Inorganic — catalytic cycle (oxo process)
Hydroformylation (oxo process)
How the Hydroformylation (oxo process) 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: Alkene, CO, H₂ (syngas), HRh(CO)(PPh₃)₃. Conditions: 10–100 bar, 40–120 °C. Bonds formed: C–C; C–H; C=O of aldehyde; bonds broken: H–H; C=C π. Stereochemistry: n:iso ratio controlled by ligand bite angle / PPh₃ excess. Rate and selectivity: Rate depends on CO and H₂ pressure; bulky phosphines favour linear aldehyde.
At a glance
Step 1: Alkene insertion into Rh–H
An alkene coordinates to an HRh(CO)(PPh₃)₃ catalyst, then undergoes migratory insertion into the Rh–H bond, forming a Rh-alkyl species.
Step 2: CO insertion: alkyl → acyl
A second migratory insertion — this time of a coordinated CO ligand into the Rh–alkyl bond — forms a Rh-acyl species, extending the carbon chain by one.
Step 3: H₂ adds, aldehyde leaves
Oxidative addition of H₂ followed by reductive elimination releases an aldehyde one carbon longer than the starting alkene, regenerating the Rh-hydride catalyst — industrially, this makes millions of tons of aldehydes annually.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- PPh3
- Triphenylphosphine. Nucleophilic phosphine; forms very strong P=O bonds, driving Wittig, Appel and Mitsunobu reactions.
- CO
- Carbon monoxide (carbonyl ligand). σ-donor / strong π-acceptor ligand; also a C1 building block.
How it compares with related reactions
| Hydroformylation (oxo process) | Homogeneous hydrogenation with Wilkinson's catalyst | Monsanto process | |
|---|---|---|---|
| Reagents | Alkene, CO, H₂ (syngas), HRh(CO)(PPh₃)₃ | RhCl(PPh₃)₃ (cat.), H₂ (1 atm) | CH₃OH + CO; [Rh(CO)₂I₂]⁻ cat., HI promoter |
| Conditions | 10–100 bar, 40–120 °C | Benzene or EtOH, 25 °C | 150–200 °C, 30–60 atm, H₂O |
| Stereochemistry | n:iso ratio controlled by ligand bite angle / PPh₃ excess | Syn addition of H₂; less-hindered alkenes react fastest | Not applicable (achiral) |
| Rate / selectivity | Rate depends on CO and H₂ pressure; bulky phosphines favour linear aldehyde | Alkene insertion into Rh–H is rate-limiting | 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 Hydroformylation (oxo process)?
Alkene, CO, H₂ (syngas), HRh(CO)(PPh₃)₃. Conditions: 10–100 bar, 40–120 °C.
Which bonds are formed and which are broken?
Formed: C–C; C–H; C=O of aldehyde. Broken: H–H; C=C π.
What is the stereochemical outcome?
n:iso ratio controlled by ligand bite angle / PPh₃ excess
What controls the rate or the selectivity?
Rate depends on CO and H₂ pressure; bulky phosphines favour linear aldehyde
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam