Alkenes and alkynes II: reactions

1. Chapter 5.
Alkenes and Alkynes II:
Reactions

2. Elimination Reactions

3. Addition Reactions

4. Addition of Halogens
C C + X2 C C
X
X

5. Examples
CH3CH2CH CH2
Br2
CH3CH2CHCH2Br
Br
CH3CH CHCH2CH3
Cl2 CH3CHCHCH2CH3
Cl
Cl

6. Mechanism

8. Halohydrin Formation
C C + X2 C C
X
OH
H2O
CH3CH CHCH3
Br2
H2O
CH3CHCHCH3
Br
OH

9. Mechanism
CH3CH CHCH3
CH3CHCHCH3
Br
OH
Br Br
CH3CH CHCH3
Br
OH2
CH3CHCHCH3
Br
OH
H

11. Orientation of the OH toward the more stable
cabocation (follows Markovnikov addition)

12. Hydrogenation
Addition of Hydrogen

13. • 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

14. C C
CH3
H
CH3
H
H2
Pt CH3CH2CH2CH3
C C
H
CH3
CH3
H
H2
Ni CH3CH2CH2CH3
H2
Ni CH3CH2CH2CH3
CH3CH2CH CH2

15. Mechanism

16. Addition of Water to Alkenes
• Acid-Catalyzed Hydration
• Oxymercuration-Demercuration
• Hydroboration-Oxydation

17. Acid-Catalyzed Hydration
C C + H2O C C
H
OH
H

19. Mechanism
CH2 CH2
H
CH2 CH3
OH2
CH2CH3
OH
H
CH3CH2OH

20. Mechanism

21. Markovnikov Addition
CH3CH CH2
H
H2O CH3CHCH3
OH

23. Oxymercuration-Demercuration
Use mercuric acetate in THF followed by sodium
borohydride
Markovnikov orientation
via mercurinium ion

24. Mechanism

25. Hydroboration-Oxidation
C C
1. BH3
.
THF
2. H2O2
C C
H
OH
•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

26. Hydroboration-Oxidation

27. BH3 Is a Lewis Acid
• Six electrons in outer shell
• Coordinates to oxygen electron pairs in ethers

28. • 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

29. 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)

30. Anti-Markovnikov Addition
C
H3C
H3C
CH2
1. BH3
.
THF
2. H2O2
CH3 CH CH2
CH3
OH

31. Syn Addition

32. 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