Definitions
- Enantiomers are non-superimposable mirror images. Every chiral centre is inverted: (R,R) and (S,S).
- Diastereomers are stereoisomers that are not mirror images. Some chiral centres are inverted but not all: (R,R) and (R,S). Cis/trans and E/Z isomers are also diastereomers.
- Meso compounds have chiral centres but are achiral overall, because of an internal mirror plane. A meso compound is identical to its mirror image.
The quick test with R and S
Assign R or S to every chiral centre in both molecules (see R and S configuration), then compare:
| If the labels are… | The molecules are… | Example |
|---|---|---|
| all the same | identical | (2R,3R) and (2R,3R) |
| all opposite | enantiomers | (2R,3R) and (2S,3S) |
| some the same, some opposite | diastereomers | (2R,3R) and (2R,3S) |
One exception: if the molecule is meso, its "enantiomer" is the same molecule. (2R,3S)-2,3-dibromobutane and (2S,3R)-2,3-dibromobutane are identical.
A worked set: 2-bromo-3-chlorobutane
Two different chiral centres, so 2² = 4 stereoisomers:
(2R,3R) and (2S,3S) are enantiomers. (2R,3S) and (2S,3R) are another pair of enantiomers. Any one from the first pair and any one from the second pair are diastereomers.
How their properties differ
| Property | Enantiomers | Diastereomers |
|---|---|---|
| Melting and boiling point | identical | different |
| Solubility, density, NMR | identical (in non-chiral conditions) | different |
| Rotation of plane-polarised light | equal and opposite | unrelated |
| Reactions with chiral molecules (enzymes, drugs) | can be very different | different |
| Separation | hard: needs a chiral method | possible by ordinary methods (distillation, chromatography) |
Quick answers
What is the difference between enantiomers and diastereomers?
Enantiomers are non-superimposable mirror images, with every chiral centre inverted. Diastereomers are stereoisomers that are not mirror images, with only some chiral centres inverted.
Are cis and trans isomers diastereomers?
Yes. Cis/trans (and E/Z) isomers are stereoisomers that are not mirror images, so they are diastereomers.
Do enantiomers have the same physical properties?
Yes, apart from the direction they rotate plane-polarised light and how they interact with other chiral molecules. Their melting points, boiling points and solubility are the same.
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