Conformations of Cyclobutane and Cyclopentane

Cyclobutane has less angle strain than cyclopropane but has more torsional strain because of its larger number of ring hydrogens. As a result, the total 

The conformation of cyclobutane. Part (a) shows computer-generated molecular models. Part (c) Is a Newman projection along the C1-C2 bond, showing that neighboring C-H bonds are not quite eclipsed. 

Problem 4.9 How many hydrogen-hydrogen eclipsing interactions would be present if cyclo-pentane were planar Assuming an energy cost of 4.0 kJ/mol for each eclipsing interaction, how much torsional strain would planar cyclopenlane have How much of this strain is relieved by puckering if the measured total strain of cyclopentane is 26.0 kJ/mol  

Problem 4.10 Draw the most stable conformation ofcis-l,3-dimcthylcydobutane. Draw the least stable conformation. 

An electron-density map provided by low-temperature X-ray studies. A top view looking down at the sample molecule shows how the electron densities in the cyclopropane bonds of the central ring are bent away from the internuclear axis. 

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Conformations of Cyclobutane and Cyclopentane Conformations of Cyclohexane 127 Axial and Equatorial Bonds in Cyclohexane 129 Conformational Mobility of Cyclohexane 131 Conformations of Monosubstituted Cyclohexanes Conformational Analysis of Disubstituted Cyclohexanes Boat Cyclohexane 140 Conformations of Polycyclic Molecules 141... 

Cycloalkanes with more than three carbon atoms are nonplanar and have puckered conformations. In cyclobutane and cyclopentane, puckering allows the molecule to adopt the most stable conformation (with the least strain energy). Puckering introduces strain by making the C—C—C angles a little smaller than they would be if the molecules were planar however, less eclipsing of the adjacent hydrogens compensates for this. 

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... 

Cyclopropane (Fig.J) is a flat molecule in respect of C-atoms, with the hydrogen atoms situated above and below the plane of the ring, so it has no conformational isomers. Cyclobutane can form three distinct shapes-a planar shape and two butterfly shapes (fig.K). Cyclopentane can also form a number of shapes or conformations. The planar structures for cyclobutane and cyclopentane are too strained to exist in practice because of eclipsed C-H bonds. 

The experimental data in Table 3.1 show that the calculated values of total angle strain are approximately correct only for cyclopropane, cyclobutane, and cyclopentane. Cyclohexane is definitely not the strained compound Baeyer s theory predicts, and the larger ring compounds are also not very strained. Any chemist today can explain the discrepancy between these calculated and experimental values of strain energy cyclohexane is not planar. In either the chair or boat conformations (Figure 3.12), all bond angles can be approximately 109.5°. In the chair conformation of cyclohexane, all bonds are staggered, and there are no apparent van der Waals repulsions in the molecule. ... 

We see that cyclopropane has the largest strain energy of any cycloalkane, which is consistent with the extreme compression of its C—C—C bond angles from 109.5° to 60°. Cyclobutane and cyclopentane each have less strain, and cyclohexane, as expected, has zero strain. What is perhaps surprising is the presence of strain in rings of from 7 to 13 carbon atoms. This strain is primarily the result of torsional and steric strain caused by the fact that these rings are constrained to conformations that cannot achieve ideal bond and torsional angles. 

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. 

FIGURE 2.15 The most stable conformations of cyclobutane, cyclopentane, and cyclohexane rings (pa and pe denote pseudoaxial and pseudoequatorial positions, respectively). 

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... 

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. 

The most important structural features that influence the conformation and reactivity of cycloalkanes differ depending on whether small (cyclopropane and cyclobutane), common (cyclopentane, cyclohexane, and cycloheptane), medium (cyclooctane through cycloundecane), or large (cyclododecane and up) rings are... 

The strain in cyclopropane and cyclobutane is largely a result of angular strain due to compression of the tetrahedral angle of 109 28 in the carbon-carbon bonding. In cyclopentane and cyclohexane this angular strain is reduced by the molecule adopting a puckered conformation. Thus, the conformation of the cyclopentane molecule is that of four carbon atoms in a plane, with one carbon atom some 0 5 A below this plane Fig. 1 (a). This is a mean value about which the puckering oscillates, and in addition there is a... 

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