Enantiomers, diastereomers and meso compounds
Enantiomers are non-superimposable mirror images. Diastereomers are stereoisomers that are not mirror images. A meso compound has stereocentres but is achiral because of an internal mirror plane, so it has fewer isomers than 2ⁿ.
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Analogy
Enantiomers are left and right gloves. Diastereomers are cousins: related but not mirror images.
Example
Two stereocentres: expect 4(R,R) and (S,S): enantiomers(R,S) is meso (internal mirror plane)Total: 3 stereoisomers
In practice
Tartaric acid in grapes has three stereoisomers: a pair of enantiomers and one meso form.
Practice
Flashcards
- What are enantiomers?Non-superimposable mirror images.
- What is a meso compound?One with stereocentres but an internal mirror plane, so it is achiral.
Questions people ask
Why does a meso compound not rotate plane-polarised light?
A meso compound has an internal mirror plane, so one half of the molecule cancels the other half's effect on light. It is achiral and optically inactive, even though it contains stereocentres. A racemic mixture is also inactive, but for a different reason: equal amounts of enantiomers cancel.
Do diastereomers have the same physical properties?
No. Unlike enantiomers, diastereomers have different melting points, boiling points and solubilities. This is useful: chemists separate a mixture of enantiomers by reacting it with a single pure chiral reagent to make diastereomers, which can then be separated by ordinary methods.