AQA A Level Chemistry

Revision Notes

3.7.5 Electrophilic Addition

Mechanism: Electrophilic Addition

Electrophilic addition

  • Alkenes undergo electrophilic addition reactions
  • In an electrophilic addition reaction, two reactants form only one product
    • So, electrophilic addition reactions will have a 100% atom economy
  • It is the double bond in an alkene which makes them so reactive
  • The C=C double bond is an electron-rich area of the molecule which is readily attacked by positively charged electrophiles
  • Alkenes will undergo electrophilic addition reactions with hydrogen halides, halogens and concentrated sulfuric acid with steam

 

General mechanism for Electrophilic Addition

Halogen Compounds Electrophilic Addition to Alkenes, downloadable AS & A Level Chemistry revision notes

Electrophilic addition of hydrogen halides or halogens at room temperatures to alkenes results in the formation of halogenoalkanes

  • When the reaction takes place with an asymmetrical alkene, then you need to determine which product will form
  • This depends on the stability of the carbocation formed as the intermediate
  • The stability of carbon carbocations is as follows:
    • Tertiary > secondary > primary
  • The major product formed will be from the intermediate with the more stable carbocation, but some of the product from the less stable carbocation intermediate will also form
  • In the mechanism above, the secondary halogenoalkane is the major product, because a secondary carbocation is more stable than a primary carbocation
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