The rate-determining step in the catalytic synthesis of acetic acid by Monsanto process is

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CSIR-UGC (NET) Chemical Science: Held on (15 Dec 2019)
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  1. oxidative addition of CH3I to [RhI2(CO)2]
  2. migration of CH3 group to CO of [RhI3(CO)2(CH3)]
  3. loss of CH3COI from [RhI3(CO)2(COCH3)]
  4. coordination of CO to [RhI3CO(COCH3)]

Answer (Detailed Solution Below)

Option 1 : oxidative addition of CH3I to [RhI2(CO)2]
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Concept:

→ The Monsanto process is an industrial method for the manufacture of acetic acid by catalytic carbonylation of methanol. 

Explanation:

F1 Quid Utakarsha S -16-2-22 D19

→ The catalytically active species is the anion cis-[Rh(CO)2I2]− .The first organometallic step is the oxidative addition of methyl iodide to cis-[Rh(CO)2I2] to form the hexacoordinate species [(CH3)Rh(CO)2I3].

→ This anion rapidly transforms, via the migration of a methyl group to an adjacent carbonyl ligand, affording the pentacoordinate acetyl complex [(CH3CO)Rh(CO)I3].

→ This five-coordinate complex then reacts with carbon monoxide to form the six-coordinate dicarbonyl complex, which undergoes reductive elimination to release acetyl iodide (CH3C(O)I).

→ The catalytic cycle involves two non-organometallic steps: conversion of methanol to methyl iodide and the hydrolysis of the acetyl iodide to acetic acid and hydrogen iodide.

→ The reaction has been shown to be first-order with respect to methyl iodide and [Rh(CO)2I2], i.e, rate of the reaction is directly proportional to the concentration of one of the reactants. Hence the oxidative addition of methyl iodide is proposed as the rate-determining step.

Conclusion:
The RDS is the oxidative addition of CH3I to [RhI2(CO)2].

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