CONFIGURATIONAL ISOMERS
109
trans
two enantiomers
cis
meso
plane of symmetry
no plane of
symmetry
three configurational isomers
When we look at the structures of trans- and cisdecalin, it is apparent that a further plane of symmetry,
through the ring fusion, is present in both structures.
This means that each isomer is superimposable on
its mirror image; consequently, there are only two
configurational isomers of decalin, one trans and
one cis.
H
H
trans-decalin
H
H
H
H
cis-decalin
H
H
≡
≡
plane of symmetry
planes of symmetry
only two configurational isomers
mirror image
mirror image
≡
H
H
H
H
≡
The situation in trans- and cis-decalin is complicated by the symmetry elements. If this symmetry is
destroyed, e.g. by introducing dimethyl substituents,
we get back to reassuringly familiar territory in which
two chiral centres lead to four configurational isomers. The same is true in the trans- and cis-1,2dimethylcyclohexane series.
trans
two enantiomers
cis
two enantiomers
H
H
trans
two enantiomers
H
H
H
H
cis
two enantiomers
H
H
four configurational isomers
four configurational isomers
destroy symmetry
dimethyl substitution removes
symmetry without adding a
new chiral centre
Fusing rings of different sizes can produce significant
restraints, especially when rings of less than six carbons
are involved. However, the characteristics of these fused
systems can be deduced logically by applying our
knowledge of single ring systems.
Fusion of a five-membered ring to a six-membered
ring gives a hydrindane system, and, as with decalins,
cis and trans forms are possible. Because the cyclopentane ring is more planar than a cyclohexane ring (see
Section 3.3.2), this causes deformation and increases
strain at the ring fusion. This deformation is more easily accommodated with the cis-fusion than the transfusion, and, in contrast to the decalins, the cis isomer
has a lower energy than the trans isomer (by about
1 kJ mol
−1 ). As in the decalins though, the cis form
is conformationally mobile, whereas the trans form
is fixed.
109
trans
two enantiomers
cis
meso
plane of symmetry
no plane of
symmetry
three configurational isomers
When we look at the structures of trans- and cisdecalin, it is apparent that a further plane of symmetry,
through the ring fusion, is present in both structures.
This means that each isomer is superimposable on
its mirror image; consequently, there are only two
configurational isomers of decalin, one trans and
one cis.
H
H
trans-decalin
H
H
H
H
cis-decalin
H
H
≡
≡
plane of symmetry
planes of symmetry
only two configurational isomers
mirror image
mirror image
≡
H
H
H
H
≡
The situation in trans- and cis-decalin is complicated by the symmetry elements. If this symmetry is
destroyed, e.g. by introducing dimethyl substituents,
we get back to reassuringly familiar territory in which
two chiral centres lead to four configurational isomers. The same is true in the trans- and cis-1,2dimethylcyclohexane series.
trans
two enantiomers
cis
two enantiomers
H
H
trans
two enantiomers
H
H
H
H
cis
two enantiomers
H
H
four configurational isomers
four configurational isomers
destroy symmetry
dimethyl substitution removes
symmetry without adding a
new chiral centre
Fusing rings of different sizes can produce significant
restraints, especially when rings of less than six carbons
are involved. However, the characteristics of these fused
systems can be deduced logically by applying our
knowledge of single ring systems.
Fusion of a five-membered ring to a six-membered
ring gives a hydrindane system, and, as with decalins,
cis and trans forms are possible. Because the cyclopentane ring is more planar than a cyclohexane ring (see
Section 3.3.2), this causes deformation and increases
strain at the ring fusion. This deformation is more easily accommodated with the cis-fusion than the transfusion, and, in contrast to the decalins, the cis isomer
has a lower energy than the trans isomer (by about
1 kJ mol
−1 ). As in the decalins though, the cis form
is conformationally mobile, whereas the trans form
is fixed.