Organic chemistry for medicine and biology studentsChem 2311Chapter 3

Alkynes

By Prof. Dr.Adel M. Awadallah

Islamic University of Gaza

Reduction of alkynes With hydrogen in presence of ordinary nickel, platinum or palladium gives alkanesHydrogentaion of alkynes with Lindlar’s catalyst gives a cis alkene

•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.•As described by its inventor,[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.[1]

•Alkyne reduction is stereoselective, occurring via syn addition to give the cis-alkene.[3]

•Lindlar’s Catalyst