Inorganic — catalytic cycle
Monsanto process: catalytic acetic acid synthesis
How the Monsanto 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: CH₃OH + CO; [Rh(CO)₂I₂]⁻ cat., HI promoter. Conditions: 150–200 °C, 30–60 atm, H₂O. Bonds formed: CH₃–Rh; Rh–acyl; CH₃COOH; bonds broken: C–I (CH₃I); Rh–acyl. Stereochemistry: Not applicable (achiral). Rate and selectivity: Oxidative addition of CH₃I is rate-determining.
At a glance
Step 1: Oxidative addition of CH₃I
Rh(I) oxidatively adds methyl iodide (generated in situ from methanol + HI), forming a Rh(III) methyl complex.
Step 2: CO migratory insertion
Migratory insertion of a coordinated CO ligand forms a Rh-acyl species — the exact same insertion step covered earlier, now inside a catalytic cycle.
Step 3: Hydrolysis releases acetic acid
Reaction with water releases acetic acid and regenerates the Rh(I) catalyst plus HI — this cycle, run industrially, produces most of the world's acetic acid.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- CO
- Carbon monoxide (carbonyl ligand). σ-donor / strong π-acceptor ligand; also a C1 building block.
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
How it compares with related reactions
| Monsanto process | Homogeneous hydrogenation with Wilkinson's catalyst | Hydroformylation (oxo process) | |
|---|---|---|---|
| Reagents | CH₃OH + CO; [Rh(CO)₂I₂]⁻ cat., HI promoter | RhCl(PPh₃)₃ (cat.), H₂ (1 atm) | Alkene, CO, H₂ (syngas), HRh(CO)(PPh₃)₃ |
| Conditions | 150–200 °C, 30–60 atm, H₂O | Benzene or EtOH, 25 °C | 10–100 bar, 40–120 °C |
| Stereochemistry | Not applicable (achiral) | Syn addition of H₂; less-hindered alkenes react fastest | n:iso ratio controlled by ligand bite angle / PPh₃ excess |
| Rate / selectivity | Oxidative addition of CH₃I is rate-determining | Alkene insertion into Rh–H is rate-limiting | Rate depends on CO and H₂ pressure; bulky phosphines favour linear aldehyde |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Monsanto process?
CH₃OH + CO; [Rh(CO)₂I₂]⁻ cat., HI promoter. Conditions: 150–200 °C, 30–60 atm, H₂O.
Which bonds are formed and which are broken?
Formed: CH₃–Rh; Rh–acyl; CH₃COOH. Broken: C–I (CH₃I); Rh–acyl.
What is the stereochemical outcome?
Not applicable (achiral)
What controls the rate or the selectivity?
Oxidative addition of CH₃I is rate-determining
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam