Cyclopentane, angle strain conformation

Cyclopentane, angle strain in. 116 conformation of, 116 molecular model of, 116 strain energy of, 114 torsional strain in, 116 Cyclopentanone. TR spectroscopy of, 731... 

Conformational analysis of rings larger than cyclohexane is more complicated. These rings are also less common than cyclohexane, so we discuss their conformations only briefly. As can be seen from Table 6.1, the seven-membered ring compound cycloheptane has only a small amount of strain. Obviously, it is nonplanar to avoid angle strain. It does have some torsional strain, but the overall strain is comparable to that of cyclopentane. It is a fairly common ring system. 

There is another conformation of cyclohexane that you might have made that looks like this. This conformation is know as the boat conformation. In this conformation there are still four carbon atoms in one plane, but the other two are both above this plane. Now all the carbon atoms are not the same—the four in the plane are different from the ones above. However, this is not a stable conformation of cyclohexane, even though there is no bond angle strain (all the angles are 109.5°). In order to understand why not, we must go back a few steps and answer our other question why is cyclopentane strained even though a planar conformation has virtually no angle strain ... 

There is minimal angle strain in cyclopentane, but considerable torsional strain is present. Cyclopentane is nonplanar and the two minimum energy geometries are the envelope and the half-chair. In the envelope conformation, one carbon atom is... 

If cyclopentane were planar, as Baeyer had predicted, it would have essentially no angle strain, but its 10 pairs of eclipsed hydrogens would be subject to considerable torsional strain. So cyclopentane puckers, allowing the hydrogens to become nearly staggered. In the process, however, it acquires some angle strain. The puckered form of cyclopentane is called the envelope conformation because the shape resembles a squarish envelope with the flap up. 

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