Lindlar catalyst

A Lindlar catalyst is a catalyst for the stereospecific hydrogenation of alkynes to alkenes. The Lindlar catalyst, palladium is used as catalyst and calcium carbonate as a carrier. The hydrogen is added in gaseous form into the reactor. To prevent the further hydrogenation to the alkane, lead acetate (or lead (II ) oxide ) as a catalyst poison. Thus, the reactivity is reduced to the extent that the alkene does not react further. The catalyst is named after Herbert Lindlar ( Hoffmann -La Roche).

Production

According to the description of the inventor, the catalyst is prepared by reduction of palladium ( II) chloride in a slurry of calcium carbonate. A first deactivated by addition of lead acetate, then disables quinoline further, which further enhances the selectivity. As an example for the selective hydrogenation of an alkyne to a styrene derivative, see.

Properties

The Lindlar catalyst (also known as palladium on calcium carbonate, poisoned referred to ) is a dark gray solid.

Use

In the hydrogenation of alkynes with Lindlar catalysts always arise cis -alkenes ( Synonym: (Z)- alkenes), as in the first reaction step, the alkyne is bound to the metallic palladium. Then in a second step, a H2 - molecule is directly bound to the triple bond in an electrophilic addition reaction. Characterized both hydrogen atoms are added at the same time and on the same side of the double bond. Subsequently, the resulting alkene from the Palladium dissolves.

One example is the reaction of phenylacetylene to styrene:

A well-known example is the synthesis of vitamin A, in which a triple bond with a Lindlar catalyst is selectively hydrogenated to a (Z)- alkene.

Recycling

The recycling of this catalyst designed to be difficult, because it must be thermally treated. The resulting exhaust gases contain some toxic lead oxides that can not hold back some scrubbing systems. Therefore, additional chemicals must be conducted in the scrubber circuit containing heavy metals (eg lead or mercury) can be precipitated.