A meso compound or meso isomer is a non-optically active member of a set of stereoisomers, at least two of which are optically active. This means that despite containing two or more stereocenters (chiral centers) it is not chiral. A meso compound is "superposable" on its mirror image (not to be confused with superimposable, as any two objects can be superimposed over one another regardless of whether they are the same. If two objects can be superposed, all aspects of the objects coincide.) and it does not produce a "(+)" or "(-)" reading when analyzed with a polarimeter. A meso compound need not even have a chiral center.
For example, there are 3 isomers of tartaric acid (depicted below): There is a meso compound (the 2R,3S and 2S,3R isomers are equivalent) and the optically active pair of dextrotartaric acid (L-(R,R)-(+)-tartaric acid) and levotartaric acid (D-(S,S)-(-)-tartaric acid). In the meso compound an internal plane of symmetry exists, bisecting the molecule which is not present in the non-meso compounds. That is, on rotating the meso compound by 180° on a plane perpendicular to the screen, the same stereochemistry is obtained, again this is not seen in the non-meso tartaric acid. (see Fischer projection).
It is a requirement for two of the stereocenters in a meso compound to have at least two substituents in common (though having this characteristic does not necessarily mean that the compound is meso). For example, in 2,4-pentanediol, both the second and fourth carbons, which are stereocenters, have all four substituents in common.
Since a meso isomer has a superposable mirror image, a compound with a total of n stereocenters cannot have 2n stereoisomers if at least one of the stereoisomers is meso.
Read more about Meso Compound: Cyclic Meso Compounds
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