Lindlar Reduction: Partial Hydrogenation of Alkynes → cis Alkenes

Understand how Lindlar catalyst partially hydrogenates alkynes to cis alkenes. Why it stops at the alkene, syn addition, and exam pitfalls.

• Reaction: Alkyne → cis alkene
• What it does: Adds one equivalent of H2 to give the Z-alkene
• Typical reagents: H2, Lindlar catalyst (Pd/CaCO3 poisoned)
• Key intermediate: Surface-adsorbed alkyne/alkene on catalyst
• Driving force: Controlled (poisoned) catalyst prevents full reduction to alkane
• Exam type: Stereochemistry
• Exam type: Reagent selection
• Exam type: Predict product