Conformational Analysis of Cyclohexane


Audio introduction to Cyclohexane conformations: 


Cyclohexane is a saturated 6-carbon ring with the formula C6H12 that features sp3 carbon atoms in nearly perfect geometry; the bond angles are very close to 109.5 degrees and there is no torsional strain associated with the cycle. This is only possible, however, if the cycle adopts a shape, the chair conformation, that takes C1 and C4 out of the plane (right).

planarcyclohexane.gif

Notice that each carbon in the cycle has two valences that are not attached to other C atoms; one of these valences is pointing either straight up or down (axial) or is pointing out at an angle away from the cycle (equatorial). The direction of these valences will alternate around the cycle. Looking at the green axial substituent, this is pointing down, on the adjacent carbons the axial H atoms are pointing up, etc. Likewise, the green equatorial group is "up" and the neighbouring equatorial groups are "down" - this alternates around the cycle.

axialequatorial.gif

Any atom or group that is larger than H will prefer to exist in the equatorial orientation to avoid destabilizing 1,3-diaxial interactions (left). The axial group will be close in space to axial substituents 3 carbons away (i.e. 1,3). The larger the groups get, the more destabilizing the interactions and so the less favourable the conformation with axial groups. The "ring-flip" process allows axial groups to become equatorial to relieve strain (see below). 

diaxialinteractions.gif

Audio commentary on the "ring-flip" process:

 
ringflip.gif

YouTube videos that describe the ring-flipping of the cyclohexane ring: