CONFORMATIONS OF 4- AND 5-MEMBERED RINGS

Although planar structures would minimise Baeyer strain in cyclobutanes and cyclopentanes, they maximise Pitzer strain.  Cyclobutane, for example, distorts slightly so that neighbouring CH bonds do not quite eclipse each other.



 


Cyclopentane also distorts, to give the "envelope" conformation (below) and the "twisted envelope" (bottom).  There are subtle but definite differenes between these two conformations.
 



 
 


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