Monsanto acetic acid process template

Rhodium cycle of the Monsanto process: SN2 oxidative addition of methyl iodide to cis-[Rh(CO)₂I₂]⁻ (rate-determining), methyl migration to CO with uptake of CO, and reductive elimination of acetyl iodide, which water hydrolyses to acetic acid and HI; HI turns methanol back into CH₃I; key step: SN2 attack of Rh(I) on CH₃I.

What the template shows

• Rh/I⁻ catalysed carbonylation of methanol: oxidative addition of CH₃I is rate-determining
• Overall: CH₃OH + CO → CH₃CO₂H (cat. [Rh(CO)₂I₂]⁻, HI/CH₃I, H₂O, 150–200 °C, 30–60 bar)
• Rh(I), 16 e⁻, square planar
• Rate = k[Rh][CH₃I]: zero order in CO and methanol, so oxidative addition of CH₃I is the slow step.
• Migratory insertion first gives the 16 e⁻ five-coordinate acyl [CH₃CORh(CO)I₃]⁻, which takes up CO to the 18 e⁻ complex drawn.
• Iodide is a co-catalyst: an organic cycle (CH₃OH → CH₃I, CH₃COI → CH₃CO₂H) feeds the metal cycle; selectivity is over 99% based on methanol.
• The BP Cativa process uses [Ir(CO)₂I₂]⁻ with a Ru promoter: oxidative addition of CH₃I is much faster at Ir, and migratory insertion becomes rate-determining.
• Key step: SN2 oxidative addition of CH₃I
• The electron-rich Rh(I) anion uses a filled d orbital (drawn as a lone pair) to attack the methyl carbon from the back; iodide leaves and then binds Rh, giving the six-coordinate Rh(III) methyl. This step is rate-determining: rate = k[Rh][CH₃I].
• methyl iodide
• CO ligands are named in the captions, not drawn · electron counts by the ionic method (I⁻ and CH₃⁻ as 2 e⁻ donors) · every part of this figure is editable

Drawn with 25 atoms, 6 arrows, 51 shapes – every one editable in MolDraw.

Sources

• Housecroft & Sharpe, Inorganic Chemistry, 5th ed., Pearson 2018, ch. 25 Textbook
• Crabtree, The Organometallic Chemistry of the Transition Metals, 6th ed., Wiley 2014, ch. 9 Textbook