Meso compound
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A meso compound or meso isomer is a chemical compound whose molecules contain 2 or more asymmetric atoms (stereocenters) but is optically inactive (or achiral). In another definition a meso compound is an optically inactive member of a set of stereoisomers, at least two of which are optically active ^[1].

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. The cis isomer of cyclooctene is also considered a meso compound. The trans form can be resolved into an enantiomeric pair.

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