Newman projection of cyclohexane

Newman projection of cyclohexane (chair and boat form)

A closer look at cyclohexane

The heats of combustion data show that cyclohexane is virtually strain-free. This must include strain

from eclipsing interaction as well as angle strain. A model of the chair conformation of cyclohexane

including all the hydrogen atoms looks like this.

The view along two of the C–C bonds clearly shows that there are no eclipsing C–H bonds in the
chair conformation of cyclohexane—in fact, all the bonds are fully staggered, giving the lowest
energy possible. This is why cyclohexane is strain-free.
Contrast this with the boat conformation. Now all the C–H bonds are eclipsed, and there is a particularly

bad interaction between the ‘flagstaff’ C–H bonds.

This explains why the boat conformation is much less important than the chair conformation.
Even though both are free from angle strain, the eclipsing interactions in the boat conformation
make it approximately 25 kJ mol–1 higher in energy than the chair conformation. In fact, as we shall
see later, the boat conformation represents an energy maximum in cyclohexane whilst the chair conformation
is an energy minimum. Earlier we saw how the eclipsing interactions in planar cyclobutane
and cyclopentane could be reduced by distortion of the ring. The same is true for the boat
conformation of cyclohexane. The eclipsing interactions can be relieved slightly if the two ‘side’ C–C

bonds twist relative to each other.

So we can say chair form is the most stable form of cyclohexane, since all bond in chair form are at perfect tetrahedral bond (109.5*) and all are staggered to each other, where as in boat form all bonds are eclipsed to each other hence, is less stable.

Effect of subsituent in Newman projection of cyclohexane (chair form)

If you want to add a substituent then equatorial position is more favorable then axial position, in order to have least steric crowding

Substituted cyclohexanes

In a monosubstituted cyclohexane, there can exist two different chair conformers: one with the substituent

axial, the other with it equatorial. The two chair conformers will be in rapid equilibrium (by

the process we have just described) but they will not have the same energy. In almost all cases, the

conformer with the substituent axial is higher in energy, which means there will be less of this form

present at equilibrium.

For example, in methylcylcohexane (X = CH3), the conformer with the methyl group axial is 7.3

kJ mol–1 higher in energy than the conformer with the methyl group equatorial. This energy difference

corresponds to a 20:1 ratio of equatorial:axial conformers at 25 °C.

There are two reasons why the axial conformer is higher in energy than the equatorial conformer.

The first is that the axial conformer is destabilized by the repulsion between the axial group X and the

two axial hydrogen atoms on the same side of the ring. This interaction is known as the 1,3-diaxial

interaction. As the group X gets larger, this interaction becomes more severe and there is less of the

conformer with the group axial.

The second reason is that in the equatorial conformer the C–X bond is anti-periplanar to

two C–C bonds, while, for the axial conformer, the C–X bond is synclinal (gauche) to two C–C bonds.