Stereochemistry of Cyclohexane Conformational Analysis

TABLE 5.2 Strain Energies for Some Cycloalkanes from Ai/° and A//7 

FIGURE 5.23 Conversion of the energy-minimum chair cyclohexane into the halfchair and then the twist form. 

FIGURE 5.24 The interconversion of two chair cyclohexanes.The two chairs and the two twist forms are intermediates (energy minima), and the two half-chairs and the full-boat are transition states (energy maxima). 

Both the half-chair and the full-boat suffer from the kinds of strain we have seen before. In the half-chair, much of the ring is planar, and there is both angle and torsional strain. The full-boat also has angle and torsional strain, but there is another hydrogen-hydrogen interaction that is destabilizing. This new interaction is between the prow and stern carbons and between the two inside hydrogens at the prow and stern of the boat. This new kind of strain is induced when two atoms come too close to each other and is called van der Waals strain (Rg. 5.25). 

FIGURE 5.25 van der Waals strain in the full-boat form of cyclohexane. Don t confuse this strain with attractive van der Waals forces (p. 87). 

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