Chair Flip
As stated above, a chair flip is a ring inversion specifically of cyclohexane (and its derivatives) from one chair conformer to another, often to reduce steric strain. The term, "flip" is misleading, because the direction of each carbon remains the same; what changes is the orientation. A conformation is a unique structural arrangement of atoms, in particular one achieved through the rotation of single bonds.Brown, William H., et al. ''Organic Chemistry''. 8th ed., Cengage Learning, 2018. A conformer is a conformational isomer, a blend of the two words.Cyclohexane
There exist many different conformations for cyclohexane, such as chair, boat, and twist-boat, but the chair conformation is the most commonly observed state for cyclohexanes because it requires the least amount of energy. The chair conformation minimizes both angle strain and torsional strain by having all carbon-carbon bonds at 110.9° and all hydrogens staggered from one another. The molecular motions involved in a chair flip are detailed in the figure on the right: The half-chair conformation (D, 10.8 kcal/mol, ''C''2 symmetry) is the energy maximum when proceeding from the chair conformer (A, 0 kcal/mol reference, ''D''3d symmetry) to the higher energy twist-boat conformer (B, 5.5 kcal/mol, ''D''2 symmetry). The boat conformation (C, 6.9 kcal/mol, ''C''2v symmetry) is a local energy maximum for the interconversion of the twoAxial and Equatorial Positions
As noted above, by transitioning from one chair conformer to another, all axial positions become equatorial and all equatorial positions become axial. Substituent groups in equatorial positions roughly follow along the equator of the cyclohexane ring and are perpendicular to the axis, while substituents in axia76l positions roughly follow the imaginary axis of the carbon ring and are perpendicular to the equator.Brown, William H., et al. ''Organic Chemistry''. 8th ed., Cengage Learning, 2018. Di-axial interactions or Axial-Axial interactions is what the steric strain between an axial substituent and another axial group, typically a hydrogen, on the same side of a chair conformation ring. The interaction is labeled by the carbon number they come from. A 1-3 di-axial interaction happens between the atoms connected to the first and third carbons. The more interactions the more strain on the molecule and the conformations with the most strain are less likely to be seen. An example is Cyclopropane which, because of its planar geometry, has six fully eclipsed carbon and axial hydrogen bonds making the strain ''116 kj (27.7 kcal)/mol.'' Strain can also be decreased when the carbon-carbon bond angles are close or at the preferred bond angle of 109.5°, meaning a ring having six tetrahedral carbons is typically lower than that of most rings.Examples
Cyclohexane is a prototype for low-energy degenerate ring flipping. Two 1H NMR signals should be observed in principle, corresponding to axial and equatorial protons. However, due to the cyclohexane chair flip, only one signal is seen for a solution of cyclohexane at room temperature, as the axial and equatorial proton rapidly interconvert relative to the NMR time scale. The coalescence temperature at 60 MHz is ca. –60 °C.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).\ Most compounds with nonplanar rings engage in degenerate ring flipping. One well-studied example is titanocene pentasulfide, where the inversion barrier is high relative to cyclohexane's. Hexamethylcyclotrisiloxane on the other hand is subject to a very low barrier. Bicycloalkanes areSee also
* Cyclohexane conformation * Conformational isomerismReferences
External links
* {{Clayden, pages=460–461