Lindlar hydrogenation (cis alkene): practice and mechanism
The Lindlar catalyst is "poisoned" and only hydrogenates to the alkene. Both H atoms come from the same side of the metal surface: the cis alkene forms.
Example
The Lindlar catalyst is partly "poisoned" and stops at the alkene. Both hydrogens come from the catalyst surface onto the same side (syn addition), so internal alkynes give the cis alkene.
Mechanism step by step
- 1
H₂ is split into H atoms on the palladium surface.
- 2
The alkyne binds to the surface; both H atoms are delivered from the same side (syn addition): cis alkene.
- 3
The alkene is bound only weakly by the poisoned catalyst (Pb, quinoline) and leaves before being hydrogenated further.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Dissolving-metal reduction of alkynes (trans alkene) · Catalytic hydrogenation
Frequently asked questions
What is "poisoned" about the catalyst?
Pd on CaCO₃ with lead acetate and quinoline. The additives block the most active sites so alkenes are no longer hydrogenated.
How do I get the trans alkene?
With sodium in liquid ammonia (Birch-type reduction): the electrons arrive one at a time and the intermediates adopt the trans arrangement.