Ring flip

Ring flipping (also known as ring inversion or ring reversal) is a phenomenon involving the interconversion (by rotation) about single bonds of cyclic conformers having equivalent ring shapes but not necessarily equivalent spatial positions of substituent atoms.

The six-membered ring of the alkane, cyclohexane, has the 'chair' as its preferred conformation and through proton NMR it would be expected that the produced spectrum would show the different sorts of protons (axial and equatorial) resonating at different frequencies, and so two would be seen, due to the protons being in slightly different chemical environments. However, only one signal can be seen as the two isomers are rapidly interconverting. This also occurs with monosubstituted cyclohexanes. This changes with the use of low temperature, as the two isomers visible interconvert much more slowly than at room temperature.

When cyclohexane undergoes a ring inversion, all axial bonds become equatorial, or vice-versa. The half-chair conformation is the energy maximum when proceeding from the 'chair' to the 'twisted boat' conformation. The 'true boat' conformation is the energy maximum for the interchanging of the two mirror image twist boat conformers, the second of which is converted to the other chair confirmation through another half-chair.

However, the chemical reactivity of cyclohexane, is inconsistent with two types of hydrogens in a stable form of the molecule (for example, there is only one monochlorocyclohexane, not two, as would be predicted if axial and equatorial hydrogens could be replaced independently), but this is due to the presence of rapid ring inversion as explained above. In substituted cyclohexanes, steric repulsion can be minimised by the substituents occupying positions along the equatorial plane rather than the axial plane, and thus would also lead to greater stability in the molecule.

References

* Clayden, J. Greeves, N. Warren, S. Wothers, P. (2001). "Organic Chemistry". Oxford. ISBN 0-19-850346-6
* [http://www.chem.uic.edu/web1/OCOL-II/WIN/STRUCT/SB/23.HTM Conformations of Alkanes & Cycloalkanes]