Organic
Catalytic Hydrogenation
Adding H₂ across double or triple bonds — using metal catalysts
Catalytic hydrogenation is the addition of hydrogen (H₂) across double or triple bonds, catalyzed by metal surfaces (Pt, Pd, Ni, Rh, Ru). H₂ adsorbs onto catalyst; bond breaks; H atoms add across alkene/alkyne. Reduces: alkenes → alkanes, alkynes → alkanes (or selectively alkenes), aromatic rings → cyclohexanes, ketones → alcohols, nitriles → amines. Industrial: hardening vegetable oils (margarine), hydrogenation of crude oil components, ammonia synthesis (in Haber process). Mechanism: surface-mediated; concerted (cis-addition); often stereoselective.
- ReactionC=C + H₂ → C-C (catalyzed by metal)
- Common catalystsPt, Pd, Ni, Rh, Ru
- Stereochemistrycis-addition (both H on same face)
- Margarine productionLiquid oils → solid via hydrogenation
- Nobel PrizeSabatier (1912) — homogeneous; Wilkinson (1973) — Wilkinson's catalyst
- Most important industrial reactionWith Haber-Bosch
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Why hydrogenation matters
- Industrial. Vast scale chemistry.
- Food. Margarine, shortening.
- Petroleum. Removing sulfur, saturating compounds.
- Pharmaceuticals. Asymmetric synthesis.
- Hydrogen economy. Storage as hydrogenated compounds.
- Synthesis. Versatile reduction.
- Catalysis. Showcase for catalyst chemistry.
Common misconceptions
- All H₂ additions same. Many variants exist.
- Heterogeneous always works. Selective reduction needs homogeneous.
- Trans-fats are saturated. Have trans double bonds.
- Hydrogenation always full. Can be partial.
- Catalyst doesn't matter. Critical for selectivity.
- Hydrogenation is slow. Industrial scale; fast with right catalyst.
Frequently asked questions
How does hydrogenation work?
H₂ adsorbs onto metal surface (Pt, Pd, Ni). H-H bond breaks; H atoms diffuse across surface. Alkene/alkyne also adsorbs on surface. H atoms transfer to carbon atoms (cis-addition; both H on same face). Saturated product desorbs. Catalyst not consumed. Mechanism: surface-mediated; many H-H and C-H bonds form/break.
Which catalyst to use?
Pt, Pd: most active; high cost; for industrial scale or specific applications. Ni: cheaper; needs higher T/P; raney Ni common. Rh, Ru: special applications. For homogeneous (Wilkinson catalyst): Rh complex; selective. Selection based on: substrate, conditions, selectivity needed, cost. Ni used in margarine; Pt/Pd in fine chemicals.
What about selectivity?
Variable. Alkene > alkyne (alkynes harder; Lindlar catalyst stops at alkene). Aromatic rings: very hard; require high T/P or special catalysts. Functional groups: nitro, aldehyde reduced before alkene with proper choice. Selective reduction critical for synthesis. Modern: chiral catalysts give stereoselective hydrogenation (Noyori — Nobel 2001).
What's margarine production?
Vegetable oils (long-chain unsaturated triglycerides) hydrogenated with Ni catalyst. Adds H₂ across C=C double bonds — converts liquid (oils) to solid (margarine, shortening). Partial hydrogenation: some double bonds remain; good for spreadability. Issue: trans-fats produced (some oil structures isomerize during hydrogenation). Trans-fats linked to heart disease — banned in many countries since 2010s.
Why are trans-fats made?
Side product of partial hydrogenation. Cis double bond can isomerize to trans during reaction (intermediate state). Trans fat = polyunsaturated trans fatty acid. Original natural fats are mostly cis. Trans configuration similar to saturated fats — solid at room T (good for processing) but: cardiovascular harm. Banned in most developed countries.
What's the Lindlar catalyst?
Selectively hydrogenates alkynes to cis-alkenes (without going to alkanes). Pd on CaCO₃, poisoned with lead acetate. Active enough to add H₂ to alkyne but not alkene. Used in: synthesis of natural products with cis double bonds (e.g., vitamin A, retinol).
How does Wilkinson's catalyst work?
Homogeneous Rh complex: RhCl(PPh₃)₃. Soluble; works in solution. Mechanism: oxidative addition of H₂ to Rh; alkene coordinates; insertion into Rh-H bond; reductive elimination gives product. Steps each catalyzed by metal complex. Modern variants enable: asymmetric hydrogenation (chiral product from prochiral substrate). Major in pharmaceutical synthesis.