Meso compound
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A meso compound or meso isomer is a chemical compound with molecules that contain 2 or more asymmetric atoms (stereocenters) but which is optically inactive (or achiral) because it contains an internal plane of symmetry. For example one of the isomers of tartaric acid is a meso compound:
Upon rotating the molecule 180° on a plane perpendicular to the screen (that is, superimposing the bottom OH with the top one), the same apparent stereochemistry is obtained. (see Fischer projection.)
For a compound with multiple asymmetric carbons, 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. If there is only one asymmetric carbon, then the compound must be chiral.
