Stereoisomers have the same chemical formula but different 3D arrangements, so special methods are needed to separate or identify them.
1. Geometrical Isomers (cis/trans, fac/mer)
- Separation Methods:
- Crystallization – cis and trans forms often have different solubilities.
- Chromatography – sometimes cis/trans isomers separate on silica gel or HPLC.
- Distinguishing Methods:
- UV-Vis or IR spectroscopy – different spatial arrangements can shift absorption bands.
- NMR spectroscopy – different environments of protons or carbons give distinct signals.
2. Optical Isomers (Enantiomers)
- Separation Methods (Enantiomeric Resolution):
- Chiral chromatography – uses a chiral stationary phase to separate enantiomers.
- Using chiral reagents – react enantiomers with another chiral compound to form diastereomers, which can be separated (diastereomers have different physical properties).
- Crystallization with chiral acids/bases – sometimes only one enantiomer crystallizes first.
- Distinguishing Methods:
- Polarimetry – measures the rotation of plane-polarized light.
- One enantiomer rotates light clockwise (+), the other counterclockwise (–).
- Circular Dichroism (CD) – shows differences in absorption of left- vs. right-circularly polarized light.
- NMR with chiral shift reagents – enantiomers give separate signals in a chiral environment.
- Polarimetry – measures the rotation of plane-polarized light.
3. Notes
- Geometrical isomers differ in position, optical isomers differ in handedness.
- Sometimes a combination of chromatography, spectroscopy, and crystallization is used to separate and identify stereoisomers.
In short:
- Geometrical isomers → separated by solubility or chromatography, identified by NMR/IR.
- Optical isomers → separated by chiral methods, identified by polarimetry or circular dichroism.