Chapter 6: Addition Reactions of Alkenes

Summary

• Alkenes can be reduced to alkanes with H₂ in the presence of metal catalysts such as Pt, Pd, Ni or Rh.
• The two new C-H σ bonds are believed to be formed simultaneously from H atoms absorbed into the metal surface.
• The reaction is stereospecific giving only the syn addition product.
• This reaction forms the basis of experimental "heats of hydrogenation" which can be used to establish the stability of isomeric alkenes.
• Since the π bonds of an alkene are stronger than those in alkynes, alkenes are less reactive than alkynes

• Hydrogenation of alkynes

CATALYTIC HYDROGENATION
The mechanism isn't fully understood, but this is a reasonable attempt to try to use the standard "curly arrow" tools to represent what seems to happen...
Step 1: Hydrogen gets absorbed onto the metal surface.
Step 2: Alkene 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.

© Dr. Ian Hunt, Department of Chemistry