Question

3. In the substitution reaction (Co(NH3)s(H20)]3+ + Y → [CO(NH3)s Y]2+ + H20, changing the entering group (Y) from SCN to Cl' results in very little change to the reaction rate. What does this tell us about the mechanism of the reaction? Draw a reaction coordinate diagram and write the rate law for the reaction.

Answer 1

The fact that Y^- if replaced by other ions does not affect the rate of reaction means that the rate is not dependent on Y^- and it is a first-order or unimolecular reaction. Y^- or any other ion just waits for H₂O to detach with the shared pair and intermediate to form.

The mechanism of this reaction can be written two steps as follows: (* Note, however, that different mechanism steps are possible but the order of reaction must match the data)

Step 1: [Co(NH₃)₅(H₂O)]³⁺$\rightarrow$ [Co(NH₃)₅]³⁺ + H₂O (slower step; rate-determining)

Step 2: [Co(NH₃)₅]³⁺ + Y^-$\rightarrow$ [Co(NH₃)₅ Y]²⁺ (faster step; does not determine rate)

The rate law for the above reaction: rate = k [[Co(NH₃)₅(H₂O)]³⁺]; where k is the reaction constant and outer square brackets denote molar concentration.

The reaction coordinate diagram of the above reaction is shown below:

Co(NH3)513++ H20 +Y- Transition state Eact Energy Reactants [Co(NH3)513+ Y- AG Products [Co(NH3)5 Y]2+ +H2O Reaction Progress