Editing Cyclohexane conformation (section)

==Dynamics==
{{main|Ring flip}}

===Chair to chair===
[[File:Cyclohexane ring flip.svg|center|thumb|540x540px|Cyclohexane chair flip (ring inversion) reaction via boat conformation '''4'''. Structures of the significant conformations are shown: chair '''1''', half-chair '''2''', twist-boat '''3''' and boat '''4'''. When ring flip happens completely from chair to chair, hydrogens that were previously axial ({{color|blue|blue}} H in upper-left structure) turn equatorial and equatorial ones ({{color|red|red}} H in upper-left structure) turn axial.<ref name=":0">{{Cite book|title=Organic chemistry|last=J|first=Clayden|year=2003|isbn=9780191666216|edition=2nd|location=Oxford|pages=373}}</ref> It is not necessary to go through the boat form.]]

The interconversion of chair conformers is called '''ring flipping''' or '''chair-flipping'''. Carbon–hydrogen bonds that are axial in one configuration become equatorial in the other, and vice versa. At room temperature the two chair conformations rapidly [[Chemical equilibrium|equilibrate]]. The [[proton NMR|proton NMR spectrum]] of cyclohexane is a singlet at room temperature, with no separation into separate signals for axial and equatorial hydrogens.

In one chair form, the [[dihedral angle]] of the chain of carbon atoms (1,2,3,4) is positive whereas that of the chain (1,6,5,4) is negative, but in the other chair form, the situation is the opposite. So both these chains have to undergo a reversal of dihedral angle. When one of these two four-atom chains flattens to a dihedral angle of zero, we have the '''half-chair''' conformation, at a maximum energy along the conversion path. When the dihedral angle of this chain then becomes equal (in sign as well as magnitude) to that of the other four-atom chain, the molecule has reached the continuum of conformations, including the twist boat and the boat, where the bond angles and lengths can all be at their normal values and the energy is therefore relatively low. After that, the other four-carbon chain has to switch the sign of its dihedral angle in order to attain the target chair form, so again the molecule has to pass through the half-chair as the dihedral angle of this chain goes through zero. Switching the signs of the two chains sequentially in this way minimizes the maximum energy state along the way (at the half-chair state) — having the dihedral angles of both four-atom chains switch sign simultaneously would mean going through a conformation of even higher energy due to [[angle strain]] at carbons 1 and 4.

The detailed mechanism of the chair-to-chair interconversion has been the subject of much study and debate.<ref name=tBu>{{cite journal|title = Conformational Study of ''cis''-1,4-Di-''tert''-butylcyclohexane by Dynamic NMR Spectroscopy and Computational Methods. Observation of Chair and Twist-Boat Conformations|author1 = Gill, G.|author2 = Pawar, D. M.|author3 = Noe, E. A.|journal = [[J. Org. Chem.]]|year = 2005|volume = 70|issue = 26|pages = 10726–10731|doi = 10.1021/jo051654z|pmid = 16355992}}</ref>  The half-chair state ('''D''', in figure below) is the key [[transition state]] in the interconversion between the chair and twist-boat conformations. The half-chair has ''C''<sub>2</sub> symmetry. The interconversion between the two chair conformations involves the following sequence: chair → half-chair → twist-boat → half-chair′ → chair′.

===Twist-boat to twist-boat===
The boat conformation ('''C''', below) is a transition state, allowing the interconversion between two different twist-boat conformations. While the boat conformation is not ''necessary'' for interconversion between the two chair conformations of cyclohexane, it is often included in the [[reaction coordinate|reaction coordinate diagram]] used to describe this interconversion because its energy is considerably lower than that of the half-chair, so any molecule with enough energy to go from twist-boat to chair also has enough energy to go from twist-boat to boat. Thus, there are multiple pathways by which a molecule of cyclohexane in the twist-boat conformation can achieve the chair conformation again.

:[[File:Cyclohexane ring flip and relative conformation energies.svg|center|thumb|550x550px|Conformations: chair ('''A'''), twist-boat ('''B'''), boat ('''C''') and half-chair ('''D'''). Energies are {{cvt|43|kJ/mol|kcal/mol|0}}, {{cvt|25|kJ/mol|kcal/mol|0}} and {{cvt|21|kJ/mol|kcal/mol|0}}.<ref name=":0" />]]