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 s bonds are 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.

• Hydrogenation of alkynes

CATALYTIC HYDROGENATION
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 s bonds.