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Reaction of Alkenes with halogens.

halogenation of alkenes

This reaction occurs rapidly it results in the loss of colour from Iodine and bromine.

Reaction type: Electrophilic Addition

Summary.

  • Overall transformation :  C=C to X-C-C-X
  • Reagent : normally the halogen (e.g. Br2) in an inert solvent like methylene chloride, CH2Cl2, but can also occur as a gas phase reaction.
  • Regioselectivity : not relevant since both new bonds are the same, C-X.
  • Reaction proceeds via cyclic halonium ion.
  • Stereoselectivity : anti since the two C-X bonds form in separate steps one from Br2 the bromide ion Br-.
MECHANISM FOR REACTION OF ALKENES WITH HALOGENS
Step 1:
The p electrons act as a nucleophile, attacking the bromine, displacing a bromide ion but forming a cationic cyclic bromonium ion as an intermediate.
addition of Br2 to C=C
Step 2:
Attack of the nucleophilic bromide from the side away from the bromonium center in an SN2 like fashion opens the cyclic bromonium ion to give overall trans addition.
   

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