- The precursor structures are called prochiral
- The ligands are designated enantiotopic or diastereotopic depending on the outcome of the change.
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".
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.
- Homotopic species have identical properties under all circumstances.
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.
- The answer to the second question is "No".
- Since enantiomers have identical chemical and physical properties, the enantiotopic hydrogens behave identically to all achiral reagents and spectroscopies.
- However, they would be different in any interaction in a chiral environment, such as with an enzyme.
The center to which the enantiotopic hydrogens are attached is said to be prochiral.
- Configurations can be assigned to the ligands on the prochiral center using the Cahn-Ingold-Prelog rules.
- One imagines each ligand in turn to be replaced by a group X that has a slightly higher CIP priority than the other enantiotopic ligand.
- If the stereogenic center that results is R, then the ligand is pro-R. The other ligand then is pro-S.
- In the example above, the hydrogen projecting to the front is pro-S, whereas the other one is pro-R.
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 front H is still pro-S and the rear one pro-R.
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.
When an sp2 carbon becomes sp3, prochirality also can be defined. The sp2 center is planar, and has two faces.
A fourth ligand can be attached to either face, resulting in enantiomeric configurations:
Generation of a Chiral Center from a Prochiral Plane 
The face that would give rise to the R configuration is labeled the re face; the other is the si face.
In the example above, attachment from the reader's side of the page gives the S configuration, so that is the si face.
The Prochiral Plane 
The R Product The S Product 
