Cyclobutanes Newman projection

Figure 4.5 The conformation of cyclobutane. Part (c) is a Newman projection along the C1-C2 bond, showing that neighboring C—H bonds are not quite eclipsed.

Problem 9.10 Depict the flexible puckered conformation of cyclobutane in a Newman projection. 

The conformation of cyclobutane is slightly folded. Folding gives partial relief from the eclipsing of bonds, as shown in the Newman projection. Compare this actual structure with the hypothetical planar structure in Figure 3-14. 

Cyclopentane is appreciably less strained than cyclobutane and cyclopropane, and the strain energy relative to cyclohexane is ca. 6.45 kJ mol-1 per CH2 group. In order to lessen the torsion strain that would occur in a planar conformation, in which every C-H bond is involved in two eclipsing interactions, cyclopentane adopts a puckered conformation (see Dunitz, Further Reading). This has four carbons approximately planar, with the fifth carbon bent out of this plane in such a way that the molecule resembles a small near-square envelope 9. A Newman projection of 9 is shown in 10. 

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