Alkene Hydrogenation with H₂ over Pd, Pt, or Ni

Molecular hydrogen reacted with a supported metal catalyst (Pd/C, Pt/C or PtO₂ “Adams”, Raney Ni, Rh/C, etc.) converts alkenes to alkanes. Both H₂ and the alkene chemisorb onto the metal surface: H₂ dissociates to surface M–H species and the alkene π-binds to the same face. Two successive surface hydride transfers deliver H atoms syn across the C=C bond (Horiuti–Polanyi mechanism). Because the reaction takes place on a metal face, new stereocenters formed under achiral conditions appear as racemic pairs. Note that Pd and Ni surfaces can also promote hydrogenolysis and other reductions; match the catalyst and conditions to the functional groups present.

Introduction

Heterogeneous hydrogenation relies on chemisorption. H₂ dissociatively adsorbs to the catalyst surface (M–H formation) while the alkene π-binds to the same face. Hydride delivery from the surface is therefore syn: in cyclic systems the two newly installed hydrogens appear cis, and in acyclic systems each new stereocenter is generated with the same relative configuration. In a cyclohexane chair, cis‑1,2 hydrogens appear as one axial and one equatorial on the same face (not trans-diaxial). Because the surface is usually achiral, any newly formed stereocenters appear as racemic mixtures.

Quick Summary

• Reagents: H₂(g) with a metal catalyst (Pd/C, Pt/C, PtO₂, Raney Ni, Rh/C). Common solvents include ethanol, ethyl acetate, acetic acid, or hexanes.
• Outcome: Reduces non-aromatic C=C bonds to C–C, delivering two hydrogens syn from one catalyst face.
• Mechanism: H₂ dissociative adsorption → alkene π-adsorption → first surface hydride transfer (half-hydrogenated intermediate) → second surface hydride transfer and desorption.
• Stereochemistry: Addition is syn. When both alkene carbons become stereocenters, the outcome is racemic unless the surface is chiral. A carbon with two identical substituents (e.g., two hydrogens) cannot be stereogenic.
• Caveats: Pd/Ni surfaces can hydrogenolyze benzyl or allylic protecting groups and reduce nitro/azide/carbonyl functions. Aromatic rings typically require harsher Ni/high P–T. Use Lindlar (Pd/BaSO₄/CaCO₃, poisoned) for partial alkyne hydrogenation to alkenes.

Mechanism (Horiuti–Polanyi Sequence)

Hydrogen molecules first dissociatively adsorb on the metal surface: the H–H bond breaks, generating adjacent metal–hydride units (M–H). The alkene approaches the same face so that both hydrogens will be delivered from that surface.

The alkene π-binds to the metal face. One surface hydride migrates to a vinylic carbon, forming a half-hydrogenated (surface alkyl) intermediate that remains bound to the catalyst.

The remaining surface hydride adds to the neighboring carbon from the same face, giving the syn dihydrogenated product. The alkane then desorbs, regenerating the catalyst surface.

The result is an alkane in which both new hydrogens originate from the same catalyst face.

Stereochemical Outcomes

• Syn addition means the two hydrogens are delivered to the same face of the alkene.

• If a carbon ends up with two identical substituents (e.g., two hydrogens), that carbon cannot be stereogenic.

• When both vinylic carbons become sp³ centers bearing four different substituents, two stereocenters are formed. With achiral catalysts, the surface imparts no chiral bias, so the outcome is a racemic pair (enantiomeric products).

• In cyclic systems, the new hydrogens are cis; in a chair conformer they occupy one axial and one equatorial position on the same face after ring relaxation.

Worked Example — Hydrogenation of pent-1-ene

+

→

Names confirmed with the OrgoSolver IUPAC Namer: C=CCCC → pent-1-ene, CCCCC → pentane.

Practical Tips & Pitfalls

• Catalyst choices: Pd/C, Pt/C or PtO₂ (Adams), Raney Ni, and Rh/C are standard. Homogeneous Rh catalysts (e.g., Wilkinson’s) achieve similar outcomes but are covered separately.
• Hydrogen source: Use H₂ gas (balloon to autoclave pressure). Ensure adequate agitation so H₂ reaches the catalyst surface.
• Functional-group sensitivity: Pd/Ni often hydrogenolyze benzyl/allylic protecting groups and reduce nitro, azide, or certain carbonyl derivatives. Protect or avoid those groups when necessary.
• Alkynes and over-reduction: Under these conditions alkynes typically reduce all the way to alkanes. Use Lindlar (poisoned Pd) for cis-alkene stops or dissolving metal reductions for trans-alkenes.
• Aromatic rings: Arenes generally resist hydrogenation under these mild Pd/Pt conditions; special high-pressure Ni systems are needed for arene saturation.
• Isomerization caveat: Some catalyst/condition sets allow double-bond migration or H-exchange via π-allyl-like surface intermediates; monitor for alkene isomerization when position matters.
• Safety: Metal catalysts are pyrophoric when dry; handle slurries under inert gas and quench spent catalysts carefully.

Exam-Style Summary

• H₂ dissociatively chemisorbs; alkene π-adsorbs to the metal surface.

• Two surface hydride transfers occur from the same face (syn addition).

• New stereocenters (when both carbons gain four different substituents) form racemic mixtures under achiral conditions.

• Pd/Ni catalysts may hydrogenolyze other functional groups; be mindful of protecting groups and nitro/azide reductions.

• Use Lindlar or alternative conditions when selective alkyne → alkene hydrogenation is desired.

FAQ / Notes

• Which catalysts are most common? Pd/C, Pt/C or PtO₂, Raney Ni, and Rh/C are routine workhorses. Wilkinson’s catalyst (RhCl(PPh₃)₃) and Crabtree’s catalyst offer homogeneous alternatives.
• Why is the addition syn? Both hydrogens originate from the same metal face; the half-hydrogenated intermediate never leaves the surface until both hydrides have added.
• Will aromatic rings hydrogenate? Not under mild Pd/Pt conditions; they usually require high-pressure Ni systems.
• What about other reducible groups? Nitro, azide, benzyl protecting groups, and some carbonyl derivatives can be reduced or cleaved under Pd/Ni hydrogenation—plan protective group strategy accordingly.
• How do I stop at an alkene when reducing an alkyne? Use Lindlar (Pd/BaSO₄/CaCO₃ with quinoline/lead) or dissolving-metal reductions; generic H₂/Pd, Pt, Ni will take alkynes all the way to alkanes.

Need More Practice?

Use our study tools to reinforce these concepts:

• IUPAC Namer — practice systematic names for the alkanes formed after hydrogenation.
• Reaction Solver — drill catalytic hydrogenation alongside related addition reactions.
• Mechanism Solver — walk through the Horiuti–Polanyi sequence and other surface-mediated additions.