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Chapter 3: Conformations
of Alkanes and Cycloalkanes
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Substituted Cyclohexanes
Substituents on cyclohexane can, in principle,
occupy either axial or equatorial positions. However, in general, equatorial
substituents are more preferred since they are more stable because of reduced
steric interactions.
This is shown in the following example for methycyclohexane.
In the equatorial system, the methyl group is anti to the C-C
bonds in the rest of the ring system. However, in the axial conformation,
a steric interaction of the methyl group with the two axial hydrogen atoms on
the same face of the ring destabilises the axial conformation. In the images
below, this 1,3-diaxial interaction can be highlighted and contrasted
with the equatorial conformer. The effect is most obvious when viewed in the
space filling model, see the JMOL images below. Make sure you can see the steric
differences.
The larger the alkyl substituent is, the greater
the preference for the equatorial position since the larger the group the greater
the steric interaction with the axial hydrogens.
