Conformations of cyclohexanes with one substituent

Draw the two chair conformations for ethylcyclohexane. Which is more stable  

The two conformations of methylcyclohexane are rapidly interconverting— they are in equilibrium. The conformer with the methyl equatorial is more stable than the conformer with the methyl axial, so the equatorial conformer is present in a larger amount in the equilibrium mixture. The axial strain energy is actually the free energy difference between the conformations and can be used to calculate the equilibrium constant for the process by using the equation A G° = —RT In K. Using the value of —1.7 kcal/mol (—7.1 kJ/mol) for AG°, the equilibrium constant is calculated to be 18 at room temperature. Therefore, at any instant, 95% of methylcyclohexane molecules have the methyl group equatorial, and only 5% have the methyl axial. 

In diagrams like this one, which do not show conformations, the ring is pictured as lying in the plane of the page. The methyl group is above the plane in this case. Of course, the geometry of the molecule is actually a chair conformation. 

The methyl group on C-l is gauche to C-3 of the ring. If a Newman projection down the C-l—C-6 bond were viewed, a similar gauche interaction between the methyl and C-5 of the ring would be found. 

Conformations of methylcyclohexane. Plat ring perspective, 0 CHAIR CONFORMATIONS, AND Q NEWMAN PROJECTION. 

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