text: Conformational analysis of medium rings begins with examination of cyclooctane. Spectroscopic methods have determined that cyclooctane possesses three main conformations: chair-boat, chair-chair, and boat-boat. Cyclooctane prefers to reside in a chair-boat conformation, minimizing the number of eclipsing ethane interactions (shown in blue), as well as torsional strain. The chair-chair conformation is the second most abundant conformation at room temperature, with a ratio of 96:4 chair-boat:chair-chair observed.
  Substitution positional preferences in the ground state conformer of methyl cyclooctane can be approximated using parameters similar to those for smaller rings. In general, the substituents exhibit preferences for equatorial placement, except for the lowest energy structure (pseudo A-value of -0.3 kcal/mol in figure below) in which axial substitution is favored.  The "pseudo A-value" is best treated as the approximate energy difference between placing the methyl substituent in the equatorial or axial positions.  The most energetically unfavorable interaction involves axial substitution at the vertex of the boat portion of the ring (6.1 kcal/mol).
  These energetic differences can help rationalize the lowest energy conformations of 8 atom ring structures containing an sp center.  In these structures, the chair-boat is the ground state model, with substitution forcing the structure to adopt a conformation such that non-bonded interactions are minimized from the parent structure. From the cyclooctene figure below, it can be observed that one face is more exposed than the other, foreshadowing a discussion of privileged attack angles (see peripheral attack).
  X-ray analysis of functionalized cyclooctanes provided proof of conformational preferences in these medium rings. Significantly, calculated models matched the obtained X-ray data, indicating that computational modeling of these systems could in some cases quite accurately predict conformations. The increased sp character of the cyclopropane rings favor them to be placed similarly such that they relieve non-bonded interactions.