Meso compound

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.^[1][2] This means that despite containing two or more stereocenters (chiral centers) it is not chiral. A meso compound is superimposable on its mirror image, and it does not produce a "(+)" or "(-)" reading when analyzed with a polarimeter.^[3] 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).^[3]

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.^[4]

Cyclic meso compounds

1,2-substituted cyclopropane has a meso cis-isomer (molecule has a mirror plane) 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 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. A meso form is a type of stereoisomer which is optically inactive. In nearly all cases at room temperature, the two cis stereoisomers of 1,2-disubstituted cyclohexanes can be treated as chemically equivalent.^[5] Also noteworthy is the fact that when a cyclohexane undergoes a ring flip, the absolute configurations of the sterocenters do not change.

References

1. ^ The Meaning of Meso Addison Ault Vol. 85 No. 3 March 2008 • Journal of Chemical Education 441
2. ^ http://goldbook.iupac.org/M03839.html
3. ^ ^{a b} McMurry, John (2008). Organic Chemistry (7th Ed.). Thomson. pp. 305–7. ISBN 978-0-495-11258-7. 
4. ^ Bruice, Paula. Organic Chemistry. 2007. Pearson Prentice Hall. Upper Saddle River NJ.
5. ^ Vollhardt, K. Peter C. Organic Chemistry: Structure and Function, Fourth Ed. 2003. W.H. Freeman and Co. New York. pp. 187.