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Prelog strain
In organic chemistry, transannular strain (also called Prelog strain after Nobel Prize–winning chemist Vladimir Prelog) is the unfavorable interactions of ring substituents on non-adjacent carbons. These interactions, called transannular interactions, arise from a lack of space in the interior of the ring, which forces substituents into conflict with one another. In medium-sized cycloalkanes, which have between 8 to 11 carbons constituting the ring, transannular strain is a major source of the overall strain, to which there is also contribution from large-angle strain and Pitzer strain.〔Smith and March, ''March's Advanced Organic Chemistry'', John Wiley & Sons Inc., 2007, ISBN 978-0-471-72091-1〕 In larger rings, transannular strain drops off until the ring is sufficiently large that it can adopt conformations devoid of any negative interactions.〔〔Anslyn and Dougherty, ''Modern Physical Organic Chemistry'', University Science Books, 2006, ISBN 978-1-891389-31-3〕
Transannular strain can also be demonstrated in other cyclo-organic molecules, such as lactones, lactams, ethers, cycloalkenes, and cycloalkynes. These compounds are not without significance, since they are particularly useful in the study of transannular strain. Furthermore, transannular interactions are not relegated to only conflicts between hydrogen atoms, but can also arise from larger, more complicated substituents interacting across a ring.
==Thermodynamics==
By definition, strain implies discomfiture, so it should follow that molecules with large amounts of transannular strain should have higher energies than those without. Cyclohexane, for the most part, is without strain and is therefore quite stable and low in energy. Rings smaller than cyclohexane, like cyclopropane and cyclobutane, have significant tension caused by small-angle strain, but there is no transannular strain. While there is no small-angle strain present in medium-sized rings, there does exist something called large-angle strain. Large-angle strain is used by rings with more than nine members to relieve some of the distress caused by transannular strain.〔〔
As the plot to the left indicates, the relative energies of cycloalkanes increases as the size of the ring increases, with a peak at cyclononane (with nine members in its ring.) At this point, the flexibility of the rings increases with increasing size; this allows for conformations that can significantly mitigate transannular interactions. Some of this strain is certainly due to Pitzer strain and large-angle strain, but a large amount is still caused by transannular strain.〔
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