AXIAL AND EQUATORIAL SUBSTITUENTS
As shown in the space-filling model below, the axial hydrogens in cyclohexane come quite close to each other without actually touching. However, if anything larger than hydrogen is placed in an axial position.....
....it can push against the the other axial atoms causing distortion and an increase in energy. Below is shown chorocyclohexane, in the chair conformation with axial Cl. Note how the C-Cl bond is angled slightly outwards due to this "steric effect".
No analogous distortion is observed for the equatorial conformer (below)
The effect is even clearer for methylcyclohexane (below).
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