Cyclohexane twist boat conformation

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. 

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... 

Cyclohexane minimizes its ring strain by being puckered rather than flat. The two extreme conformations are the more stable chair and the less stable boat. The twist-boat conformer is less stable than the chair by about 23 kJ/mol, but is more stable than the boat. It is formed from the boat by moving one flagpole" to the left and the other to the right. See Fig. 9-5. 

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). 

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)... 

Cyclohexane sometimes adopts a twist-boat conformation, but never a true boat structure, which represents an energy maximum, But boat structures are important in some bicyclic compounds where the compound simply doesn t have any choice in the conformation it adopts. The simplest compound locked into a boat structure is norbornane. The CH2 bridge has to be diaxial (otherwise it can t reach), which means that the cyclohexane ring part of the structure has no choice but to be a boat,... 

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. 

D-(-f )-Glucose contains the six-membered, pyranose ring. Since the C—O—C bond angle (11 L) is very nearly equal to the tetrahedral angle (109.5°), the pyranose ring should be quite similar to the cyclohexane ring (Sec. 9.14). It should be puckered and, to minimize torsional and van der Waals strain, should exist in chair conformations in preference to twist-boat conformations. X-ray analysis shows this reasoning to be correct. 

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. 

The boat form is an alternate conformation of cyclohexane. Actually, by a slight twist, the nonbonded interactions in the boat form can be reduced (twist boat conformation). 

Boat cyclohexane is approximately 29 kJ/mol (7.0 kcal/mol) less stable than chair cyclohexane, although this value is reduced to about 23 kJ/mol (5.5 kcal/mol) by twisting slightly, thereby relieving some torsional strain (Figure 4.24). Even this twist-boat conformation is still much more strained than the chair conformation, though, and molecules adopt this geometry only under special circumstances. 

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