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Achiral meso compounds

German: achirale Mesoverbindungen

1 Definition

Achiral meso compounds are chemical compounds that have a stereocenter but are nonetheless achiral. Meso forms have two or more stereocenters which is why they exist in at least one spatial arrangement that features a plane of symmetry (rotating mirror axis, mirror plane, point reflection).

2 General rule

Compounds that are achiral even though they do have multiple chirality centers are referred to as meso compounds.

3 Example

A common example is tartaric acid. It has two stereocenters. Tartaric acid has 3 isomers, two enantiomers and the achiral meso form.

The first two are the (S,S) and (R,R) forms according to the Cahn–Ingold–Prelog priority rules (CIP). Those two isomers are chiral and relate to each other as image and mirror image, which means that they are enantiomers.

The last isomer is the meso compound. In this form one stereocenter has an (R)- and the other an (S)-configuration. Because an intramolecular plane exists, the two stereocenters are identical and apparently achiral. As a result they are optically inactive. The meso compound is often also described as a diastereomer, a special case with at least one chiral center but still achiral. They can be aligned congruently.

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