Editing Cyclohexane conformation (section)

{{short description|Structures of cyclohexane}}
[[Image:Cyclohexane-chair-colour-coded-3D-balls.png|250px|thumb|A cyclohexane molecule in ''chair'' conformation. Hydrogen atoms in axial positions are shown in red, while those in equatorial positions are in blue.]]

'''Cyclohexane conformations''' are any of several three-dimensional shapes adopted by [[cyclohexane]]. Because many [[chemical compound|compound]]s feature structurally similar six-membered [[Ring (chemistry)|rings]], the structure and dynamics of cyclohexane are important prototypes of a wide range of compounds.<ref>{{cite book|title=Stereochemistry of Organic Compounds|first1=Ernest Ludwig|last1=Eliel|first2=Samuel H.|last2=Wilen|publisher=Wiley India|year=2008|isbn=978-8126515707}}</ref><ref name=March6th>{{March6th}}</ref>

The [[internal angle]]s of a [[regular polygon|regular]], flat [[hexagon]] are 120°, while the preferred [[molecular geometry|angle between successive bonds]] in a [[carbon chain]] is about 109.5°, the [[tetrahedral angle]] (the [[arc cosine]] of −{{sfrac|1|3}}). Therefore, the cyclohexane ring tends to assume non-planar (warped) [[Conformational isomerism|conformations]], which have all angles closer to 109.5° and therefore a lower [[strain energy]] than the flat hexagonal shape.

Consider the carbon atoms numbered from 1 to 6 around the ring. If we hold carbon atoms 1, 2, and 3 stationary, with the correct bond lengths and the tetrahedral angle between the two bonds, and then continue by adding carbon atoms 4, 5, and 6 with the correct bond length and the tetrahedral angle, we can vary the three [[dihedral angle]]s for the sequences (2,3,4), (3,4,5), and (4,5,6). The next bond, from atom 6, is also oriented by a dihedral angle, so we have four [[degrees of freedom]]. But that last bond has to end at the position of atom 1, which imposes three conditions in three-dimensional space. If the bond angle in the chain (6,1,2) should also be the tetrahedral angle then we have four conditions. In principle this means that there are no degrees of freedom of conformation, assuming all the bond lengths are equal and all the angles between bonds are equal. It turns out that, with atoms 1, 2, and 3 fixed, there are two solutions called '''''chair''''', depending on whether the dihedral angle for (1,2,3,4) is positive or negative, and these two solutions are the same under a rotation. But there is also a continuum of solutions, a [[Circle (topology)|topological circle]] where [[angle strain]] is zero, including the '''''twist boat''''' and the '''''boat''''' conformations. All the conformations on this continuum have a twofold axis of symmetry running through the ring, whereas the chair conformations do not (they have [[Point groups in three dimensions|''D''{{sub|3d}}]] symmetry, with a threefold axis running through the ring). It is because of the symmetry of the conformations on this continuum that it is possible to satisfy all four constraints with a range of dihedral angles at (1,2,3,4). On this continuum the energy varies because of [[Pitzer strain]] related to the dihedral angles. The twist-boat has a lower energy than the boat. In order to go from the chair conformation to a twist-boat conformation or the other chair conformation, bond angles have to be changed, leading to a high-energy '''''half-chair''''' conformation. So the relative energies are: {{nowrap|''chair'' < ''twist-boat'' < ''boat'' < ''half-chair''}} with ''chair'' being the most stable and ''half-chair'' the least. All relative conformational energies are shown below.<ref name=":0" /><ref name='Nelson2011'>{{cite journal|last1 = Nelson|first1 = Donna J.|year = 2011|title = Toward Consistent Terminology for Cyclohexane Conformers in Introductory Organic Chemistry|journal = [[J. Chem. Educ.]]|volume = 88|pages=292–294|doi = 10.1021/ed100172k|last2 = Brammer|first2 = Christopher N.|issue = 3|bibcode = 2011JChEd..88..292N}}</ref> At room temperature the molecule can easily move among these conformations, but only ''chair'' and ''twist-boat'' can be isolated in pure form, because the others are not at local energy minima.

The boat and twist-boat conformations, as said, lie along a continuum of zero angle strain. If there are substituents that allow the different carbon atoms to be distinguished, then this continuum is like a circle with six boat conformations and six twist-boat conformations between them, three "right-handed" and three "left-handed". (Which should be called right-handed is unimportant.) But if the carbon atoms are indistinguishable, as in cyclohexane itself, then moving along the continuum takes the molecule from the boat form to a "right-handed" twist-boat, and then back to the same boat form (with a permutation of the carbon atoms), then to a "left-handed" twist-boat, and then back again to the achiral boat. The passage from boat → right-twist-boat → boat → left-twist-boat → boat constitutes a full [[pseudorotation]].