Chapter 4 - Stereochemistry of Alkanes and Cycloalkanes
Conformations of Cyclohexane
If the cyclohexane ring is substituted with subsituents like a methyl group, the two chair conformations are no longer equal in energy (like the all H cyclohexane). The conformation with the methyl group in the axial position is higher in energy because the methyl group has steric interactions with the other axial groups (1,3-diaxial interactions).
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H
H
C H
3
H H
H axial methylcyclohexane side and top views
H
H
H
H
H
C H
3
equatorial methylcyclohexane side and top views
The chair conformations of disubstituted cyclohexanes will have different interactions depending on where on the ring they are substituted and their stereochemistry.
In cis-1,2-dimethylcyclohexane one can draw both chair conformations. In each conformation, one methyl is in an axial position and one is in an equatorial position. Thus, both conformations have identical energy. In the trans stereoisomer, there is a clear difference in the two
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Cook
North Dakota State University page 1 Chem 341

conformations. One places both methyl groups in the more crowded axial positions and one in the more stable (lower energy) equatorial positions.
cis-1,2-dimethylcyclohexane
C H
3
1
2
C H
3 trans-1,2-dimethylcyclohexane
C H
3
1
2
C H
3
2
H ax
C H
3
1
C
H
H eq
3 ax
C H
3 ring flip
1 C H
3 eq
2
H
H same interactions in both conformations -- equal in energy
2
C H ax
C H
3
1
H
H
3 ax
H ring flip different interactions in both conformations -- NOT equal in energy
C H
3 eq
1
2 eq
C H
3
H lower in energy
In the 1,3-isomers, the opposite is the case. The cis isomer has different energy conformations while the conformations of the trans isomer are equal in energy. Note that on one side of the ring or the other, the axial and equatorial positions alternate from one carbon to the next.
cis-1,3-dimethylcyclohexane
1
C H
3 ax
C H
3 ax
C H
3
1
H ring flip eq
H
3
C
3 1 C H
3 eq
3
C H
3
H
3 different interactions in both conformations -- NOT equal in energy
H H lower in energy trans-1,3-dimethylcyclohexane
1
C H
3
H ax
C H
3
1
H ring flip
H 3 1 C H
3 eq
3
C H
3 eq H
3
C
3 ax
C H
3
H same interactions in both conformations -- equal in energy
The 1,4-isomers have the same issues as the 1,2-isomers. The cis conformation places one group equatorial and one axial in both conformations while the trans isomer has different energy conformations.
© 2007 Gregory R .
Cook
North Dakota State University page 2 Chem 341

H
3
C cis-1,4-dimethylcyclohexane
C H
3
1
4 H
3
C eq
H
4
H
3
C trans-1,4-dimethylcyclohexane
C H
3
1
4 H
4 ax C H
3 ax
C H
3
1
H ring flip ax
H
C H
4
3
1 same interactions in both conformations -- equal in energy
H
C H
3 eq ax
C H
3
1
H ring flip
H
3
C eq different interactions in both conformations -- NOT equal in energy
H
4
1 lower in energy
H
C H
3 eq
The size of the groups can also influence the energy of the different ring flip conformers. For example, if the groups are different and one is bigger than the other, the cis-1,4-isomer favors the one in which the largest group is equatorial. You can find a chart with different substituents and their size effects in your textbook.
H cis-1-isopropyl-4-methylcyclohexane
C H
3
1
C H
C H
3
3
C
4 H
3 eq
C
4
H
H
3
C ax
C H
C H
3
1
H ring flip ax
H
The larger group would prefer to be in the equatorial position --
NOT equal in energy
C H
3
4 lower in energy
1
C H
3 eq
C H
C H
3
H
Cyclohexane can exist in another higher energy conformation, but this is obviously not preferred. In the Boat conformation, cyclohexane has several additional eclipsing interactions that increase the torsional strain in the molecule.
boat cyclohexane
H
H H
H
H
H
H H
For on-line models of alkane and cycloalkane conformations, see the Models Page on the web .
For a good site for drawing chairs, see: http://www.csub.edu/Chemistry/331/cychex/chair.html
© 2007 Gregory R .
Cook
North Dakota State University page 3 Chem 341

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© 2007 Gregory R .
Cook
North Dakota State University page 4 Chem 341