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Organic Chemistry 1 · Alkene and Alkyne Chemistry

Catalytic Hydrogenation

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

converts an alkene to an alkane by adding H₂ across the C=C bond in the presence of a heterogeneous metal catalyst (Pt, Pd, Ni, or Pd/C). Both hydrogens are delivered from the metal surface to the same face of the alkene, so the reaction is a . For a disubstituted cycloalkene, this places the two new hydrogens cis, which makes the two substituents end up cis to each other. The reaction is conceptually clean — no rearrangements and no regiochemical ambiguity — but it requires flammable and is taught here only conceptually.

Why this matters

Catalytic hydrogenation is a cornerstone of industrial and pharmaceutical chemistry: it saturates unsaturated fats (partial hydrogenation) and reduces carbon–carbon double bonds in drug intermediates. The metal surface's control of syn delivery is also the conceptual model behind stereoselective reductions. (Hydrogen gas is highly flammable and explosive in air, and pressurized reactions require specialized equipment and trained personnel; safety and operational details are governed by approved institutional documentation and are not provided here.)

The college version

1. Catalytic Hydrogenation and Heterogeneous Catalysts

Catalytic hydrogenation is the addition of H₂ to reduce an alkene to an alkane. The catalyst is a metal — platinum (Pt), palladium (Pd), or nickel (Ni), often as finely divided metal or on a carbon support (Pd/C). These are heterogeneous catalysts: the solid catalyst is in a different phase from the gaseous H₂ and the liquid (or dissolved) alkene. The catalyst is not consumed; it lowers the activation energy by providing a surface where H₂ and the alkene come together.

2. The Catalytic Surface Model

The accepted picture has several steps: (1) H₂ adsorbs onto the metal surface and the H–H bond dissociates into surface-bound hydrogen atoms; (2) the alkene adsorbs, using its π electrons to bind the metal surface; (3) the two hydrogens transfer to the same face of the alkene (syn); and (4) the alkane desorbs, freeing the surface for another cycle. Because both hydrogens come from the same surface, hydrogenation is stereospecifically syn.

3. Alkene-to-Alkane Reduction and Stereochemistry

Hydrogenation converts C=C to C–C, decreasing the degree of unsaturation by one per double bond and producing a saturated alkane. The reaction is exothermic; the heat released (heat of hydrogenation) is a direct measure of alkene stability (Topic 26). Stereochemically, the cis product forms from a cyclic or cis-disubstituted alkene because both H atoms add from one face. There is no regiochemistry problem (both carbons simply gain an H) and no rearrangement (no carbocation intermediate).

How it works

  1. Adsorb the alkene onto the metal surface through its π electrons.
  2. Dissociate H₂ into surface-bound hydrogen atoms.
  3. Transfer both hydrogens to the same face of the alkene (syn addition).
  4. Release the alkane and recycle the catalyst surface.

Common confusions

Do not confuseWithDifference
Syn additionAnti additionSyn = same face; anti = opposite faces
Heterogeneous catalystHomogeneous catalystSolid surface in a different phase vs catalyst dissolved in the same phase
Reduction (gain of H)Oxidation (gain of O / loss of H)Hydrogenation is a reduction, not an oxidation
HydrogenationHydrohalogenationH₂ adds two H atoms; HX adds H and a halogen
Heat of hydrogenationRate of hydrogenationThermodynamic stability vs kinetic speed

Memory aids

"Hydrogenation is Helpful and Handles both sides the same" — both H atoms add to the same face (syn). Or simply: "Surface = Syn."

Quick review

Topic Recap

Catalytic hydrogenation reduces an alkene to an alkane using H₂ and a heterogeneous metal catalyst (Pt, Pd, Ni, Pd/C). Through the , both hydrogens add to the same face (syn), giving cis products from cyclic alkenes, with no rearrangements and no regiochemical ambiguity. The reaction is conceptually straightforward but requires flammable hydrogen gas and trained handling.

Knowledge Check

  1. What reagents and catalyst convert an alkene to an alkane?
  2. Why is catalytic hydrogenation a syn addition?
  3. What is the product of hydrogenating 1,2-dimethylcyclohexene, and what is its stereochemistry?
  4. What is a , and why is the catalyst not consumed?
  5. Why is there no regiochemical (Markovnikov) question in hydrogenation?

Answers and Rationales

  1. H₂ gas with a metal catalyst such as Pt, Pd, Ni, or Pd/C.
  2. Both alkene and H₂ adsorb onto the metal surface; the two hydrogens are delivered from that same surface to one face of the alkene.
  3. cis-1,2-dimethylcyclohexane. Both hydrogens add from the same face, so the two methyls remain cis.
  4. A solid catalyst in a different phase from the reactants. It lowers activation energy and is regenerated each cycle, so it is reused rather than consumed.
  5. Both alkene carbons become equivalent sp³ carbons each gaining one H, so there is no "more substituted" preference to decide — only syn stereochemistry matters.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Picture a flat metal table with sticky spots. The alkene lies down flat on the table and grabs hold; the H₂ molecule also lands on the table and splits into two separate H atoms sitting on the surface. Because the alkene is stuck face-down on the table, both H atoms can only reach it from the top side — they both attach from the same side, which is syn addition. Then the finished alkane pops off, leaving the table clean for the next alkene.

A comparison: it is like stamping two stickers onto the front of a flat card that is lying on a desk. You can only press both stickers onto the upward face. Where it stops being exact: the alkene does not literally lie "face-down" on a flat, rigid table; adsorption is a dynamic balance of metal–π bonding, and the H atoms are not truly free atoms floating on the surface but adsorbed species that migrate. The model is a useful picture, not a quantum-mechanical description.

Simple Example

1,2-dimethylcyclohexene + H₂ over Pd → cis-1,2-dimethylcyclohexane. Both hydrogens add to the same face, so the two methyl groups, which started on the same side of the double bond, stay on the same face (cis) in the product.

Worked example

  1. The alkene adsorbs to the metal surface through its π bond; this weakens the π bond and holds the alkene in a fixed geometry.
  2. H₂ adsorbs and dissociates on the surface, giving two hydrogen atoms coordinated to the metal.
  3. One surface hydrogen adds to each alkene carbon, both from the side facing the surface — the defining syn step. Electron movement can be pictured as the π electrons and the surface-bound H atoms rearranging into two new C–H σ bonds.
  4. The saturated alkane desorbs, regenerating the empty metal surface. Atom balance: CₙH₂ₙ + H₂ → CₙH₂ₙ₊₂; every carbon keeps an octet and gains one H. No charges or ions are involved at any stage.

Key takeaways

  • High yield: Hydrogenation = alkene → alkane, using H₂ and Pt, Pd, Ni, or Pd/C.
  • High yield: It is a syn addition: both hydrogens add to the same face.
  • High yield: A disubstituted cycloalkene gives the cis-dialkyl product.
  • High yield: No rearrangements and no regiochemical ambiguity (both carbons just gain H).
  • The catalyst is heterogeneous and is regenerated, not consumed.
  • Heat of hydrogenation measures alkene stability (more stable alkene = less heat released).
  • Hydrogenation is not limited to alkenes; alkynes can also be reduced (Topic 35), but the alkene case is the focus here.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice Organic Chemistry 1

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Describe catalytic hydrogenation of alkenes and identify the heterogeneous catalysts used.
  • Explain the catalytic surface model and why hydrogenation is a syn addition.
  • Predict the stereochemical outcome of hydrogenating a substituted cyclic alkene.
  • Distinguish catalytic hydrogenation from other alkene additions and state the safety limitations of working with hydrogen gas.

Key vocabulary

Catalytic hydrogenation
Addition of H₂ across an alkene using a metal catalyst
Heterogeneous catalyst
Solid catalyst in a different phase from the reactants
Hydrogen gas
H₂, the reductant that adds across the π bond
Alkene reduction
Conversion of C=C to C–C (gain of H)
Syn addition
Both hydrogens add to the same face
Catalytic surface model
Picture of adsorption, dissociation, addition, desorption

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