Rotation around the stereogenic axis is restricted since the transition state for the rotation will have one of the two planar structures shown below. In these transition states, there is a steric repulsion between the R-group and one of the carbonyl oxygens. As the size of the R-group increases from ethyl (compound a) to isopropyl (compound b) to trifluoromethyl (compound c; NB fluorine atoms are significantly larger than hydrogen atoms) to tert-butyl (compound d), this steric repulsion will increase. Hence, the energy of the transition state will increase on going from a-d. The energy of the atropisomers themselves (i.e. of the non-planar form as drawn above) does not change significantly as the size of the R-group increases however, since there is no steric repulsion when the structure is non-planar. As a result, the activation energy for rotation will increase and the rate of rotation (i.e. the rate at which the atropisomers interconvert) will decrease as the size of the R-group increases.
No rate constant is quoted for R=H, since this compound is not chiral and so does not exist as a pair of atropisomers. The reason for the achiral nature of this compound is that the two ortho-substituents on the aromatic ring are now identical, as are the two meta-substituents.
back to simple answer to question 6 back to CHAPTER 3 answers
back to answers to problems back to STEREOCHEMISTRY home page