A Lindlar catalyst is a heterogeneous catalyst that consists of palladium deposited on calcium carbonate and treated with various forms of lead. The lead additive serves to deactivate the palladium sites. A variety of "catalyst poisons" have been used including lead acetate and lead oxide. The palladium content of the catalyst is usually 5% by weight. The catalyst is used for the hydrogenation of alkynes to alkenes (i.e. without further reduction into alkanes).Thus if a compound contains a double bond as well as a triple bond , only the triple bond reduces to a double bond as opposed to both bonds being reduced down to a single bond.[citation needed]
As described by its inventor Herbert Lindlar,[1][2] the catalyst is prepared by reduction of palladium chloride in a slurry of calcium carbonate followed by adding lead acetate. By this approach, one obtains a catalyst with a large surface area. Further deactivation of the catalyst with quinoline enhances its selectivity, preventing formation of alkanes. An example of alkyne reduction is the reduction of phenylacetylene to styrene. A recent description on how the poisons act on the selectivity of the Lindlar catalyst identifies a complementary role of the poisons while Pb reduces the amount of hydrogen content in the bulk of Pd, quinoline blocks the formation of oligomers. [3]
Alkyne reduction is stereoselective, occurring via syn addition to give the cis-alkene.[4] An example of commercial use is the organic synthesis of vitamin A which involves an alkyne reduction with the Lindlar catalyst.
Other heterogeneous catalysts useful for hydrogenation are Adams' catalyst, Palladium black, and Raney nickel.
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