Twist conformation of cyclohexane

Figure 8-6. Comparison of the radial distribution function of the ctiair, boat, and twist conformations of cyclohexane (hydrogen atoms are not considered).

Figure 4.16 (a) Carbon skeleton and (b) line drawing of the twist conformation of cyclohexane. 

The energy barrier between the chair, boat, and twist conformations of cyclohexane are low enough to make separation of the conformers impossible at room temperature. 

The energy barriers between the chair, boat, and twist conformations of cyclohexane are low enough (Fig. 4.16) to make separation of the conformers impossible at room temperature. At room temperature the thermal energies of the molecules are great enough to cause approximately 1 million interconversions to occur each second. 

A twist conformation of cyclohexane may be obtained Ity slightly twisting carbon atoms 2 and 5 of the boat conformation as shown. 

A modified boat conformation of cyclohexane, known as the twist boat (Fig. 1.8), or skew boat, has been suggested to minimize torsional and nonbounded interactions. This particular conformation is estimated to be about 1.5 kcal moE (6 kJ moE ) lower in energy than the boat form at room temperature. 

The easiest way to visualize chair cyclohexane is to build a molecular model. (In fact do it now.) Two-dimensional drawings like that in Figure 4.7 are useful, but there s no substitute for holding, twisting, and turning a three-dimensional model in your own hands. The chair conformation of cyclohexane can be drawn in three steps. 

In addition to the chair conformation of cyclohexane, a second arrangement called the twist-boat conformation is also nearly free of angle strain. It does, however, have both sleric strain and torsional strain and is about 23 kj/mol (5.5 kcal/mol) higher in energy than the chair conformation. As a result, molecules adopt the twisl-boat geometry only under special circumstances. 

Twist-boat conformation (Section 4.5) A conformation of cyclohexane that is somewhat more stable than a pure boat conformation. 

An interesting case of conformational analysis comes to play when we consider a six-membered ring (cyclohexane). There are many conformations that this compound can adopt. You will see them all in your textbook the chair, the boat, the twist-boat. The most stable conformation of cyclohexane is the chair. We call it a chair, because when you draw it, it looks like a chair ... 

Refer to Figure 24-9 for the chair and twist boat conformations of cyclohexane. 

In general, things are simpler than that, much to our advantage. Within the limits set by the precision of the present estimates, structural features like the chair, boat, or twist-boat conformations of cyclohexane rings, as well as the butane-gawc/ze effects or the cis-tmns isomerism of ethylenic compounds leave no recognizable distinctive trace in zero-point plus heat content energies. Indeed, whatever residual, presently... 

The chair conformer can undergo conformational isomerism to a second chair conformer which is degenerate in energy with the first. Cyclohexane is thus a dynamic molecule which exists largely in one of two chair isomers. These are the lowest energy conformations. Other higher energy conformations of cyclohexane include the boat form, which is 10.1 kcal/mol (42.3 kJ/mol) above the chair form, and the twist boat form, which lies 3.8 kcal/mol (15.9 kJ/mol) above the chair form. 

Figure 8.9. One octant of a sphere on which the conformations of cyclohexane can be mapped. Special conformations C, chair B, boat TB, twist boat HB, half-boat and HC, half-chair. (From Cremer and Pople [1975a,b].)...

The chair conformation of cyclohexane is not rigid. It can convert to a twist boat conformation and then to a new chair conformation in a process termed ring-flipping, as shown Figure 6.14 (not all the hydrogens are shown for clarity). 

In the symmetrical boat conformation of cyclohexane, eclipsing of bonds results in torsional strain. In the actual molecule, the boat is skewed to give the twist boat, a conformation with less eclipsing of bonds and less interference between the two flagpole hydrogens. 

The less stable puckered conformation of cyclohexane, with both parts puckered upward. The most stable boat is actually the twist boat (or simply twist) conformation. Twisting minimizes torsional strain and steric strain, (p. 113)... 

This twisting gives rise to a slightly different conformation of cyclohexane called the twist-boat conformation, which, although not as low in energy as the chair form, is lower in energy (by 4 kjmol-1)... 

In contrast to the results for monocyclic compounds, in bicyclic compounds the rigidity of bridged structure frequently permits more substantial effects than in acyclic or monocyclic compounds. The bridging locks the cyclohexane ring into the boat and/or twist-bond conformation. For example, bicyclo [2.2.1] heptane and bicyclo [2.2.2] octane have one and two boat cyclohexane rings respectively. The enthalpies of the boat and twist-boat conformers of cyclohexane are 27 and 23 kJ/mol higher than that of the chair conformer. Therefore, bicyclo [2.2.1] heptane and bicyclo [2.2.2] octane have ring strains of 64 and 54 kJ/mol, respectively, as shown in Table 4. 

Until recently knowledge of twist conformation of simple cyclohexanes could be said to be limited to certain di-f-butylcyclohexanes which if they existed as chair conformations would have an axial f-butyl group, to cyclohexane-1,4-dione and to certain highly substituted cyclohexanes. Undoubtedly twist conformations are quite common, it is unfortunate that reasonably direct evidence is often not available. 

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