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Chapter 3: Conformations
of Alkanes and Cycloalkanes
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Substituted Cyclohexanes
Cyclohexane substituents can
be found in either axial or equatorial positions. However, in general, equatorial
substituents tend to be preferred because they are more stable because of reduced
steric interactions.
Let's consider an example, methycyclohexane.
In the equatorial system, the methyl group has space around it as it is pointed away from the rest of the ring. The C-C bond that connects the methyl group is anti to the two C-C
bonds in the rest of the ring system which means there is minimal torsional strain.
However, in the axial conformation,
the methyl group is closer to the rest of the ring. There is an unfavourable steric interaction between the methyl group with the two axial hydrogen atoms on
the same face of the ring. This destabilises the axial conformation. In the JSMOL images
below, this 1,3-diaxial interaction can be highlighted and contrasted
with the equatorial conformer. The close proximity of the substituent and the H atoms is most obvious when looking at the
space filling model. In addition, the C-C bond that connects the methyl group is gauche to the two C-C bonds in the rest of the ring system which means there is also some torsional strain.
Make sure you can see the steric
differences between the axial and equatorial situations.
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.
