CHAPTER 8: Question 3
3D Explanations
Cyclobutanes adopt non-planar butterfly conformations. If the two substituents
are cis to one another, then both can adopt the less hindered, pseudo-equatorial
positions as shown below. If however, the substituents are trans to one
another then one of them must adopt a pseudo-axial position which is more
sterically hindered. Thus, the cis-isomer will be thermodynamically
more stable.
If the cyclobutane ring was to adopt a planar conformation, then in
the cis-isomer, the two substituents would be on the same face of
the ring and there would be a steric repulsion between them as shown in
the 3D structure below. In the trans-isomer however, the two substituents
will be on opposite faces of the four-membered ring and there will be no
interaction between them as shown below. Thus, in this case the trans-isomer
would be thermodynamically more stable.
The reaction occurs because the hydrogen atom next to the carbonyl of
the ester (in both the starting material and product) is acidic and can
be removed by a base such as sodium methoxide. This generates an enolate
which can be reprotonated from either face to either regenerate the starting
material or give the product. The product is the thermodynamically more
stable isomer, so eventually the trans-isomer (starting material)
is converted into the cis-isomer (product).
back to CHAPTER 8 answers
back to answers to problems
back to STEREOCHEMISTRY home page
