Bromine & Alkenes (Edexcel IGCSE Chemistry: Double Science)
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Bromine & Alkenes
- Alkenes are a homologous series of hydrocarbon compounds with at least one double bond between two of the carbon atoms on the chain
- The double bond can be written as carbon carbon double bond or as C=C
- The general formula for alkenes is:
CnH2n
- Alkenes are generally more desirable than alkanes as they are more reactive due to the presence of the carbon-carbon double bond, so they can take part in reactions in which alkanes cannot, making them more useful than alkanes
- They are used to make polymers and are the starting materials for the production of many other chemicals
- Two useful reactions are the bromination of alkenes and polymerisation
Bromination of Ethene
- Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond
- The reaction between bromine and ethene is an example of an addition reaction
- 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
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
Diagram showing the result of the test using bromine water with alkanes and alkenes
- 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, hence the solution no longer contains free bromine so it loses its colour
Exam Tip
Alkenes are more reactive than alkanes due to the presence of the carbon-carbon double bond which contains an area of high electron density.
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