Partial Hydrogenation using Lindlar's Catalyst

$$\text{R--C}\equiv\text{C--R'} + \text{H}_2 \xrightarrow{\text{Pd / CaCO}_3 + \text{Quinoline}} cis\text{-alkene}$$

Concept: Alkynes undergo selective stereospecific reduction with hydrogen gas ($$\text{H}_2$$) in the presence of poisoned palladium catalyst to form cis-alkenes (syn-addition).

Reagents & Conditions: Palladium supported on calcium carbonate ($$\text{Pd / CaCO}_3$$) partially deactivated ("poisoned") with quinoline or lead acetate (Lindlar's Catalyst).

Key Notes:

• Deactivation of the catalyst prevents further hydrogenation of the formed alkene into an alkane.
• Hydrogen atoms add from the same side of the triple bond (syn-addition), selectively forming the cis isomer.