CHAPTER 9: Question 2

 

Simple Answers

The radioactive iodide displaces the iodide in an SN2 reaction. Each reaction incorporates one radioactive iodine atom into the product. When 50% of the starting material has reacted, all of the starting material / product will be racemic since 50% of the material will have the same configuration as the starting material, and 50% will have the opposite configuration. Since only 50% of the strating material needs to react to form a racemic product, but all of the strating material needs to react to become radioactive, it follows that the rate of racemization will be twice the rate of incorporation of radioactivity.

The significance of this data is that it shows that the rate of inversion (which is half the rate of racemization) is equal to the rate of reaction as required by the mechanism of an SN2 reaction.
 

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