- Ring strain
Ring strain is an
organic chemistryterm that describes the destabilization of a cyclic molecule—such as a cycloalkane—due to the non-favorable high energyspatial orientations of its atoms. Non-cyclic molecules do not exhibit ring strain because their terminal (end) atoms are not connected to force a particular type of spatial orientation.
Ring strain results from a combination of
angle strain, conformational strain or Pitzer strain, and transannular strain or van der Waals strain.
Molecules with a high amount of
ring strainconsist of three, four, and some five-membered rings, including: cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, [1,1,1] propellanes, [2,2,2] propellanes, epoxides, aziridines, cyclopentenes, and norbornenes. These molecules have bond anglesbetween ring atoms which are more acute than the optimal tetrahedral (109.5°) and trigonal planar (120°) bond anglesrequired by their respective sp3 and sp2 bonds. Because of the smaller bond angles, the bonds have higher energy and adopt more p-character to reduce the energy of the bonds. In addition, the ring structures of cyclopropanes/enes and cyclclobutanes/enes offer very little conformational flexibility. Thus, the substituents of ring atoms exist in an eclipsed conformationin cyclopropanesand between gauche and eclipsed in cyclobutanes, contributing to higher ring strainenergy in the form of Van der Waals repulsion.
The potential energy and unique bonding structure contained in the bonds of molecules with ring strain can be used to drive reactions in
organic synthesis. Examples of such reactions are Ring opening metathesis polymerisation, photo-induced ring opening of cyclobutenes, and nucleophilicring-opening of epoxidesand aziridines.
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