Lindlar catalyst is a heterogeneous catalyst consisting of palladium deposited on calcium carbonate and poisoned with lead acetate and quinoline. It is specifically designed for the selective hydrogenation of alkynes to cis-alkenes (Z-alkenes). The lead and quinoline additives act as catalyst poisons that deactivate the palladium, preventing over-reduction of the alkyne to the fully saturated alkane. The primary and most significant application of Lindlar catalyst is in organic synthesis for the stereoselective synthesis of cis-olefins. This transformation is a cornerstone in the synthesis of complex natural products, pharmaceuticals, and fine chemicals where the specific geometry of the double bond is crucial for biological activity or material properties. It is a standard reagent for the partial hydrogenation of terminal and internal alkynes. The catalyst is typically used under mild conditions (atmospheric pressure of hydrogen gas, room temperature). Due to the presence of lead, it is considered highly toxic and requires careful handling. It is commercially available as a gray or black powder and is often used in suspension. Its selectivity makes it irreplaceable for many synthetic pathways, despite the development of alternative homogeneous catalysts.