Flagpole hydrogen

FIGURE 3 14 (a) A ball and spoke model and (b) a space filling model of the boat confor mation of cyclohexane Torsional strain from eclipsed bonds and van der Waals strain involving the flagpole hydrogens (red) make the boat less stable than the chair... 

Like the chair conformation, all of the C—C—C bond angles of the boat conformation are 109.5°, so it has no angle strain. However, it does have other types of strain. The two red hydrogens, called flagpole hydrogens, approach each other too closely and cause some sceric strain. In addition, the conformations about the green bonds are eclipsed. This can be seen more easily in the Newman projection down these bonds ... 

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. 

Two hydrogens (blue) in the boat conformation point upward like flagpoles. The twist boat reduces the steric repulsion of the flagpole hydrogens, (p. 113)... 

The chair forms of cyclohexane are 7 kcal/mol more stable than the boat forms. The boat conformation is destabilized by torsional strain because the hydrogens on the four carbon atoms in the plane are eclipsed. Additionally, there is steric strain because two hydrogens at either end of the boat—the flagpole hydrogens—are forced close to each other, as shown in Figure 4.14. 

Flagpole hydrogens (Section 4.12B) Hydrogens in the boat conformation of cyclohexane that are on either end of the boat and are forced into close proximity to each other. 

FIGURE 3.12 (a) The boat and (fa) skew boat conformations of cyclohexane. A portion of the torsional strain In the boat Is relieved by rotation about C—C bonds In the skew boat. Bond rotation Is accompanied by movement of flagpole hydrogens away from each other, which reduces the van der Waals strain between them. 

Cyclohexane can also exist in a boat conformation, shown in Figure 2.9. Like the chair conformer, the boat conformer is free of angle strain. However, the boat conformer is not as stable as the chair conformer because some of the bonds in the boat conformer are eclipsed, giving it torsional strain. The boat conformer is further destabilized by the close proximity of the flagpole hydrogens (the hydrogens at the bow and stern of the boat), which causes steric strain. 

Flagpole hydrogens, 99-100 Fluorinated hydrocarbons, boiling points, 130, 132 Fluorine... 

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