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 methylcyclohexane (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 con former. The fi rst 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.

The table shows the preference of a number of substituted cyclohexanes for the equatorially substituted conformer over the axially substituted conformer at 25 °C.

Note the following points (also referred to in Chapter 12).

• The three columns in the table are three different ways of expressing the same information. However, just looking at the percentage’s column, it is not immediately obvious to see how much more of the equatorial conformer there is—after all, the percentages of equatorial conformer for methyl, ethyl, isopropyl, t-butyl, and phenyl cyclohexanes are all 95% or more. Looking at the equilibrium constants gives a much clearer picture.

• The amount of equatorial conformer present does increase in the order Me < Et < i-Pr < t-Bu, but perhaps not quite as expected. The ethyl group must be physically larger than a methyl group but there is hardly any difference in the equilibrium constants. The increase in the proportion of equatorial conformer on going from Et to i-Pr is only a factor of two, but for t-butyl cyclohexane it is estimated that there is about 3000 times more of the equatorial conformer than the axial conformer.

• The same anomaly occurs with the methoxy group—there is a much greater proportion of the conformer with a methoxy group axial than with a methyl group axial. This is despite the fact that a methoxy group is physically larger than a methyl group.

• The equilibrium constant does not depend on the actual size of the substituent, but rather its interaction with the neighbouring axial hydrogens. In the axial conformer of methylcyclohexane there is a direct interaction between the methyl group and the axial hydrogen atoms.

• In the case of the methoxy group, the oxygen acts as link and removes the methyl group away from the ring, lessening the interaction.

• The groups Me, Et, i-Pr, and t-Bu all need to point some atom towards the other axial hydrogens, and for Me, Et, and i-Pr this can be H.

• Only for t-Bu must a methyl group be pointing straight at the axial hydrogens, so t-Bu has a much larger preference for the equatorial position than the other alkyl groups. In fact, the interactions between an axial t-Bu group and the axial hydrogen atoms are so severe that the group virtually always stays in the equatorial position. As we shall see later, this can be very useful.

مواضيع ذات صلة

E1 reactions can be stereoselective

The role of the leaving group

Substrate structure may allow E1

E1 and E2 mechanisms

Size

Elimination happens when the nucleophile attacks hydrogen instead of carbon

Axially and equatorially substituted rings react differently

Steroid

Decalins

What happens with more than one substituent on the ring

The ring inversion (flipping) of cyclohexane

A closer look at cyclohexane