Reactions of Alkenes | Organic Chemistry 2

The reactions of alkenes are studied in this chapter: the hydrogenation, the electrophilic addition of hydrogen halides with the Markovnikov rule, the electrophilic hydration, the electrophilic halogenation, the hydroboration-oxidation with the formation of anti-Markovnikov products, the epoxide synthesis, the vicinal syn dihydroxylation, the ozonolysis – an oxidative cleavage, the radical bromination.

Alkenes Reactivity

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

Typical reactions of alkenes: addition reactions
 

Hydrogenation

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):
 


Mechanism:
 


The electrophilic addition of hydrogen halides follows the Markovnikov rule.

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

Electrophilic Hydration

Addition of H2O:
 


Mechanism:
 


 

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

Electrophilic Halogenation

Addition of dihalogen (Cl2, Br2, I2):
 


Mechanism:
 

 
 

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

 

Hydroboration - Oxidation

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.

Epoxide Synthesis

 

Mechanism:


syn stereospecificity (trans alkene gives trans epoxide).

Vicinal Syn Dihydroxylation

With Osmium Tetroxide:
 

 

With Potassium Permanganate:
 

Oxidative Cleavage: Ozonolysis

Radical Bromination

Addition of HBr:

 

 

Mechanism: Anti-Markovnikov addition of HBr
 

- Initiation steps:

 
 

- Propagation steps: