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Also known as: Heterogeneous Hydrogenation, Sabatier Reaction
Both H atoms add to the same face of the double bond (syn addition) because the reaction occurs on the surface of the metal catalyst. The alkene adsorbs onto the catalyst surface, H₂ dissociates on the metal, and both hydrogens are delivered from the same face.
H₂ and the alkene both adsorb onto the catalyst surface. H₂ undergoes dissociative chemisorption, breaking into two metal-bound H atoms.
Heterogeneous — reaction on solid surface
Both hydrogen atoms are transferred sequentially from the catalyst surface to the same face of the alkene, giving syn addition.
Syn addition — both H's from same face
Cyclohexene
H₂, Pd/C
Cyclohexane
2-Butyne
H₂, Lindlar's catalyst
cis-2-Butene
2-Butyne
H₂ (excess), Pd/C
Butane
Essential method for reducing C=C and C≡C bonds. Lindlar's catalyst for partial reduction of alkynes to cis-alkenes is particularly valuable.