CHAPTER 8: Question 9

 

Simple Answers

 The slow step (and hence the rate determining step) of this reaction involves attack of hydroxide on the carbonyl bond. Hence, in the transition state of this step of the reaction, the acetate group is converted into a larger substituent since it is interacting with the hydroxide. In the trans-isomer (A), the acetate is in an equatorial
position, whilst in the cis-isomer (A) the acetate group has to adopt an axial position. Substituents in axial positions are less stable than those in equatorial positions for steric reasons, and this destabilisation increases as the size of the group increases. Hence, although isomer B is less stable than isomer A, the energy difference between the starting material and transition state for B is greater than the corresponding energy difference for isomer A. Since it is the energy difference between the starting material and transition state (the activation energy) that determines the rate of reaction, A is hydrolysed more rapidly than B. This is illustrated graphically
below.
 
 

back to CHAPTER 8 answers          back to answers to problems          back to STEREOCHEMISTRY home page