EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:4.4.2 Bromine & Alkenes

EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:4.4.2 Bromine & Alkenes

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-Addition-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-Test-Alkenes

 

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.

转载自savemyexams

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