Envelope form of cyclopentane

Figure 4.11 (a) The folded or bent conformation of cyclobutane, (b) The bent or envelop form of cyclopentane. In this structure the front carbon atom is bent upward. In actuality, the molecule is flexible and shifts conformations constantly... 

Aspects of cycloalkane conformational analysis. A. The two puckered conformations of cyclobutane. Note how the substituents (X and Y) exchange positions when puckered forms interconvert. B. Left The envelope form of cyclopentane—the conformer contains a mirror plane in the plane of the page. Right The half-chair form of cyclopentane, looking down the two-fold rotation axis. 

In cyclopentane, as opposed to cyclobutane and cyclopropane, the bond angles have values close to the optimum. Therefore, the strain in the molecule arises essentially from bond opposition and is partly relieved by puckered conformations. Two flexible forms of cyclopentane exist, namely the so-called envelope (LXXVlIIa) and half-chair (LXXVIIIb) forms. The former has four carbons in the same plane, and... 

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. 

Neither the puckered envelope form nor the twist form of cyclopentane is static. In the envelope form the flap moves around the ring, generating the five possible puckered isomers. This motion requires only a series of rotations around carbon-carbon bonds and closely resembles the motions in cyclobutane shown in Figure 5.10. A model will help you to visualize this motion. Hold two adjacent carbons, sight down the bond that connects them, and convert one form of the envelope into another. 

In contrast to the findings for the cyclobutanes where the large amplitude motions mainly consist of conversion between rather rigid forms, the cyclopentanes exhibit more complex conformational and dynamic properties. Pseudorotation is a prominent large amplitude motion prevailing not only in cyclopentane but also in other five-membered rings. If the barrier to pseudorotation is high, distinct conformations may exist. In this case, the envelope conformation which has maximum Cs symmetry... 

The two conformers LXXVlIIa and LXXVIIIb are extremes of symmetry in what is known as the pseudorotational circuit of cyclopentane. If in structure LXXVlIIa, the out-of-plane carbon (arbitrarily designated C-l here) is pushed down together with C-2, a half-chair is obtained (C-l above, C-2 below the plane). If the motion is continued, another envelope is reached (C-2 below the plane). The process then repeats with C-2 and C-3, and so on 10 envelope forms and 10 half-chair forms interconvert by this process, which is not unlike a wave on a water surface ([100], and refs, therein). 

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. 

Figure 4.14. Attempted perspective views of cyclopentane (a) as a flattened envelope showing bond lengths and angles, (b) as envelopes in equilibrium, and (c) in the twist form.

In the envelope conformation, one carbon atom is displaced from the plane of the other four. In the half-chair conformation, three carbon atoms are coplanar, while one atom is displaced above and another below the plane. The energy differences between conformers are small, and cyclopentane exists in a shallow potential well in which rapid interconversion of conformers occurs. All the carbon atoms in effect take turns in being the out-of-plane carbon in the envelope form. This low-energy motion by which five envelope conformations interconvert is called a pseudorotation. The same kind of motion interconverts the various equivalent half-chair conformations. Substituted cyclopentanes also adopt puckered geometries, which may be either envelope or half-chair, depending on the nature of the substitution. 

Cyclopentane is a twisted ring in the form of an envelope so that one of the carbon atoms is out of the plane of the ring. This decreases the number of echpsing interactions of hydrogen atoms on adjacent carbons. 

Any one of the five atoms of the ring can be either above or below the plane defined by the other four in the envelope conformation. The energy barriers separating them are very low, and in cyclopentane or in proline all of the envelope conformations are freely interconvertible through intermediate skew forms.32 Furanose sugar rings are very flexible but the presence of the bulky substituents reduces the number of possible conformations.33-363 See Chapter 5 for further discussion. 

Cyclobutane derivatives are usually, but not always, nonplanar. Cy-clobutane itself exists as two butterflylike conformers that are easily interconvertible by inversion. Cyclopentane is a cyclic five-membered ring structure that can exist as a set of half-chair (twist-boat) forms (C2) and a set of envelope (C5) conformations. These are low-energy conformations and are readily interconverted by twists about bonds without any bond angle changes, only changes in torsion angles. These interconversions are called pseudorotations. 

MM calculations of germacyclopentanes indicate that the symmetric twist form is the most stable in contrast to cyclopentane where the envelope and twist forms have much... 

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