Reactions of Alkenes | Organic Chemistry 2

The reactions of alkenes are studied in this chapter: hydrogenation, electrophilic addition of hydrogen halides, electrophilic hydration, electrophilic halogenation, hydroboration-oxidation, epoxide synthesis, vicinal syn dihydroxylation, ozonolysis, radical bromination

Alkenes Reactivity

Reactivity of alkenes is 'all about the π bond'
The π bond of alkenes is electron-rich ⇒ it behaves as a nucleophile and will attack an electrophile
Typical reactions of alkenes are addition reactions

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Hydrogenation

Addition of H2:


A catalyst (Pd or Pt) is necessary to decrease the activation energy of the reaction
​​​​​​​Syn addition: the 2 hydrogens of H2 are added on the same side (the least hindered) ⇒ hydrogenation of alkenes is stereospecific

Electrophilic Addition of Hydrogen Halides

Addition of Hydrogen Halide (HCl, HBr, HI):
 


Mechanism:

The electrophilic addition of hydrogen halides follows the Markovnikov rule: formation of the most stable carbocation (most substituted carbocation)
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Electrophilic Hydration

Addition of H2O:
 


Mechanism:

Acid catalyzed addition (H2SO4), reversible reaction, Markovnikov addition

Electrophilic Halogenation

Addition of dihalogen X​​​​​​​2 (X = Cl, Br, I):
 


Mechanism:


​​​​​​​Bromonium ion can also be trapped by other nucleophiles (for example H2O):

 

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Hydroboration - Oxidation

Hydroboration (addition of BH3):
 

 

Oxidation:
 


Mechanism:

Addition of BH3: addition + regioselectivity (boron on the less hindered carbon)
Oxidation step: formation of Anti-Markovnikov alcohols (least substituted alcohol as a major product)

Epoxide Synthesis


Mechanism:

syn stereospecificity ⇒ trans alkene gives trans epoxide​​​​​​​

 

Vicinal Syn Dihydroxylation

With osmium tetroxide OsO4:
 

 

With potassium permanganate KMnO​​​​​​​4:
 

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Oxidative Cleavage: Ozonolysis

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Radical Bromination

Addition of HBr:
 


Mechanism:

Anti-Markovnikov addition of HBr through a radical process

  1. Initiation steps:


     
  2. Propagation steps: