Analysis of Substituted Cyclohexanes

Interchange of axial and equatorial positions of substituents in cyclohexane conformers. 

Axial and equatorial conformations of a methyl substituent on cyclohexane. 

Newman projections of mono-substituted cyclohexane conformations. 

Viewing the molecule from another orientation would show that there is one more such interaction. 

Since there are two 1,3-diaxial interactions for each axial methyl, we predict the axial conformation to be less stable than the equatorial conformation by about 1.8kcal/mol, which is close to the experimental value of 1.74kcal/mol. Thus, 1.74kcal/mol is the equatorial preference or Avalue for a methyl substituent. If we treat the distribution of a molecule between two conformations as a chemical equilibrium, we can use equation 3.2 to calculate that 6% of the methyl groups will be axial and 94% will be equatorial at 25°. 

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