Gauche interaction, substituted cyclohexane

When the chair-boat-chair interconversion occurs with a substituted cyclohexane such as methylcyclohexane, the two cliair conformations are no longer equivalent because the methyl is equatorial in one and axial in the other. The axial methyl group introduces two gauche-butane interactions that are not present in the equatorial-methyl conformation ... 

Figure 6.13 Traditional explanations of equatorial preference of alkyl-substituted cyclohexanes based on the 1,3-diaxial repulsive interactions or the presence of gauche interactions.

To predict the more stable conformer of a more highly substituted cyclohexane, the cumulative effect of placing substituents either axially or equatorially must be considered, in addition to their potential mutual 1,3-diaxial or 1,2-gauche (Section 2-9) interactions. For... 

In substituted cyclohexanes, the chair conformation with the most and the largest substituents equatorial is the most stable this is attributed to gauche and 1,3-diaxial interactions. The difference in energy between axial and equatorially substituted forms is dependent mainly on the size of the substituent. 

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