chapter 5. alkenes and alkynes ii: reactions. elimination reactions
TRANSCRIPT
Chapter 5.Alkenes and Alkynes II:
Reactions
Elimination Reactions
Addition Reactions
Addition of Halogens
C C + X2 C C
X
X
Examples
CH3CH2CH CH2Br2 CH3CH2CHCH2Br
Br
CH3CH CHCH2CH3Cl2 CH3CHCHCH2CH3
Cl
Cl
Mechanism
Halohydrin Formation
C C + X2 C C
X
OH
H2O
CH3CH CHCH3Br2
H2OCH3CHCHCH3
Br
OH
Mechanism
CH3CH CHCH3
CH3CHCHCH3
Br
OH
Br Br
CH3CH CHCH3
Br
OH2
CH3CHCHCH3
Br
OHH
Orientation of the OH toward the more stable cabocation (follows Markovnikov addition)
Hydrogenation
Addition of Hydrogen
• 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
C CCH3
H
CH3
H
H2
Pt CH3CH2CH2CH3
C CH
CH3
CH3
H
H2
Ni CH3CH2CH2CH3
H2
Ni CH3CH2CH2CH3CH3CH2CH CH2
Mechanism
Addition of Water to Alkenes
• Acid-Catalyzed Hydration
• Oxymercuration-Demercuration
• Hydroboration-Oxydation
Acid-Catalyzed Hydration
C C + H2O C C
H
OHH
Mechanism
CH2 CH2
H
CH2 CH3
OH2
CH2CH3
OHH
CH3CH2OH
Mechanism
Markovnikov Addition
CH3CH CH2H
H2O CH3CHCH3
OH
Oxymercuration-DemercurationUse mercuric acetate in THF followed by sodium borohydrideMarkovnikov orientation
via mercurinium ion
Mechanism
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
Hydroboration-Oxidation
BH3 Is a Lewis Acid
• Six electrons in outer shell• Coordinates to oxygen electron pairs in ethers
• 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
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
C
H3C
H3C
CH21. BH3
.THF2. H2O2
CH3 CH CH2
CH3
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