Meso compound

A meso compound or meso isomer is a chemical compound whose molecules contain 2 or more asymmetric atoms (stereocenters) but it is optically inactive (or achiral) because it contains an internal plane of symmetry.

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For example one of the isomers of tartaric acid depicted below is a meso compound. An internal mirror plane exists bisecting the molecule; on rotating the molecule 180° on a plane perpendicular to the screen (that is, superposing the bottom OH with the top one), the same apparent stereochemistry is obtained. (see Fischer projection.)

For a compound with multiple asymmetric carbons, the presence of an internal plane of symmetry indicates that the compound is achiral. However, it is still possible for a compound with multiple asymmetric carbons and no internal plane of symmetry to be meso.