A meso compound is a special type of stereoisomer that possesses an internal plane of symmetry. This internal symmetry results in the compound being optically inactive, even though it may contain chiral centers. The term "meso" is derived from the Greek word for "middle" or "intermediate."
The structure of a meso compound can be visualized as having two or more identical chiral centers, but with a plane of symmetry passing through the molecule, effectively dividing it into two halves that are mirror images of each other. This plane of symmetry allows for the superimposition of one half onto the other half.
Here is an example to illustrate the structure of a meso compound. Consider tartaric acid, specifically the meso-tartaric acid isomer:
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