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Mechanism of the Rhodium Catalysed Reaction

The reaction takes place in three stages.

CH3OH + HI          CH3I + H2O

CH3I + CO CH3COI

CH3COI + H2O     CH3COOH + HI

The first and last stages are not catalytic

 

Question

a) What is the coordination number, and oxidation state of the rhodium in the initial catalyst?

b) Work out how they change in step 1. Explain why this step is called oxidative addition.

c) What could you expect the rate of step 1 to depend on?

d) What happens to the oxidation state of the rhodium in step 4?

e) What kind of reaction is happening when the leaving molecule CH3COI reacts with water?

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The Rhodium Cycle
click below to see the cycle build up step by step

 

 

 

 

 

 

 

 

Question

Inspect the animated cycle carefully.  What is the rate-determining step?

Experimental work has shown that the reaction is first order with respect to the concentrations of both species involved in the rate-determining step. Write a rate equation for the reaction.

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click here for energy profile.
There are other side reactions involving methanol which lead to low levels of high boiling impurities like propanoic acid and these need to be removed in a final distillation column.

 

 

 

 

 

The catalyst is a complex ion which can dissolve in the reaction mixture and so this is an example of homogeneous catalysis. There are some disadvantages. Rhodium is expensive at $700 per troy ounce.

The water concentration in the reaction vessel needs to be 10% (by weight) to keep the rate of the desired reaction high, and to prevent the catalyst being lost from the reaction mixture as insoluble RhI3. The presence of this water means that a distillation column is needed to remove it from the product.
There are a number of side reactions. The rhodium catalysis the reaction

CO + H2O  CO2 + H2

which has two undesired effects

  • It reduces the concentration of CO in the reaction vessel and so the mixed gases have to be vented from the vessel and replaced by more CO.
  • It reduces the yield on CO