Chapter 9 : Alkynes

Summary

• Alkynes can be partially reduced to cis-alkenes with H₂ in the presence of poisoned catalysts.

(eg. Pd / CaCO₃ / quinoline which is also known as Lindlar's catalyst)
• Alkynes can be reduced to alkanes with H₂ in the presence of catalysts (Pt, Pd, Ni etc.)
• The new C-H σ bonds are formed simultaneously from H atoms absorbed onto the metal surface.
• The reaction is stereospecific giving only the syn addition product.
• The "poisoned" catalyst prevents over-reduction, which would give the alkane by reducing the alkene

• Dissolving metal reduction of alkynes
• Hydrogenation of alkenes

CATALYTIC HYDROGENATION
Step 1: Hydrogen gets absorbed onto the metal surface.
Step 2: Alkyne approaches the H atoms absorbed on the metal surface.
Step 3: C≡C  reacts with the H atoms on the surface forming the two new C-H σ bonds generating the alkene.

© Dr. Ian Hunt, Department of Chemistry, University of Calgary