4.3 Ring inversion of substituted cyclohexanes
4.3 Ring inversion of substituted cyclohexanes
Methylcyclohexane, which is obtained by replacing one of the hydrogen atom of cyclohexane with a methyl group, is a very intriguing compound. The two chair forms of methylcyclohexane are not identical as is the case with cyclohexane itself. The equatorial methyl derivative 21 and the axial methyl derivative 22 form a pair of stereoisomers (Fig. 4.3)
Figure 4.3 Ring inversion of methylcyclohexane
Two chair forms of cyclohexane 11 and 13 are identical, but two chair forms of methylcyclohexane 21 and 22 are stereoisomers. Then, how we can characterize the difference between 21 and 22? Let us return to cyclohexane and examine twelve hydrogen atoms preferably with the aid of a molecular model. The interatomic distances among three axial hydrogen atoms bonded to C1, C3 and C5 are relatively short. If one of the three hydrogen atoms, e.g., the hydrogen atom bonded to C1, is substituted by a large group such as a methyl, the distance between the methyl group and the axial hydrogen atoms bonded to C3 or C5 become much shorter to generate severe interaction. This will make the energy of the axial methylcyclohexane higher than that of the equatorial one and this interaction is referred to as 1,3-diaxial interaction.