The Larger Cycloalkanes and Their Conformations

Possible conformations for cycloheptane include the comfortable appearing chair form, 7. However, this form has eclipsed hydrogens at C4 and C5 as well as nonbonded interactions between the axial-like hydrogens on C3 and C6. The best compromise conformation is achieved by a 30°-40° rotation around the C4-C5 bond to relieve the eclipsing of the hydrogens. This spreads the interfering hydrogens at C3 and C6 and results in a somewhat less strained conformation called the twist chair. The twist chair, 8, is very flexible and 

Exercise 12-22 If the twist-chair conformation 8 were rigid rather than flexible, how many different monochlorocycloheptanes would you expect (a) excluding mirror-image isomers and (b) including mirror image isomers  

There are several more or less reasonable looking cyclooctane conformations. After much research it now is clear that the favored conformation is the boat-chair, 9, which is in equilibrium with a few tenths percent of the crown conformation, 10  

The activation energy for interconversion of these two forms is about 10 kcal mole-1. The boat-chair conformation 9 is quite flexible and movement of its CH2 groups between the various possible positions occurs with an activation energy of only about 5 kcal mole-1. 

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