Are a meso compound and its mirror image isomers?

How are *meso* isomers even isomers? By definition, *meso* isomers are compounds that are superposable on their mirror images, but doesn't that mean that the mirror image is just the same compound?


Can someone explain the concept of *meso* isomers and diastereomers to me? 





> 
> ***meso*-compound**  
> 
> A term for the achiral member(s) of a set of diastereoisomers which also includes one or more chiral members.[[1](http://dx.doi.org/doi:10.1351/pac199668122193), [2](http://dx.doi.org/10.1351/goldbook.M03839)]
> 
> 
> **isomer**  
> 
> One of several species (or molecular entities ) that have the same atomic composition (molecular formula) but different line formulae or different stereochemical formulae and hence different physical and/or chemical properties.[[1](http://dx.doi.org/doi:10.1351/pac199668122193), [3](http://dx.doi.org/doi:10.1351/pac199466051077), [4](http://dx.doi.org/10.1351/goldbook.I03289)]
> 
> 
> 


Your notion is correct in so far as the *meso*-compound and its mirror image are not isomers of each other since they are actually the same compound.


For example, *meso*-tartaric acid (preferred IUPAC name: (2​*R*,3​*S*)-2,3-dihydroxybutanedioic acid) can be superimposed on its mirror image “(2​*S*,3​*R*)-2,3-dihydroxybutanedioic acid”; i.e. both structures actually describe the same compound. Thus, they are not isomers of each other. (And hence, “(2​*S*,3​*R*)-2,3-dihydroxybutanedioic acid” is not a correct IUPAC name.)


[![(2R,3S)-2,3-dihydroxybutanedioic acid](https://i.stack.imgur.com/MxxGx.png)](https://i.stack.imgur.com/MxxGx.png)  [![“(2S,3R)-2,3-dihydroxybutanedioic acid”](https://i.stack.imgur.com/V4HCV.png)](https://i.stack.imgur.com/V4HCV.png)


However, two further stereochemical configurations are possible, namely (2​*R*,3​*R*)-2,3-dihydroxybutanedioic acid and (2​*S*,3​*S*)-2,3-dihydroxybutanedioic acid.


[![2R,3R)-2,3-dihydroxybutanedioic acid](https://i.stack.imgur.com/n1oTD.png)](https://i.stack.imgur.com/n1oTD.png)  [![2S,3S)-2,3-dihydroxybutanedioic acid](https://i.stack.imgur.com/3Tlcl.png)](https://i.stack.imgur.com/3Tlcl.png)


These structures are non-superimposable mirror images of each other, i.e. they are enantiomers, and the above-mentioned *meso*-tartaric acid is a diastereomer of both of them.


Therefore, there are a total of three isomers of tartaric acid:


* (2​*R*,3​*R*)-2,3-dihydroxybutanedioic acid,
* (2​*S*,3​*S*)-2,3-dihydroxybutanedioic acid, and
* the *meso*-compound (2​*R*,3​*S*)-2,3-dihydroxybutanedioic acid.


**References**


[1] [Basic terminology of stereochemistry (IUPAC Recommendations 1996). *Pure Appl. Chem.* **1996,** *68,* 2193](http://dx.doi.org/doi:10.1351/pac199668122193)  

[2] [*meso*-compound. In *IUPAC Compendium of Chemical Terminology,* (Gold Book)](http://dx.doi.org/10.1351/goldbook.M03839)  

[3] [Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994). *Pure Appl. Chem.* **1994,** *66,* 1077](http://dx.doi.org/doi:10.1351/pac199466051077)  

[4] [isomer. In *IUPAC Compendium of Chemical Terminology* (Gold Book)](http://dx.doi.org/10.1351/goldbook.I03289)


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