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CHM 136H

Synthesis and Reactions of Alkenes
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Chapter 8: Alkene Reactions
- alkene synthesis
- stereochemistry and regiochemistry of addition reactions
- addition of HBr, Br2, Br2/H2O
- addition of H2O: 1. oxymercuration
2. hydroboration
- hydrogenation: H2
- formation of 3-membered rings: 1. dichlorocarbene
2. Simmons-Smith
3. Epoxidation
- formation of 1,2-diols: OsO4
- cleavage of alkenes: O3/Zn, HIO4
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2
Synthesis of alkenes: elimination reactions
addition
elimination
+ A-B
Elimination with base:
Acid-catalyzed elimination:
KOH
H2SO4
+ Br– + H2O
+ H2O
CH3CH2OH
H2O,
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syn addition:
A and B add to
same face of alkene
anti addition:
A and B add to
opposite faces of alkene
Reactions of alkenes: addition reactions
- selectivity in addition:
1. regioselectivity of addition: Markovnikov's rule
2. stereochemisty of addition: not selective, carbocation intermediate
A-B = H-X
A-B A-B
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3
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Halogenation:
What mechanism leads to this anti addition?
+ Br2
CCl4
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- selectivity in addition: A-B = X2
The mechanism:
reaction at either face
of alkene equally likely
Br- attacks backside of the
more substituted carbon
anti addition
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5
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(Z)-2-butene
Addition of Br2 to (Z)-2-butene
Stereochemistry of alkene addition reactions
(E)-2-butene
Addition of Br2 to (E)-2-butene
Stereochemistry of alkene addition reactions
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bromonium ion
Mechanism very similar to
halogenation except
nucleophile in second step is H2O
++
Same reaction of cyclohexene and X2 IN H2O:
a bromohydrin
deprotonation
Who wins?
Br2/H2OCl2/H2O
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Overall reaction for the formation of a bromohydrin:
- water as solvent and reactant!
H2O
Br2 +