Bromine & Alkenes (Edexcel IGCSE Chemistry (Modular))

Bromine & alkenes

Bromination of alkenes

• Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond

• When bromine is reacted with an alkene a dibromoalkane is formed

• The reaction between bromine and ethene is an example of an addition reaction and forms dibromoethane 

• The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms across the C=C double bond

Addition of bromine to ethene

Bromine atoms add across the C=C in the addition reaction of ethene and bromine

Testing for alkenes

Bromine water test

• Alkanes and alkenes have different molecular structures

• All alkanes are saturated and alkenes are unsaturated

• The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot

• This allows us to tell alkenes apart from alkanes using a simple chemical test called the bromine water test

  • Bromine water is used in the test for alkenes as it is safer and easier to handle than bromine

• Bromine water is an orange coloured solution

• When bromine water is added to an alkane, it will remain as an orange solution as alkanes do not have double carbon bonds (C=C) so the bromine remains in solution

• But when bromine water is added to an alkene:

  • The bromine atoms add across the C=C bond

  • The solution no longer contains free bromine so it loses its colour / decolourises

Bromine water test

Diagram showing the result of the test using bromine water with alkanes and alkenes