Chirality and Topicity

Ideas That Already Should Be Familiar to You:

Plane of Symmetry: an infinitely thin plane that divides an object into halves that are mirror images of each other
Chirality: the property of lacking a plane of symmetry; most often but not always associated in molecules with the presence of a carbon having four different ligands
Any object that lacks a plane of symmetry is chiral; one that has one or more planes is achiral
Stereoisomers: a pair of molecules that differ in the way the same collection of atoms is arranged in three dimensional space
Enantiomers: stereoisomers that are nonsuperposable mirror images of each other; enantiomers are chiral
Diastereoisomers: stereoisomers that are not enantiomers; diastereomers may or may not be chiral

Stereoisomers (and other types of isomers) can be classified using the following decision tree:

Topicity is potential stereoisomerism.

Molecules having this property have structural elements that can be altered so as to create stereoisomers.

If the stereoisomers that result are enantiomers, the elements are labeled enantiotopic.

If the resulting stereoisomers are diastereomers, the elements labeled diastereotopic.
Making decisions about topicity in organic molecules can be facilitated by use of a simple decision tree (adapted from Mislow and Siegel, J. Am. Chem. Soc., 1984, 106, 3319):

Consider first the two CH₂ hydrogens in propane:

The plane defined by the bonds to the two methyl groups (bright green) is a symmetry plane of the molecule. Reflection through it interconverts the two hydrogens. Thus, they are related by symmetry and the answer to the first question is "Yes".
Furthermore, the plane defined by the bonds to the two hydrogens themselves (cyan) also is a symmetry plane. The answer to the second question is "Yes", and the hydrogens are homotopic.
Next, replace one of the methyls with an ethyl:

The green plane is still a plane of symmetry that interchanges the two hydrogens.

But now, the plane defined by the hydrogens (magenta) is no longer a plane of symmetry. The answer to the second question is "No". The hydrogens are enantiotopic.

Since enantiomers have identical chemical and physical properties, the enantiotopic hydrogens behave identically to all achiral reagents and spectroscopies.

However, they would be different toward an enzymatic oxidation, for example, or would have different chemical shifts if observed in a chiral solvent.
The third situation is obtained by replacing a hydrogen at the CH₂ of the ethyl by a chlorine, thus introducing a chiral center into the molecule.

Now neither plane is a symmetry plane. The answer to the first question is "No", and since the hydrogens have the same connectivity, the answer to the second question is "Yes". The hydrogens are diastereotopic.
They will be removed at different rates in a radical substitution, for example, and will have different chemical shifts in ordinary achiral solvents.
The test of substitution is another simple way to classify the hydrogens. In the first molecule, replacing the hydrogens alternately with an X group generates identical structures. In the second, it generates enantiomers. In the third, diastereomers are produced. Draw out the structures and check this for yourself.

This page last modified 12:15 PM on Tuesday August 16th, 2011.
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