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.
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). The meso compound is bisected by an internal plane of symmetry that 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 (not the case for 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.
Cyclic meso compounds
The two cis stereoisomers of 1,2-substituted cyclohexanes behave like meso compounds at room temperature in most cases. At room temperature, most 1,2-disubstituted cyclohexanes undergo rapid ring flipping (exceptions being rings with bulky substituents), and as a result, the two cis stereoisomers behave chemically identically with chiral reagents. At low temperatures, however, this is not the case, as the activation energy for the ring-flip cannot be overcome, and they therefore behave like enantiomers. Also noteworthy is the fact that when a cyclohexane undergoes a ring flip, the absolute configurations of the sterocenters do not change.
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