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Aqueous halogen
reacton
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
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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. |
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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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