Definition of Meso Compounds:
- A meso compound is a special type of stereoisomer that contains multiple chiral centers but is nevertheless achiral as a whole.
- In other words, it has chiral centers, but due to an internal plane of symmetry (or another element of symmetry), the molecule is superimposable on its own mirror image.
Identifying a Meso Compound
- Multiple Chiral Centers: You might see more than one carbon each bearing four different substituents.
- Internal Plane of Symmetry: There is at least one plane that bisects the molecule into two halves that are mirror images of each other.
- Achirality as a Whole: Even though individual carbons are “chiral centers,” the entire molecule does not rotate plane-polarized light because the two “halves” cancel each other’s optical rotation.
Examples of Meso Compounds
- The classic example is meso tartaric acid. It has two chiral centers, but an internal plane of symmetry makes the overall molecule achiral.
- Hence, meso tartaric acid does not rotate plane-polarized light and exists as just one achiral compound—despite the presence of what appear to be “chiral” carbons.
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