## Organic and Inorganic Chemistry Lesson of the Day – Meso Isomers

A molecule is a meso isomer if it

Meso isomers have an internal plane of symmetry, which arises from 2 identically substituted but oppositely oriented stereogenic centres.  (By “oppositely oriented”, I mean the stereochemical orientation as defined by the Cahn-Ingold-Prelog priority system.  For example, in a meso isomer with 2 tetrahedral stereogenic centres, one stereogenic centre needs to be “R”, and the other stereogenic centre needs to be “S”. )  This symmetry results in the superimposability of a meso isomer’s mirror image.

By definition, a meso isomer and an enantiomer from the same stereoisomer are a pair of diastereomers.

Having at least 2 stereogenic centres is a necessary but not sufficient condition for a molecule to have meso isomers.  Recall that a molecule with $n$ tetrahedral stereogenic centres has at most $2^n$ stereoisomers; such a molecule would have less than $2^n$ stereoisomers if it has meso isomers.

Meso isomers are also called meso compounds.

Here is an example of a meso isomer; notice the internal plane of symmetry – the horizontal line that divides the 2 stereogenic carbons:

(2R,3S)-tartaric acid

Image courtesy of Project Osprey from Wikimedia (with a slight modification).

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