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


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).

Electrophilic Hydration

Addition of H2O:



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

Electrophilic Halogenation

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.

Epoxide Synthesis



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: