Catalytic hydrogenation: practice and mechanism
On the Pd catalyst all C=C and C≡C bonds are fully saturated with H₂; aromatic rings stay.
Example
On the palladium catalyst, hydrogen adds to C=C and C≡C bonds (syn addition) and the saturated alkane forms. Alkynes are fully hydrogenated; only the poisoned Lindlar catalyst stops at the cis alkene. Aromatic rings survive these conditions.
Mechanism step by step
- 1
H₂ is adsorbed on the palladium surface and split into hydrogen atoms.
- 2
The alkene binds to the surface with one face.
- 3
Both hydrogens are transferred one after the other from the same face (syn addition); the alkane leaves the surface.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Only the Lindlar catalyst (poisoned Pd) stops at the cis alkene. With ordinary Pd/C the alkyne is hydrogenated all the way to the alkane.
Often confused with: Addition of bromine · Addition of HBr (Markovnikov)
Frequently asked questions
Why isn't the benzene ring hydrogenated?
The aromatic stabilisation is large; it needs high pressure, high temperature or Rh/Pt.
How do I get a cis alkene from an alkyne?
With the Lindlar catalyst (Pd poisoned with lead and quinoline); a trans alkene with Na in liquid NH₃.