Chapter 5.
Alkenes and Alkynes II:
Reactions
Elimination Reactions
Addition Reactions
Addition of Halogens
X
C C
+ X2
C C
X
Examples
Br
CH3CH2CH CH2
Br2
CH3CH2CHCH2Br
Cl
CH3CH CHCH2CH3
Cl2
CH3CHCHCH2CH3
Cl
Mechanism
Halohydrin Formation
OH
C C
+ X2 H O
2
C C
X
OH
CH3CH CHCH3
Br2
H2O
CH3CHCHCH3
Br
Mechanism
Br Br
CH3CH CHCH3
Br
CH3CH CHCH3
OH2
Br
CH3CHCHCH3
OH
H
Br
CH3CHCHCH3
OH
Orientation of the OH toward the more stable
cabocation (follows Markovnikov addition)
Hydrogenation
Addition of Hydrogen
Reduction of Alkenes
•
•
•
•
•
Addition of H-H across C=C
Reduction in general is addition of H2 or its equivalent
Requires Pt, Pd, or Ni as powders on carbon and H2
Hydrogen is first adsorbed on catalyst
Reaction is heterogeneous
C C
+ H2
M
C C
H H
M = Pt, Pd, or Ni
CH3CH2CH CH2
CH3
H
C C
H
CH3
CH3
CH3
H
C C
H
H2
Ni
CH3CH2CH2CH3
H2
Ni
CH3CH2CH2CH3
H2
Pt
CH3CH2CH2CH3
Mechanism
Addition of Water to Alkenes
• Acid-Catalyzed Hydration
• Oxymercuration-Demercuration
• Hydroboration-Oxydation
Acid-Catalyzed Hydration
OH
C C + H2O
H
C C
H
Mechanism
H
CH2 CH2
OH2
CH2 CH3
H
OH
CH2CH3
CH3CH2OH
Mechanism
Markovnikov Addition
CH3CH CH2
H
H2O
CH3CHCH3
OH
Oxymercuration-Demercuration
Use mercuric acetate in THF followed by sodium
borohydride
Markovnikov orientation
via mercurinium ion
Mechanism
Hydroboration-Oxidation
C C
.
BH3 THF
1.
2. H2O2
OH H
C C
•Herbert Brown invented hydroboration (received the
1979 Nobel Chemistry Prize.)
•Borane (BH3) is electron deficient is a Lewis acid
•Borane adds to an alkene to give an organoborane
Hydroboration-Oxidation
BH3 Is a Lewis Acid
• Six electrons in outer shell
• Coordinates to oxygen electron pairs in ethers
Hydroboration-Oxidation Forms an
Alcohol from an Alkene
• Addition of H-BH2 (from BH3-THF complex) to three
alkenes gives a trialkylborane
• Oxidation with alkaline hydrogen peroxide in water
produces the alcohol derived from the alkene
Orientation in Hydration via
Hydroboration
• Regiochemistry is opposite to Markovnikov
orientation (Anti-Markovnikov)
– OH is added to carbon with most H’s
• H and OH add with syn stereochemistry, to
the same face of the alkene (opposite of anti
addition)
Anti-Markovnikov Addition
H3C
C CH2
H3C
.
BH3 THF
1.
2. H2O2
CH3
CH3 CH CH2
OH
Syn Addition
Mechanism of Hydroboration
• Borane is a Lewis acid
• Alkene is Lewis base
• Transition state involves
anionic development on
B
• The components of BH3
are across C=C