CONFIGURATIONAL ISOMERS
91
This is essentially the same as the tartaric acid
example, without the conformational complication.
Thus, there are two chiral centres, and the groups around
each centre are the same. Again, we get only three
stereoisomers rather than four, since the cis compound
is an optically inactive meso compound. There is a plane
of symmetry in this molecule, and it is easy to see that
one chiral centre is mirrored by the other, so that we
lose optical activity.
Conformational mobility, such as we get in cyclohexane rings, makes the analysis more difficult, and
manipulating molecular models provides the clearest vision of the relationships. Let us look at 1,2dimethylcyclohexane as an example. Again, we have
met the cis and trans isomers when we looked at conformational aspects (see Section 3.3.2). Here, we need
to consider both configuration and conformation.
H 3 C
H 3 C
CH 3
CH 3
H 3 C
CH 3
CH 3
CH 3
mirror
(+)- and (–)- trans enantiomers; two chair
conformations are shown for each, the favoured
diequatorial and the unfavoured diaxial − note that
the mirror image relationship is readily apparent in
both conformers
Care: this shows two interconvertible conformers for
each of the two non-interconvertible enantiomers
trans-1,2-dimethylcyclohexane
2
1
2
1
1
2
In the trans compound, two mirror image enantiomeric forms can be visualized. These will be the
(+)- and (−)-trans isomers. Note particularly that conformational changes may also be considered, but these
do not change configuration, so we are only seeing different conformers of the same compound. The
above scheme thus shows two interconvertible conformers (upper and lower structures) for each of the
two non-interconvertible enantiomers (left and right
structures).
The cis compound provides the real challenge,
however. If we draw version A, together with its
mirror image C, they do not look capable of being
H 3 C
CH 3
CH 3
CH 3
mirror
CH 3
CH 3
H 3 C
CH 3
this is the difficult one!
the cis isomer is an optically inactive meso
compound
the picture shows mirror images of the
equal-energy interconvertible conformers
however, consider a 120° rotation of A about the
central axis which produces D; 120° rotation of C
produces B; therefore, they are all the same
compound, but different conformers
cis-1,2-dimethylcyclohexane
2
1
A
B
C
D
superimposed. However, conformer A may be ringflipped to an equal-energy conformer B, and this will
have a corresponding mirror image version D. Now
consider a 120
◦ rotation of version A about the central
axis; this will give D. A similar 120
◦ rotation of version
C about the central axis will give B. It follows, therefore,
that if simple rotation of one structure about its axis
gives the mirror image of a conformational isomer,
then we cannot have enantiomeric forms but must
have the same compound. These are thus two different
conformers of an optically inactive meso compound. It
may require manipulation of models to really convince
you about this!
Now, although the cyclohexane ring is not planar, the overall consequences for trans- and cisdimethylcyclohexane can be predicted by looking at the
two-dimensional representations.
trans
cis
plane of
symmetry
no plane of
symmetry
the meso nature of cis-1,2-dimethylcyclohexane
can be deduced from the plane of symmetry in the
2D representation:
91
This is essentially the same as the tartaric acid
example, without the conformational complication.
Thus, there are two chiral centres, and the groups around
each centre are the same. Again, we get only three
stereoisomers rather than four, since the cis compound
is an optically inactive meso compound. There is a plane
of symmetry in this molecule, and it is easy to see that
one chiral centre is mirrored by the other, so that we
lose optical activity.
Conformational mobility, such as we get in cyclohexane rings, makes the analysis more difficult, and
manipulating molecular models provides the clearest vision of the relationships. Let us look at 1,2dimethylcyclohexane as an example. Again, we have
met the cis and trans isomers when we looked at conformational aspects (see Section 3.3.2). Here, we need
to consider both configuration and conformation.
H 3 C
H 3 C
CH 3
CH 3
H 3 C
CH 3
CH 3
CH 3
mirror
(+)- and (–)- trans enantiomers; two chair
conformations are shown for each, the favoured
diequatorial and the unfavoured diaxial − note that
the mirror image relationship is readily apparent in
both conformers
Care: this shows two interconvertible conformers for
each of the two non-interconvertible enantiomers
trans-1,2-dimethylcyclohexane
2
1
2
1
1
2
In the trans compound, two mirror image enantiomeric forms can be visualized. These will be the
(+)- and (−)-trans isomers. Note particularly that conformational changes may also be considered, but these
do not change configuration, so we are only seeing different conformers of the same compound. The
above scheme thus shows two interconvertible conformers (upper and lower structures) for each of the
two non-interconvertible enantiomers (left and right
structures).
The cis compound provides the real challenge,
however. If we draw version A, together with its
mirror image C, they do not look capable of being
H 3 C
CH 3
CH 3
CH 3
mirror
CH 3
CH 3
H 3 C
CH 3
this is the difficult one!
the cis isomer is an optically inactive meso
compound
the picture shows mirror images of the
equal-energy interconvertible conformers
however, consider a 120° rotation of A about the
central axis which produces D; 120° rotation of C
produces B; therefore, they are all the same
compound, but different conformers
cis-1,2-dimethylcyclohexane
2
1
A
B
C
D
superimposed. However, conformer A may be ringflipped to an equal-energy conformer B, and this will
have a corresponding mirror image version D. Now
consider a 120
◦ rotation of version A about the central
axis; this will give D. A similar 120
◦ rotation of version
C about the central axis will give B. It follows, therefore,
that if simple rotation of one structure about its axis
gives the mirror image of a conformational isomer,
then we cannot have enantiomeric forms but must
have the same compound. These are thus two different
conformers of an optically inactive meso compound. It
may require manipulation of models to really convince
you about this!
Now, although the cyclohexane ring is not planar, the overall consequences for trans- and cisdimethylcyclohexane can be predicted by looking at the
two-dimensional representations.
trans
cis
plane of
symmetry
no plane of
symmetry
the meso nature of cis-1,2-dimethylcyclohexane
can be deduced from the plane of symmetry in the
2D representation: