Specification Point 4.27:
Describe the reactions of Alkenes with Bromine to produce Dibromoalkanes

Reactions of Alkenes with Bromine

Alkenes: A homologous series of hydrocarbon compounds with double carbon bonds that has the general formula CnH2n.

Reaction between Alkenes and Bromine:

The reaction between bromine and alkenes is an example of a type of reaction called an addition reaction. This reaction is an addition reaction as the diatomic bromine atom has been added across the double bond of the ethene.

Example: Addition reaction of ethene and bromine

Ethene and bromine reaction, Edexcel IGCSE Chemistry

Ethene   +   Bromine   →   Dibromoethane

 

    C2H4               Br2                      C2H4Br2

Specification Point 4.28:
Describe how Bromine water can be used to distinguish between an Alkane and an Alkene

Alkanes: A homologous series of hydrocarbon compounds with only single carbon bonds that has the general formula CnH2n+2

Alkenes: A homologous series of hydrocarbon compounds with double carbon bonds that has the general formula CnH2n

Use of Bromine Water to distinguish between Alkane & Alkene

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:

Distinguish alkanes and alkenes, Edexcel IGCSE Chemistry

The result of the test using bromine water with alkanes and alkenes

Explanation:

  • Bromine water is an orange solution of bromine.
  • When bromine water is shaken with an Alkane, it will remain as an orange solution as alkanes do not have double carbon bonds (C=C).
  • But when bromine water is shaken with an alkene, the alkene will decolourise the bromine water and turn colourless as alkenes do have double carbon bonds (C=C).
  • This reaction between alkenes and bromine is called an addition reaction.

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.