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Aqueous halogen reacton

halohydration of alkenes

Reaction type: Electrophilic Addition

The reaction is initially similiar to the reaction with the simple halogen, however there are many more water molecules than there are halide ions so step 2 of the reaction is different. 

Summary.

  • Overall transformation :  C=C to HO-C-C-X
  • Reagents : X2 / H2O or HOX (hypohalous acid)
  • Reaction proceeds via cyclic halonium ion (compare with halogenation)
  • In the presence of an alternative nucleophile, water opens the halonium ion.
  • Regioselectivity : X reacts as the electrophile so the C-O bond forms at the more stable cation center.
  • Stereoselectivity : anti since the two new s bonds form in separate steps.
MECHANISM FOR REACTION OF ALKENES WITH Br2 / H2O
 
Step 1:
Same first step as for the reaction of Br2/CH2Cl2.
The p electrons act as a nucleophile, attacking the
bromine, displacing a bromide ion but forming a
cationic cyclic bromonium ion as an intermediate.
formation of a bromohydrin by reaction of Br2 in H2O with C=C
Step 2:
Attack of the nucleophilic water molecule from the side away from the bromonium center in an SN2 like fashion opens the cyclic bromonium ion to give overall trans addition.
Step 3:
An acid / base reaction converts the oxonium into the alcohol.
   

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