Partial hydrogenation — syn-selective
Lindlar semihydrogenation of an alkyne
How the Lindlar semihydrogenation of an alkyne works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: H₂, Pd/CaCO₃, Pb(OAc)₂, quinoline. Conditions: Hexane or EtOAc, 25 °C, 1 atm H₂. Bonds formed: Two C–H bonds (syn, same face); bonds broken: H–H; one C≡C π bond. Stereochemistry: syn addition gives the cis (Z) alkene. Rate and selectivity: Poisoned Pd: alkyne fast, alkene slow.
At a glance
Step 1: A deliberately poisoned catalyst
Lindlar's catalyst is palladium on CaCO₃ that has been 'poisoned' with lead and quinoline. The poison blocks the most active sites, so the catalyst reduces alkynes but is too weak to reduce the resulting alkene.
Step 2: Syn delivery of both hydrogens
The alkyne binds more strongly to the surface than the alkene does, and both hydrogens add to the same face of the alkyne from surface-bound H₂ — the same syn delivery seen in ordinary hydrogenation. This gives a cis (Z) alkene.
Step 3: cis-Alkene desorbs
The cis alkene desorbs before it can be reduced further. This is the standard way to make Z-alkenes from alkynes, and it contrasts with Na/NH₃ (dissolving-metal) reduction, which gives the trans (E) alkene through radical-anion intermediates.
Abbreviations and terms
- OAc
- Acetoxy. –OC(=O)CH₃, an acetate group.
- EtOAc
- Ethyl acetate. Moderately polar solvent.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
How it compares with related reactions
| Lindlar semihydrogenation of an alkyne | Rosenmund reduction | Br₂ addition to an alkene (vicinal dihalide) | |
|---|---|---|---|
| Reagents | H₂, Pd/CaCO₃, Pb(OAc)₂, quinoline | Acid chloride, H₂, Pd/BaSO₄ + quinoline–S (poison) | Br₂ |
| Conditions | Hexane or EtOAc, 25 °C, 1 atm H₂ | Toluene or xylene, 25–110 °C, 1 atm H₂ | CH₂Cl₂ or CCl₄, 25 °C, dark |
| Stereochemistry | syn addition gives the cis (Z) alkene | Not relevant (aldehyde carbon is not a stereocentre) | Anti addition via bromonium; trans product (racemic) |
| Rate / selectivity | Poisoned Pd: alkyne fast, alkene slow | Poison slows aldehyde over-reduction; HCl scavenger (base) sometimes added | Bromonium formation rate-limiting; fast |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Lindlar semihydrogenation of an alkyne?
H₂, Pd/CaCO₃, Pb(OAc)₂, quinoline. Conditions: Hexane or EtOAc, 25 °C, 1 atm H₂.
Which bonds are formed and which are broken?
Formed: Two C–H bonds (syn, same face). Broken: H–H; one C≡C π bond.
What is the stereochemical outcome?
syn addition gives the cis (Z) alkene
What controls the rate or the selectivity?
Poisoned Pd: alkyne fast, alkene slow
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go
Related mechanisms
- Rosenmund reduction
- Br₂ addition to an alkene (vicinal dihalide)
- Halohydrin formation from an alkene + Br₂/H₂O
- Still-Gennari modification (Z-selective HWE)
- Acetic acid transfers a proton to water
- Acid-catalyzed amide hydrolysis
- Acid-catalyzed epoxide opening with methanol
- Acid-catalyzed hydration of an alkene