Hydrogenation of Alkynes with the Lindlar catalyst
Hydrogenation of Alkynes with Lindlar Catalyst: Mechanism
Constitution of Lindlar catalyst:
Lindlar catalysts consist of finely distributed palladium as the catalyst deposited on calcium carbonate as support. The reactivity of the palladium is reduced by treatment with lead acetate and quinoline. The events happening at the surface of the catalyst during hydrogenation are not fully understood.
Hydrogenation of alkynes follows the same principle as that of alkenes with platinum. Hydrogen and alkyne are bound to the surface of the catalyst and H atoms are being added cis-specific by insertion into the C metal bond.
The selective reduction of triple bonds is the result of stronger bonding to the metal surface but not higher reactivity of the bond. As long as the reaction mixture contains alkyne molecules, available active metal centers will be occupieded by alkyne. Only after all alkyne is reduced to alkene, further hydrogenation of the alkene can take place. As a rule, reduction to the alkane is faster than formation of the alkene. Therefore, it is very important to stop the reaction after uptake of one equivalent of H2
- Stereochemistry of the hydrogenation with Lindlar catalyst
- Hydrogenation of alkynes with Lindlar catalyst proceeds stereoselectively in syn fashion. Cis alkenes are being formed.