How to spot one
- Check there are at least two chiral centres with the same set of groups (for example, both carbons have H, Br and CH₃).
- Look for an internal mirror plane that cuts the molecule into two halves that are reflections of each other. Draw it in an eclipsed or Fischer conformation, where the plane is easier to see.
- Or use R and S: for two matching centres, (R,S) is meso, while (R,R) and (S,S) are a pair of enantiomers.
Examples
In a Fischer projection, meso compounds are easy to recognise: you can draw a horizontal line through the middle and the top half reflects onto the bottom half.
Why it matters: counting stereoisomers
Two chiral centres usually give 2² = 4 stereoisomers. If the two centres carry the same groups, one of those four is meso and two of them are the same molecule, so there are only three:
| 2,3-Dibromobutane | Chiral? | Relationship |
|---|---|---|
| (2R,3R) | yes | enantiomers of each other |
| (2S,3S) | yes | |
| (2R,3S) = (2S,3R) | no: meso | diastereomer of both of the others |
Properties
A meso compound is achiral, so it is optically inactive: it doesn't rotate plane-polarised light. This is different from a racemic mixture, which is also inactive but is a 50:50 mixture of two chiral molecules. A meso compound is a single compound.
Adding Br₂ to (E)-but-2-ene gives the meso dibromide, because the two bromine atoms add to opposite faces (anti addition). The (Z) alkene gives the racemic (2R,3R)/(2S,3S) pair.
Quick answers
What is a meso compound?
A meso compound has two or more chiral centres but is achiral overall, because it has an internal mirror plane. It is identical to its own mirror image and doesn't rotate plane-polarised light.
Is a meso compound optically active?
No. A meso compound is achiral, so it is optically inactive.
What is the difference between a meso compound and a racemic mixture?
Both are optically inactive. A meso compound is a single achiral compound. A racemic mixture is a 50:50 mix of two enantiomers, each of which is chiral.
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