Ring flip

A conformation is a unique structural arrangement of atoms, in particular one achieved through the rotation of single bonds.

As a consequence of the chair flip, the axially-substituted and equatorially-substituted conformers of a molecule like chlorocyclohexane cannot be isolated at room temperature.

However, in some cases, the isolation of individual conformers of substituted cyclohexane derivatives has been achieved at low temperatures (–150 °C).

Substituent groups in equatorial positions roughly follow along the equator of the cyclohexane ring and are perpendicular to the axis, while substituents in axial positions roughly follow the imaginary axis of the carbon ring and are perpendicular to the equator.

An example is cyclopropane which, because of its planar geometry, has six fully eclipsed carbon and axial hydrogen bonds making the strain 116 kJ/mol (27.7 kcal/mol).

Two 1H NMR signals should be observed in principle, corresponding to axial and equatorial protons.

[6] As a consequence of the chair flip, the axially-substituted and equatorially-substituted conformers of a molecule like chlorocyclohexane cannot be isolated at room temperature.

However, in some cases, the isolation of individual conformers of substituted cyclohexane derivatives has been achieved at low temperatures (–150 °C).

The conformer of methylcyclohexane with equatorial methyl is favored by 1.74 kcal/mol (7.3 kJ/mol) relative to the conformer where methyl is axial.
The conformational changes that occur in a cyclohexane ring flip take place over several stages. Structure D (10.8 kcal/mol) is the highest energy transition state of the process.
Numbered six carbon ring demonstrating the changes made axially and equatorially when a ring is "flipped."In the left side representation, red hydrogens are equatorial, then become axial upon ring flip.
The 1 H NMR spectrum of titanocene pentasulfide features two signals at room temperature, a consequence of its relative rigidity.
Bicycloalkane with two "bridgehead carbons"
Skeletal structures of 1,8-dimethyl naphthalene and 4,5-dimethyl phenanthrene annotated with the steric effects between the two methyl groups .