Chemistry

Hydrogenation of alkynes with the Lindlar catalyst


Mechanism

Composition of the Lindlar catalyst:

Lindlar catalysts consist of calcium carbonate as a carrier with finely divided palladium as a catalyst. The reactivity of the palladium is reduced by treatment with lead acetate and quinoline. The exact process of hydrogenation on the catalyst surface has not yet been clearly clarified.

Mechanism:

The hydrogenation of alkynes proceeds on the same principle as the hydrogenation of alkenes with platinum. Hydrogen and alkyne are bound to the surface of the catalyst and the hydrogen atoms are selectively bonded to the C-metal bond cis added up.

The reason for the selective reduction of the triple bond is not the higher reactivity with respect to hydrogenation, but the stronger bond to the metal surface. As long as there is alkyne in the reaction mixture, all active metal centers are occupied by the alkyne. Only when the alkyne has been completely converted can the alkene formed be further hydrogenated. The reduction to the alkane is usually faster than the formation of the alkene. Therefore it is important to check the reaction after taking up one equivalent of H2 to stop.

Stereochemistry in the hydrogenation with Lindlar catalyst
The hydrogenation of alkynes with the Lindlar catalyst is stereoselective syn . It arise cis-Alkenes.


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