Inorganic — catalytic cycle
Homogeneous hydrogenation with Wilkinson's catalyst
How the Homogeneous hydrogenation with Wilkinson's catalyst 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: RhCl(PPh₃)₃ (cat.), H₂ (1 atm). Conditions: Benzene or EtOH, 25 °C. Bonds formed: two C–H bonds (syn); bonds broken: H–H; C=C π. Stereochemistry: Syn addition of H₂; less-hindered alkenes react fastest. Rate and selectivity: Alkene insertion into Rh–H is rate-limiting.
At a glance
Step 1: Oxidative addition of H₂
Rh(I) oxidatively adds H₂ across the metal, forming a Rh(III) dihydride — the same oxidative addition mechanism covered earlier, just with H₂ instead of an alkyl halide.
Step 2: Alkene binds and inserts into Rh–H
The alkene substrate coordinates, then undergoes migratory insertion into one Rh–H bond, forming a Rh–alkyl species.
Step 3: Reductive elimination releases the alkane
Reductive elimination releases the saturated alkane and regenerates Rh(I) — the same three mechanistic steps you've already learned individually, chained into a full catalytic cycle.
Abbreviations and terms
- EtOH
- Ethanol. Polar protic solvent.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- PPh3
- Triphenylphosphine. Nucleophilic phosphine; forms very strong P=O bonds, driving Wittig, Appel and Mitsunobu reactions.
- Wilkinson
- Wilkinson’s catalyst, RhCl(PPh₃)₃. Homogeneous hydrogenation catalyst.
How it compares with related reactions
| Homogeneous hydrogenation with Wilkinson's catalyst | Hydroformylation (oxo process) | Monsanto process | |
|---|---|---|---|
| Reagents | RhCl(PPh₃)₃ (cat.), H₂ (1 atm) | Alkene, CO, H₂ (syngas), HRh(CO)(PPh₃)₃ | CH₃OH + CO; [Rh(CO)₂I₂]⁻ cat., HI promoter |
| Conditions | Benzene or EtOH, 25 °C | 10–100 bar, 40–120 °C | 150–200 °C, 30–60 atm, H₂O |
| Stereochemistry | Syn addition of H₂; less-hindered alkenes react fastest | n:iso ratio controlled by ligand bite angle / PPh₃ excess | Not applicable (achiral) |
| Rate / selectivity | Alkene insertion into Rh–H is rate-limiting | Rate depends on CO and H₂ pressure; bulky phosphines favour linear aldehyde | Oxidative addition of CH₃I is rate-determining |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Homogeneous hydrogenation with Wilkinson's catalyst?
RhCl(PPh₃)₃ (cat.), H₂ (1 atm). Conditions: Benzene or EtOH, 25 °C.
Which bonds are formed and which are broken?
Formed: two C–H bonds (syn). Broken: H–H; C=C π.
What is the stereochemical outcome?
Syn addition of H₂; less-hindered alkenes react fastest
What controls the rate or the selectivity?
Alkene insertion into Rh–H is rate-limiting
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam