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 stereogenic centers, the molecule is not chiral. A meso compound is "superposable" on its mirror image. Two objects can be superposed if all aspects of the objects coincide and it does not produce a "" or "" reading when analyzed with a polarimeter. For example, tartaric acid can exist as any of three stereoisomers depicted below in a Fischer projection. Of the four colored pictures at the top of the diagram, the first two represent the meso compound, followed by the optically active pair of levotartaric acid - and dextrotartaric acid -. The meso compound is bisected by an internal plane of symmetry that is not present for the non-meso isomers. That is, on reflecting the meso compound through a mirror plane perpendicular to the screen, the same stereochemistry is obtained; this is not the case for the non-meso tartaric acid, which generates the other enantiomer. The meso compound must not be confused with a 50:50 racemic mixture of the two optically-active compounds, although neither will rotate light in a polarimeter.
It is a requirement for two of the stereocenters in a meso compound to have at least two substituents in common. For example, in 2,4-pentanediol, both the second and fourth carbon atoms, which are stereocenters, have all four substituents in common. Since a meso isomer has a superposable mirrorimage, a compound with a total of nchiral centers cannot attain the theoretical maximum of 2n stereoisomers if one of the stereoisomers is meso.
1,2-substituted cyclopropane has a meso cis-isomer and two trans-enantiomers:
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 rapidring flipping, and as a result, the two cis stereoisomers behave chemically identically with chiral reagents. At lowtemperatures, 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 stereocenters do not change.
