Chapter 5. Alkenes and Alkynes II: Reactions. Elimination Reactions

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