Ameso compound or meso isomer is anoptically inactiveisomer in a set ofstereoisomers, at least two of which are optically active.[1][2] This means that despite containing two or morestereocenters, the molecule is notchiral. 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). Two objects can be superposed if all aspects of the objects coincide and it does not produce a "(+)" or "(-)" reading when analyzed with apolarimeter.[3] The name is derived from the Greekmésos meaning “middle”.
For example,tartaric acid can exist as any of three stereoisomers depicted below in aFischer projection. Of the four colored pictures at the top of thediagram, the first two represent the meso compound (the 2R,3S and 2S,3R isomers are equivalent), followed by the optically active pair oflevotartaric acid (L-(R,R)-(+)-tartaric acid) anddextrotartaric acid (D-(S,S)-(-)-tartaric acid). The meso compound is bisected by an internalplane of symmetry that is not present for the non-meso isomers (indicated by an X). 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,[3] which generates the otherenantiomer. The meso compound must not be confused with a 50:50racemic mixture of the two optically-active compounds, although neither will rotate light in apolarimeter.
It is a requirement for two of the stereocenters in a meso compound to have at least two substituents in common (although having this characteristic does not necessarily mean that the compound is meso). 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 mirror image, a compound with a total ofn chiral centers cannot attain the theoretical maximum of 2n stereoisomers if one of the stereoisomers is meso.[4]
A meso isomer need not have a mirror plane. It may have aninversion or arotoreflexion symmetry such as S4. For example, there are two meso isomers of 1,4-difluoro-2,5-dichlorocyclohexane but neither has a mirror plane, and there are two meso isomers of 1,2,3,4-tetrafluorospiropentane (see figure).
1,2-substitutedcyclopropane has a mesocis-isomer (molecule has a mirror plane) and twotrans-enantiomers:
The two cis stereoisomers of 1,2-substitutedcyclohexanes 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.[5] At low temperatures, however, this is not the case, as theactivation 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.
