Reactions of Alkenes | Organic Chemistry 2
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
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)
Electrophilic Hydration
Addition of H2O:
Mechanism:
Acid catalyzed addition (H2SO4), reversible reaction, Markovnikov addition
Electrophilic Halogenation
Addition of dihalogen X2 (X = Cl, Br, I):
Mechanism:
Bromonium ion can also be trapped by other nucleophiles (for example H2O):
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 KMnO4:
Oxidative Cleavage: Ozonolysis
Radical Bromination
Addition of HBr:
Mechanism:
Anti-Markovnikov addition of HBr through a radical process
- Initiation steps:
- Propagation steps:
