92
STEREOCHEMISTRY
It is clear that this representation of cis-dimethylcyclohexane shows a plane of symmetry, and we
can deduce it to be a meso compound. No such
plane of symmetry is present in the representation
of trans-dimethylcyclohexane. Why does this approach
work? Simply because the transformation of planar
cyclohexane (with eclipsed bonds) into a non-planar
form (with staggered bonds) is a conformational change
achieved by rotation about single bonds. The fact that
cyclohexane is non-planar means we may have to invoke
the conformational mobility to get the three-dimensional
picture.
Our consideration of meso compounds leads us to
generalize:
• a molecule with one chiral centre is chiral;
• a molecule with more than one chiral centre may be
chiral or achiral.
Now let us extend this generalization with a further
statement:
• a molecule may be chiral without having a chiral
centre.
This is the subject of the next section.
3.4.6 Chirality without chiral centres
We shall restrict discussions here to three types of
compound. In the first we get what is termed torsional
asymmetry, where chirality arises because of restricted
rotation about single bonds. The commonest examples
involve two aromatic rings bonded through a single
bond (biphenyls). If large groups are present in the
ortho positions, these prevent rotation about the interring single bond, and the most favourable arrangement
to minimize interactions is when the aromatic rings
are held at right angles to each other. As a result,
two enantiomeric forms of the molecule can exist.
Because of the size of the ortho groups, it is not
possible to interconvert these stereoisomers merely by
rotation. Even when we only have two different types of
substituent, as shown, we get two enantiomeric forms.
CO 2 H
Cl
HO 2 C
Cl
HO 2 C
Cl
CO 2 H
Cl
chirality via restricted rotation − torsional asymmetry
mirror
large ortho groups prevent rotation
two enantiomeric forms exist
rotate
structure 90º ≡
CO 2 H
Cl
Cl
HO 2 C
The second type of compound is called an allene;
these compounds contain two double bonds involving
the same carbon. These compounds exist, but are often
difficult to prepare and are very reactive. It is the
concept of chirality which is more important here than
the chemistry of the compounds. If a carbon atom is
involved in two double bonds, it follows that the π
bonds created must be at right angles to each other.
The consequence of this is that the substituents on the
other carbons of the allene are also held at right angles
to each other. Again, two enantiomeric forms of the
molecule can exist.
STEREOCHEMISTRY
It is clear that this representation of cis-dimethylcyclohexane shows a plane of symmetry, and we
can deduce it to be a meso compound. No such
plane of symmetry is present in the representation
of trans-dimethylcyclohexane. Why does this approach
work? Simply because the transformation of planar
cyclohexane (with eclipsed bonds) into a non-planar
form (with staggered bonds) is a conformational change
achieved by rotation about single bonds. The fact that
cyclohexane is non-planar means we may have to invoke
the conformational mobility to get the three-dimensional
picture.
Our consideration of meso compounds leads us to
generalize:
• a molecule with one chiral centre is chiral;
• a molecule with more than one chiral centre may be
chiral or achiral.
Now let us extend this generalization with a further
statement:
• a molecule may be chiral without having a chiral
centre.
This is the subject of the next section.
3.4.6 Chirality without chiral centres
We shall restrict discussions here to three types of
compound. In the first we get what is termed torsional
asymmetry, where chirality arises because of restricted
rotation about single bonds. The commonest examples
involve two aromatic rings bonded through a single
bond (biphenyls). If large groups are present in the
ortho positions, these prevent rotation about the interring single bond, and the most favourable arrangement
to minimize interactions is when the aromatic rings
are held at right angles to each other. As a result,
two enantiomeric forms of the molecule can exist.
Because of the size of the ortho groups, it is not
possible to interconvert these stereoisomers merely by
rotation. Even when we only have two different types of
substituent, as shown, we get two enantiomeric forms.
CO 2 H
Cl
HO 2 C
Cl
HO 2 C
Cl
CO 2 H
Cl
chirality via restricted rotation − torsional asymmetry
mirror
large ortho groups prevent rotation
two enantiomeric forms exist
rotate
structure 90º ≡
CO 2 H
Cl
Cl
HO 2 C
The second type of compound is called an allene;
these compounds contain two double bonds involving
the same carbon. These compounds exist, but are often
difficult to prepare and are very reactive. It is the
concept of chirality which is more important here than
the chemistry of the compounds. If a carbon atom is
involved in two double bonds, it follows that the π
bonds created must be at right angles to each other.
The consequence of this is that the substituents on the
other carbons of the allene are also held at right angles
to each other. Again, two enantiomeric forms of the
molecule can exist.