Chapter 5. Alkenes and Alkynes II: Reactions. Elimination Reactions.
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Transcript of Chapter 5. Alkenes and Alkynes II: Reactions. Elimination Reactions.
![Page 1: Chapter 5. Alkenes and Alkynes II: Reactions. Elimination Reactions.](https://reader030.fdocuments.in/reader030/viewer/2022033017/56649ef15503460f94c02c45/html5/thumbnails/1.jpg)
Chapter 5.Alkenes and Alkynes II:
Reactions
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Elimination Reactions
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Addition Reactions
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Addition of Halogens
C C + X2 C C
X
X
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Examples
CH3CH2CH CH2Br2 CH3CH2CHCH2Br
Br
CH3CH CHCH2CH3Cl2 CH3CHCHCH2CH3
Cl
Cl
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Mechanism
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Halohydrin Formation
C C + X2 C C
X
OH
H2O
CH3CH CHCH3Br2
H2OCH3CHCHCH3
Br
OH
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Mechanism
CH3CH CHCH3
CH3CHCHCH3
Br
OH
Br Br
CH3CH CHCH3
Br
OH2
CH3CHCHCH3
Br
OHH
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Orientation of the OH toward the more stable cabocation (follows Markovnikov addition)
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Hydrogenation
Addition of Hydrogen
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• 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
Reduction of Alkenes
C C + H2 C C
H H
M
M = Pt, Pd, or Ni
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C CCH3
H
CH3
H
H2
Pt CH3CH2CH2CH3
C CH
CH3
CH3
H
H2
Ni CH3CH2CH2CH3
H2
Ni CH3CH2CH2CH3CH3CH2CH CH2
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Mechanism
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Addition of Water to Alkenes
• Acid-Catalyzed Hydration
• Oxymercuration-Demercuration
• Hydroboration-Oxydation
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Acid-Catalyzed Hydration
C C + H2O C C
H
OHH
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Mechanism
CH2 CH2
H
CH2 CH3
OH2
CH2CH3
OHH
CH3CH2OH
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Mechanism
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Markovnikov Addition
CH3CH CH2H
H2O CH3CHCH3
OH
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Oxymercuration-DemercurationUse mercuric acetate in THF followed by sodium borohydrideMarkovnikov orientation
via mercurinium ion
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Mechanism
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Hydroboration-Oxidation
C C1. BH3
.THF2. H2O2
C C
HOH
•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
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Hydroboration-Oxidation
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BH3 Is a Lewis Acid
• Six electrons in outer shell• Coordinates to oxygen electron pairs in ethers
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• 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
Hydroboration-Oxidation Forms an Alcohol from an Alkene
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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)
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Anti-Markovnikov Addition
C
H3C
H3C
CH21. BH3
.THF2. H2O2
CH3 CH CH2
CH3
OH
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Syn Addition
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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
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