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Organic Chemistry: Stereochemistry

Other Forms of Stereoisomerism

Fischer Projections

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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

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