Planar conformation of cyclopentane

Envelope, twist, and (hypothetical) planar conformations of cyclopentane. 

Solution 12 In the planar conformation of cyclopentane, aU five pairs of methylene hydrogen atoms are eclipsed. That produces 10 eclipsed hydrogen interactions. Some torsional strain is relieved in the envelope conformation since there are only six eclipsed hydrogen interactions. 

FIGURE 3 12 The (a) planar (b) envelope and (c) half chair conformations of cyclopentane... 

Figure 4.6 The conformation of cyclopentane. Carbons 1, 2, 3, and 4 are nearly planar, but carbon 5 is out of the plane. Part (c) is a Newman projection along the C1-C2 bond, showing that neighboring C-H bonds are nearly staggered.

Structural Considerations and Spectroscopic Properties.—The geometry of three conformations of cyclopentane, planar half chair (C2), and envelope (CJ, has been investigated by means of a floating spherical Gaussian orbital model which allows for extensive optimization of geometry. Relative to the half-chair the and planar forms were calculated to be 2.6 and 9.8 kcal mol less stable, values which are larger than found previously. 

Cyclic alkanes undergo pseudorotation because rotation by 360° is not possible. Pseudorotation in cyclic alkanes leads to many conformations. Cyclopropane is planar, with relatively weak banana bonds. The lowest energy conformation of cyclobutane is a puckered conformation. The lowest energy conformation of cyclopentane is an envelope conformation. The lowest energy conformation of cyclohexane is an equilibrating mixture of two chair conformations. 

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

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 ... 

The pseudorotation circuit of cyclopentane is essentially of constant strain, and therefore without maxima and minima in contrast, the fully planar conformer is less stable by ca. 5 kcal/mol [101]. An extensive treatment of the conformational aspects of cyclopentane and of many five-membered-ring compounds has been published by Legon [92]. 

Pseudorotation The progression of one conformer of a five-membered ring to another conformer. In the case of cyclopentane there is no planar intermediate all conformers have at least one carbon atom out of the plane of the other carbon atoms. The maximum pucker can, in this case, rotate with almost no potential energy barrier between conformers. Each of the multitude of possible conformers can be described in terms of the maximum pucker and the pseudorotation phase angle, that is, where the conformer lies on a pseudorotation cycle (with an arbitrarily chosen origin). 

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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