.The test works with any C=C bond, so it is also used to show unsaturation in larger molecules such as unsaturated fats.