Reactions of Alkenes | Organic Chemistry 2

π bond of alkenes is weak and electronically rich
⇒ behaves as a nucleophile (attacks electrophile).

Typical reactions of alkenes: addition reactions
 

Addition of H₂:
 

- Need a catalyst (Pd or Pt) to decrease the activation energy of the reaction.
- Stereospecific reaction: syn addition ⇒ the 2 H (H and H) are added on the same side (the less hindered one).

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

Mechanism:
 

The electrophilic addition of hydrogen halides follows the Markovnikov rule.

Markovnikov rule: Formation of the most stable carbocation (most substituted carbocation).

Addition of H₂O:
 

Mechanism:
 

Catalytic in acid (H₂SO₄), reversible reaction, Markovnikov addition.

Addition of dihalogen (Cl₂, Br₂, I₂):
 

Mechanism:
 

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

Hydroboration = Addition of BH₃:
 

Mechanism:

Syn addition of BH₃ + regioselective addition: boron on the less hindered carbon.

Oxidation:
 

 Anti-Markovnikov alcohols: least substituted alcohol as a major product.

Mechanism:

syn stereospecificity (trans alkene gives trans epoxide).

With Osmium Tetroxide:
 

With Potassium Permanganate:
 

Addition of HBr:

Mechanism: Anti-Markovnikov addition of HBr
 

- Initiation steps:

- Propagation steps: