Cyclopentane, angle strain

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... 

Cyclopropane (115 kj/mol strain) and cyclobutane (110.4 kj/mol strain) have both angle strain and torsional strain. Cyclopentane is free of angle strain but has a substantial torsional strain due to its large number of eclipsing interactions. Both cyclobutane and cyclopentane pucker slightly away from planarity to relieve torsional strain. 

Cyclopentane has little torsional strain and angle strain. 

Cyclopentane with a planar ring would have five pairs of eclipsed H s producing considerable eclipsing strain. This strain is reduced, at the expense of some increase in angle strain, when one CHj pushes out of the plane of the ring (Fig. 9-4). The puckering is not fixed but alternates around the ring. 

Because cyclopentane and cyclobutane (Sections 12-3E and 12-3F) also have nonplanar carbon rings, it is clear that the Baeyer postulate of planar rings is not correct. Nonetheless, the idea of angle strain in small rings is important. There is much evidence to show that such strain produces thermodynamic instability and usually, but not always, enhanced chemical reactivity. 

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 ... 

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