Twist Boat Conformation of Cyclohexane

Because the boat conformation of cyclohexane is 29 kj mole less stable than the chair conformation, only a tiny fraction of cyclohexane molecules exist in a boat conformation. The steric energy of the boat conformation can be lowered by rotation around the C-2 to C 3 and C-5 to C-6 bonds, which decreases the hydrogen—hydrogen eclipsing interactions. The result is a twist boat conformation. It is about 22 kJ mole less stable than the chair conformation. This energy difference means that only about 0.01% of the cyclohexane molecules are in the twist boat conformation at room temperature. 

The chair—chair interconversion of cyclohexane passes through twist boat conformations that are in equilibrium with a boat conformation. 

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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... 

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. 

Boat and twist-boat conformations of cyclohexane. The twist-boat conformation is lower in energy than the boat conformation by 6 kj/mol. Both conformations are much more strained than chair cyclohexane. 

Fig. 22. Ring inversion of the twist-boat conformation of cyclohexane...

Squillacote M, Sheridan RS, Chapman OL, Anet FAL. Spectroscopic detection of the twist-boat conformation of cyclohexane. A direct measurement of the free energy difference between the chair and the twist-boat. J Am Chem Soc 1975 97 3244—3246. 

Given that the twist-boat conformer of cyclohexane is 3.8 kcal/mole (or 15.9 kJ/mol) higher in free energy than the chair... 

The twist boat conformation of cyclohexane has a lower energy than the pure boat conformation, but is not as stable as the chair conformation. 

The boat and twist-boat conformations of cyclohexane are in equilibrium. There are six bonds in cyclohexane, so pseudorotation around each of these bonds will induce a boat-twist-boat equilibration, leading to six boat and six twist conformations. Because all have the same energy, only the maximum and minimum are shown in the middle portion of Figure 8.10. 

Given that the twist-boat conformer of cyclohexane is 5.3 kcaiymol higher in free energy than the chair conformer, calculate the percentage of twist-boat conformers present in a sample of cyclohexane at 25 °C. Does your answer agree with the statement made in Section 3.12 about the relative numbCT of molecules in these two conformations ... 

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