Hydrogenation of Alkenes
Hydrogenation of Alkenes: Catalytic hydrogenation turns an alkene into an alkane. Step-by-step mechanism, reagents and practice.
What happens
Catalytic hydrogenation turns an alkene into an alkane. Hydrogen gas (H₂) is added across the C=C double bond using a metal catalyst such as Pd, Pt, or Ni. The double bond becomes a single bond, and each carbon gains a hydrogen.
Reaction summary
Alkene + H₂ → Alkane (Pd/C, Pt, or Ni catalyst)
Reagents and conditions
- Starting material: Propene (alkene)
- Reagents: H₂, Pd/C (or Pt, or Ni)
- Conditions: Metal catalyst surface
- Products: Propane (alkane)
Mechanism
- Adsorption: H₂ and the alkene bind to the metal surface.
- H–H Cleavage: The H–H bond breaks on the metal surface.
- Delivery: The two hydrogen atoms are delivered to the double bond, from the same face.
- Desorption: The alkane leaves the catalyst surface.
Selectivity
- Stereoselectivity: Syn addition: both hydrogen atoms are added to the same face of the double bond.
Common mistakes
- Hydrogenation does not add H and OH, that's hydration, not hydrogenation.
Memory trick
"Hydrogenation = Hydrogen goes ON": C=C → C–C, both new bonds are to H, nothing else.