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

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General mechanism for Electrophilic Addition

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