Topicity
Topicity
Our interest here is in exploring potential chirality: * Can we change one ligand of four on a tetrahedral atom to generate a chiral center? * Can we add one ligand to a trigonal atom to create a chiral center? * To answer these questions, we need to find planes of symmetry (mirror planes) and consider how to destroy them. * 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. * 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. *
Our interest here is in exploring potential chirality: * Can we change one ligand of four on a tetrahedral atom to generate a chiral center? * Can we add one ligand to a trigonal atom to create a chiral center? * To answer these questions, we need to find planes of symmetry (mirror planes) and consider how to destroy them. * 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. * 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. *