Inorganic — heterogeneous surface catalysis
Catalytic hydrogenation of an alkene (Pd/C, H₂)
How the Catalytic hydrogenation of an alkene (Pd/C, H₂) works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: H₂ (1 atm), Pd/C. Conditions: EtOH or EtOAc, 25 °C. Bonds formed: two C–H (syn); bonds broken: H–H; C=C π. Stereochemistry: Syn addition from the face bound to Pd. Rate and selectivity: Fast; limited by H₂ and substrate access to the surface.
At a glance
Step 1: H₂ chemisorbs on Pd
H₂ dissociatively chemisorbs onto the palladium surface, splitting into two separate hydrogen atoms bound to adjacent metal sites — the same dissociative chemisorption strategy covered in the Haber process, just at room temperature on a noble metal surface.
Step 2: Alkene binds flat on the surface
The alkene coordinates flat against that same surface, positioning both alkene carbons near surface-bound hydrogen atoms simultaneously.
Step 3: Both H added to the same face
Both hydrogens transfer to the same face of the alkene in a concerted-feeling surface process, giving the saturated alkane with syn addition — because both new C–H bonds form on the metal-facing side, this delivers predictable syn-selective hydrogenation, unlike the anti-selective addition seen in halogenation.
Abbreviations and terms
- EtOH
- Ethanol. Polar protic solvent.
- EtOAc
- Ethyl acetate. Moderately polar solvent.
- Pd/C
- Palladium on carbon. Heterogeneous hydrogenation catalyst.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
How it compares with related reactions
| Catalytic hydrogenation of an alkene (Pd/C, H₂) | Contact process | Haber process | |
|---|---|---|---|
| Reagents | H₂ (1 atm), Pd/C | SO₂, O₂, V₂O₅ catalyst | N₂, H₂, promoted Fe catalyst (K₂O, Al₂O₃) |
| Conditions | EtOH or EtOAc, 25 °C | 400–450 °C, 1–2 atm | 400–500 °C, 150–300 atm |
| Stereochemistry | Syn addition from the face bound to Pd | Not applicable (surface reaction) | Not applicable (surface reaction) |
| Rate / selectivity | Fast; limited by H₂ and substrate access to the surface | Mars–van Krevelen: re-oxidation of V(IV) is rate-limiting; ΔH = −99 kJ/mol | Rate-limiting: dissociative chemisorption of N₂; ΔH = −92 kJ/mol |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Catalytic hydrogenation of an alkene (Pd/C, H₂)?
H₂ (1 atm), Pd/C. Conditions: EtOH or EtOAc, 25 °C.
Which bonds are formed and which are broken?
Formed: two C–H (syn). Broken: H–H; C=C π.
What is the stereochemical outcome?
Syn addition from the face bound to Pd
What controls the rate or the selectivity?
Fast; limited by H₂ and substrate access to the surface
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam