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).
Electrophilic Addition of Hydrogen Halides
Addition of Hydrogen Halide (HCl, HBr, HI):
The electrophilic addition of hydrogen halides follows the Markovnikov rule.
Markovnikov rule: Formation of the most stable carbocation (most substituted carbocation).
Addition of H2O:
Catalytic in acid (H2SO4), reversible reaction, Markovnikov addition.
Addition of dihalogen (Cl2, Br2, I2):
Bromonium ion can also be trapped by other nucleophiles (for example H2O):
Hydroboration - Oxidation
Hydroboration = Addition of BH3:
Syn addition of BH3 + regioselective addition: boron on the less hindered carbon.
Anti-Markovnikov alcohols: least substituted alcohol as a major product.
syn stereospecificity (trans alkene gives trans epoxide).
Vicinal Syn Dihydroxylation
With Osmium Tetroxide:
With Potassium Permanganate:
Oxidative Cleavage: Ozonolysis
Addition of HBr:
Mechanism: Anti-Markovnikov addition of HBr
- Initiation steps:
- Propagation steps: