catalytic hydrogenation
Catalytic hydrogenation is the addition of hydrogen gas (H2) across a double or triple bond, turning an alkene into an alkane (or an alkyne first into an alkene, then an alkane). On its own, H2 is unreactive toward a pi bond; the magic ingredient is a metal catalyst — finely divided platinum, palladium, or nickel — on whose surface the reaction takes place. It is the reaction that hardens vegetable oils into margarine and shortening.
The mechanism is a surface story. Hydrogen molecules stick to the metal and split into hydrogen atoms lying on the surface; the alkene also lands on the surface, lying flat against the metal with its pi bond facing down. The two hydrogen atoms are then delivered to the two alkene carbons from the same side — the side touching the metal. Because both hydrogens come from one face, the addition is syn: the two new C-H bonds form on the same face of the former double bond.
Catalytic hydrogenation matters as a clean, stereospecifically syn way to saturate double bonds, and the heat it releases (the heat of hydrogenation) is also a sensitive ruler for comparing alkene stabilities. For alkynes, choosing the catalyst lets you stop halfway: a poisoned palladium catalyst (Lindlar's catalyst) reduces an alkyne only to the cis alkene, while a dissolving-metal route (Na in ammonia) gives the trans alkene instead — so the same alkyne can be funneled to either geometry.
1,2-Dimethylcyclohexene plus H2 over a platinum catalyst gives cis-1,2-dimethylcyclohexane, because both hydrogens are delivered to the same face of the ring (syn addition).
Both H atoms add from the metal surface, so hydrogenation is syn.
A catalyst speeds up the reaction and is regenerated; it does not change the position of equilibrium. Hydrogenation is favourable because making two C-H bonds from H2 and a pi bond releases energy, not because the metal pushes the equilibrium.