SolutionsCyclohexane
1. Why is it not planar?
If cyclohexane were planar, what would its internal angle be? What two problems would that structure have?
120°. That angle is far from the 109.5° of an sp3 carbon (angle strain) and, in addition, all neighbouring hydrogens would be eclipsed (eclipsing strain). The chair solves both problems.
2. Axial and equatorial
How many axial and how many equatorial hydrogens does the cyclohexane chair have? What happens to them when the chair inverts?
Six axial (three up and three down, alternating) and six equatorial. When the chair inverts, all axial positions become equatorial and vice versa.
3. Inversion profile
Arrange in order of increasing energy: chair, boat, twist boat and the transition state of the inversion.
Chair (0) < twist boat (≈5.0 kcal/mol) < boat (≈6.5 kcal/mol) < transition state (≈10.8 kcal/mol).
At room temperature the barrier is overcome thousands of times per second, so the two chairs are in rapid equilibrium.
4. Newman
In the Newman projection of the chair, viewed along two parallel C–C bonds, how are the hydrogens on neighbouring carbons arranged?
All staggered, as in the staggered conformation of ethane or the anti conformation of butane. That is why the chair has no eclipsing strain.