Ring strain nonbonded atom interactions

There are many other nonplanar conformations of cyclohexane, one of which is the boat conformation. You can visualize the interconversion of a chair conformation to a boat conformation by twisting the ring as illustrated in Figure 3.9. A boat conformation is considerably less stable than a chair conformation. In a boat conformation, torsional strain is created by four sets of eclipsed hydrogen interactions, and steric strain is created by the one set of flagpole interactions. Steric strain (also called nonbonded interaction strain) results when nonbonded atoms separated by four or more bonds are forced abnormally close to each other—that is, when they are forced closer than their atomic (contact) radii allow. The difference in potential energy between chair and boat conformations is approximately 27 kj/mol (6.5 kcal/mol), which means that, at room temperature, approximately 99.99% of all cyclohexane molecules are in the chair conformation. 

The medium-size rings (7 to 12 ring atoms) are relatively free of angle strain and can easily take a variety of spatial arrangements. They are not large enough to avoid all nonbonded interactions between atoms. 

The extent of substitution of the carbon atoms a to the C=C bond is the same in 132 as in 130a, but some nonbonded interactions are removed by ring closure in 132, and the C=C torsion angle is only 11.8° with R(C=C) 134.9 pm (194) in good agreement with calculations (187). The strain energies in 131b and 132 were calculated to be 41.58 and 60.67 kcal/mol, respectively. 

The conformational preference of perhydropyrrolo[l,2-c][l,3]oxazine (Section X) for the O-inside cis-conformation (501) must be due to its stabilization relative to the trans-conformation (500) and the O-outside cis-conformation (502) by the generalized anomeric effect and the presence of ring fusion strain (see previous discussion) in the trans-conformer (500). The nonbonded interaction between the oxygen atom... 

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