Twist-boat conformation steric strain

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

There are many other nonplanar conformations of cyclohexane, one of which is the boat conformation. You can visualize the interconversion of a chair conformation to a boat conformation by twisting the ring as illustrated in Figure 3.9. A boat conformation is considerably less stable than a chair conformation. In a boat conformation, torsional strain is created by four sets of eclipsed hydrogen interactions, and steric strain is created by the one set of flagpole interactions. Steric strain (also called nonbonded interaction strain) results when nonbonded atoms separated by four or more bonds are forced abnormally close to each other—that is, when they are forced closer than their atomic (contact) radii allow. The difference in potential energy between chair and boat conformations is approximately 27 kj/mol (6.5 kcal/mol), which means that, at room temperature, approximately 99.99% of all cyclohexane molecules are in the chair conformation. 

The conformers that cyclohexane assumes when interconverting from one chair conformer to the other are shown in Figure 3.12. To convert from the boat conformer to a chair conformer, one of the two topmost carbons of the boat conformer must be pulled down so that it becomes the bottommost carbon of the chair conformer. When the carbon is pulled down just a little, the twist-boat conformer is obtained, which is more stable than the boat conformer because the flagpole hydrogens have moved away from each other, thus relieving some steric strain. When the carbon is pulled... 

A number of 1,3-dithianyl-substituted 1,4-disilacyclohexanes were synthesized (98TL3197) and one molecular structure (176) was studied by X-ray diffraction (cf. Scheme 57). The two 1,3-dithianyl rings adopt strain-free chair conformations, whereas the conformation of the 1,4-disilacyclo-hexane ring was a twist-boat. The twist form was attributed to steric repulsion between the exo-cyclic C-S bonds the interaction between the silicon and the two sulfur atoms was also considered. 

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)... 

You can visualize interconversion of chair and boat conformations by twisting about two carbon-carbon bonds as illustrated in Figure 2.18. A boat conformation is considerably less stable than a chair conformation because of the torsional strain associated with four pairs of eclipsed C—H interactions and the steric strain between the two"flagpole"hydrogens. The difference in energy between chair and boat conformations is approximately 27 kj (6.5 kcal)/mol. 

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