Conformational Stereoisomerism
Consider two possible gauche
conformations of
butane, which are mirror images of each other. If you construct models of
butane and put them in these conformations, you will see that these
conformations are not superimposable. What we have are distinct right- and
left-handed versions of gauche-butane. One could say that these
conformations of butane are conformational enantiomers.
Figure %: Chiral gauche conformations of butane
Since butane exhibits this type of enantiomerism, does this mean that butane is
chiral? It does not. Individual conformations of butane may be chiral, but
the conformations of butane interconvert so rapidly that neither chiral
conformation can be isolated. On average, butane is still achiral. The
general rule concerning conformational enantiomers is as follows:
any molecule analyzed for chirality must be conformationally stable.
A molecule is not considered chiral if it is in rapid equilibrium with its
mirror image.
Chirality Without Stereogenic Carbons
The example of conformational enantiomers of gauche butane suggests that
conformational enantiomers could be isolated if interconversion between
the two gauche forms is prevented. There are some systems where this
does occur. For example, the conformations of many biphenyl systems are
restricted with regards to rotation about the central σ bond. The two
conformational enantiomers of the biphenyl molecule can be isolated
individually.
Figure %: A chiral biphenyl system
Cumulated dienes (allenes) also exhibit chirality without having
asymmetric carbon atoms. A cumulated diene is a molecule that contains two
double bonds on a single carbon. The central carbon atoms of allene are
sp-hybridized. The two ends of allene are perpendicular because the two
contiguous Π bonds use perpendicular p orbitals on the central
carbon:
Figure %: Orbital view of allene, whose two ends are perpendicular.
The twisted shape of allene makes substituted allenes chiral. Consider the
following allene with methyl substituents. The mirror images are not
superimposable:
Figure %: Enantiomers of chiral allenes
