Reaction of Alkenes with Halogens

AQA AS-level ChemistryAlkenesLesson 14 of 23

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.

Key terms in this lesson

unsaturated

Containing one or more carbon-carbon double (or other multiple) bonds, able to undergo addition reactions.

More in Alkenes

• Electrophilic Addition
• Reaction with Halogens
• Reaction with Hydrogen Halides
• Asymmetrical Alkenes
• Reaction with COncnetrated Sulfuric Acid
• Asymmetrical Alkenes
• Reaction with Water
• Addition Polymers

All 23 lessons in Alkenes · All AQA AS-level Chemistry topics