Carbonyl Reduction Hydrogenation H2 Metal

Aldehyde/Ketone → Alcohol with H₂ over Pd, Pt, or Ni

Carbonyl Reductions: Aldehyde/Ketone → Alcohol with H₂ over Pt, Pd, or Ni


H₂ with a heterogeneous metal catalyst (Pd/C, Pt/C or PtO₂, Raney Ni, Rh/C) adds across C=O to give primary (aldehyde) or secondary (ketone) alcohols. The hydrogenation follows a surface-mediated Horiuti–Polanyi pathway: H₂ dissociates on the metal to surface hydrides, the carbonyl adsorbs, and two hydrogens are delivered sequentially to the carbonyl carbon and oxygen. Aldehydes typically reduce faster than ketones. In α,β-unsaturated systems, C=C reduction usually outpaces C=O with Pd or Ni, while Pt more readily reduces both. Mild conditions avoid arene saturation but hydrogenolysis of benzyl/allyl groups or nitro reduction can occur.

Studying related surface additions? Review the companion guides on alkene hydrogenation with H₂ and alkyne hydrogenation with H₂ to compare syn delivery and catalyst tuning.

Quick Summary


  • Reagents/conditions: H₂ (balloon → a few atm), Pt/C or PtO₂ (Adams’), Pd/C, Raney Ni; solvents EtOH, MeOH, EtOAc, THF; 0–60 °C.
  • Outcome: Aldehyde → primary alcohol; ketone → secondary alcohol.
  • Chemoselectivity: α,β-Unsaturated carbonyls hydrogenate at C=C faster than C=O with Pd/Ni; Pt reduces both. Aromatic rings usually survive mild conditions.
  • Cautions: Pd/Ni can hydrogenolyze benzyl/allyl groups or reduce nitro; free thiols or coordinating amines poison catalysts.
  • Mechanism: Surface H₂ dissociation → hydride to carbon → proton/hydride to oxygen → desorption (no radicals/carbocations).

Mechanism (4 Frames)


H₂ adsorbs to the metal surface and cleaves into two hydrides.
**Step 1 — H₂ activation:** Molecular hydrogen chemisorbs on the catalyst (Pd, Pt, Ni) and splits into surface M–H species.
Carbonyl π-binds to the metal surface adjacent to the hydrides.
**Step 2 — Carbonyl adsorption:** The aldehyde or ketone coordinates through the C=O, aligning the π* orbital with the surface hydrides.
Surface hydride adds to the carbonyl carbon while the oxygen bears the new O–H bond and metal coordination.
**Step 3 — Hydride delivery to C:** A surface hydride attacks the carbonyl carbon (with O already protonated on the surface), creating the new C–H bond and a bound alkoxide.
Primary or secondary alcohol leaving the catalyst surface.
**Step 4 — Alcohol product:** The reduced alcohol (1° for aldehydes, 2° for ketones) desorbs; achiral catalysts deliver racemic mixtures when a stereocenter forms.

Worked Examples


Benzaldehyde → Benzyl Alcohol

Reactant: benzaldehyde Reagent button: H₂ over Pd/Pt/Ni Product: benzyl alcohol

Benzaldehyde + H₂/Pd-C (EtOH, rt) → benzyl alcohol. Fast, chemoselective carbonyl reduction.

Cyclohexanone → Cyclohexanol

Reactant: cyclohexanone Reagent button: H₂ over Pd/Pt/Ni Product: cyclohexanol

Cyclohexanone + H₂/Raney Ni (EtOH, 30 °C) → racemic cyclohexanol.

Cinnamaldehyde (Chemoselectivity)

Reactant: cinnamaldehyde Reagent button: H₂ over Pd/Pt/Ni Product: hydrocinnamaldehyde (C=C reduced)

Cinnamaldehyde + H₂/Pd-C (balloon, rt) → hydrocinnamaldehyde (C=C reduced first). Prolonged exposure → 3-phenyl-1-propanol (use non-H₂ methods for allylic alcohol).

Scope & Limitations


  • Great: Aliphatic/benzylic aldehydes and ketones; cyclic ketones; α-halo carbonyls (watch for hydrogenolysis of benzylic halides).
  • Slower: Hindered dialkyl ketones; substrates with strongly coordinating heteroatoms (pyridyl, thioethers) unless catalyst tuned.
  • Caution: Benzyl/allylic C–O and C–X groups cleave on Pd; nitro groups reduce to amines; S-containing compounds poison catalysts (Raney Ni cleaves thioacetals).
  • Chemoselectivity knobs: Catalyst choice (Pd vs Pt vs Ni), H₂ pressure, temperature, and solvent polarity. Pd/Ni favor C=C hydrogenation first; PtO₂ hits both more rapidly.
  • Labeling: Using D₂ installs D at carbon and oxygen (common exam question).

Practical Tips & Pitfalls


  • Degas solvent and keep Pd/C or Raney Ni wet; dry Pd/C can ignite on exposure to air.
  • Start with rt and balloon pressure; raise temperature or pressure only if conversion stalls.
  • Filter catalysts through celite before concentrating; avoid metal fines in rotavap flasks.
  • Decide upfront whether you want the C=C intact (use NaBH₄/Luche) or fully saturated (use Pd/Ni).
  • Protect benzyl/allylic groups if hydrogenolysis would be problematic.

Exam-Style Summary


H₂ with Pd, Pt, or Ni adds across C=O on a metal surface, delivering hydride to carbon then proton to oxygen. Aldehydes → 1° alcohols, ketones → 2° alcohols, racemic when new stereocenters form. In α,β-unsaturated systems, C=C usually hydrogenates before C=O (Pd/Ni). Watch for catalyst poisoning (S/N donors) and hydrogenolysis of benzyl/allyl groups.

Interactive Toolbox


  • Mechanism Solver — View H₂ dissociation, hydride delivery, protonation, and desorption; toggle catalyst icons to highlight chemoselectivity.
  • Reaction Solver — Choose substrate class and catalyst to preview C=C vs C=O reduction priority and hydrogenolysis risks.
  • IUPAC Namer — Confirm names for the resulting primary/secondary alcohols (no raw SMILES shown).