Conformations of Monosubstituted Cyclohexanes
Unlike cyclohexane, which has two equivalent chair conformers, the two chair conformers of a monosubstituted cyclohexane such as methylcyclohexane are not equivalent. The methyl substituent is in an equatorial position in one conformer and in an axial position in the other (Figure 2.11), because substituents that are equatorial in one chair conformer are axial in the other (Figure 2.8).
Figure 2.11 A substituent is in an equatorial position in one chair conformer and in an axial position in the other. The conformer with the substituent in the equatorial position is more stable.
The chair conformer with the methyl substituent in an equatorial position is the more stable conformer because a substituent has more room and, therefore, fewer steric interactions when it is in an equatorial position. This can be best understood by examining Figure 2.12, which shows that when the methyl group is in an equatorial position, it is anti to the C-3 and C-5 carbons. Therefore, the substituent extends into space, away from the rest of the molecule.
In contrast, when the methyl group is in an axial position, it is gauche to the C-3 and C-5 carbons (Figure 2.13). As a result, there are unfavorable steric interactions between the axial methyl group and both the axial substituent on C-3 and the axial substituent on C-5 (in this case, hydrogens). In other words, the three axial bonds on the same side of the ring are parallel to each other, so any axial substituent will be relatively close to the axial substituents on the other two carbons. Because the interacting substituents are on 1,3-positions relative to each other, these unfavorable steric interactions are called 1,3-diaxial interactions. If you take a few minutes to build models, you will see that a substituent has more room if it is in an quatorial position than if it is in an axial position.
“Build a model of methylcyclohexane, and convert it from one chair conformer to the other.”
Figure 2.12 An equatorial substituent on the C-1 carbon is anti to the C-3 and C-5 carbons.
Figure 2.13 An axial substituent on the C-1 carbon is gauche to the C-3 and C-5 carbons.
The gauche conformer of butane and the axial-substituted conformer of methylcyclohexane are compared in Figure 2.14. Notice that the gauche interaction is the same in both—an interaction between a methyl group and a hydrogen bonded to a carbon gauche to the methyl group. Butane has one such gauche interaction and methylcyclohexane has two.
Figure 2.14 The steric strain of gauche butane is the same as the steric strain between an axial methyl group and one of its axial hydrogens. Butane has one gauche interaction between a methyl group and a hydrogen; methylcyclohexane has two.
In Section 2.10, we saw that the gauche interaction between the methyl groups of butane caused the gauche conformer to be 0.9 kcal mol (3.8 kJ mol) less stable than the anti conformer. Because there are two such gauche interactions in the chair conformer of methylcyclohexane when the methyl group is in an axial position, this chair conformer is 1.8 kcal mol (7.5 kJ mol) less stable than the chair conformer with the
methyl group in the equatorial position. Because of the difference in stability of the two chair conformers, at any one time more monosubstituted cyclohexane molecules will be in the chair conformer with the substituent in the equatorial position than in the chair conformer with the substituent in the axial position. The relative amounts of the two chair conformers depend on the substituent (Table 2.10). The substituent with the greater bulk in the area of the 1,3- diaxial hydrogens will have a greater preference for the equatorial position because it will have stronger 1,3-diaxial interactions. For example, the equilibrium constant (Keq) for the conformers of methylcyclohexane indicates that 95% of methylcyclohexane molecules have the methyl group in the equatorial position at 25 °C:
“The larger the substituent on a cyclohexane ring, the more the equatorialsubstituted conformer will be favored.”
In the case of tert-butylcyclohexane, where the 1,3-diaxial interactions are even more destabilizing because a tert-butyl group is larger than a methyl group, more than 99.9% of the molecules have the tert-butyl group in the equatorial position.