Chapter 9 : Alkynes

Summary

• Alkynes are reduced to alkanes with H₂ (which is normally in excess due to the way in which the reaction is performed) 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.
• Alkynes can be partially reduced to cis-alkenes with H₂ in the presence of poisoned catalysts
  
(e.g. Pd / CaCO₃ / quinoline which is also known as Lindlar's catalyst)
• The "poisoned" catalyst prevents over-reduction, which would give the alkane by reducing the alkene
• The reaction is stereospecific giving only the syn addition product.
• Since the π bond in an alkyne is weak than that in an alkene, alkynes reduce more rapidly than alkenes

• 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 cis-alkene. Since the 2 atoms are transferred to the same face of the alkens at the same time, the addition is syn.

© Dr. Ian Hunt, Department of Chemistry