Meso compound

A meso compound is an achiral molecule containing stereocenters whose internal symmetry makes it superimposable on its mirror image.

Stereocenter: An atom at which exchanging two groups produces a stereoisomer. Meso compounds contain stereocenters, even though the whole molecule is achiral.
R–S notation: A system that labels a stereocenter’s absolute configuration as R or S using priority rules. Meso structures often have corresponding stereocenters with opposite configurations.
Meso-tartaric acid: A tartaric acid stereoisomer with two stereocenters and an internal plane of symmetry. It is a standard example of a molecule that is achiral despite having stereocenters.
Stereoisomer count: The number of distinct stereoisomers possible for a molecule with specified connectivity. Internal symmetry can reduce the count below the maximum predicted from the number of stereocenters.
Chirality: A property of an object that is not superimposable on its mirror image. Internal symmetry prevents a meso compound from being chiral.
Cahn–Ingold–Prelog priority rules: Rules for ranking substituents to assign absolute configuration at stereocenters. These rules establish the configurations compared when identifying internal symmetry.
Meso-2,3-dibromobutane: The achiral stereoisomer of 2,3-dibromobutane with opposite configurations at its two stereocenters. Its paired stereocenters and symmetry provide a compact textbook example.
Optical resolution: The separation of a racemic mixture into its individual enantiomers. A meso compound is not an enantiomer in a racemate and cannot be resolved into mirror-image forms.
Plane of symmetry: A plane that divides an object into mirror-image halves. A plane of symmetry commonly accounts for a meso compound’s achirality.
Molecular symmetry: The symmetry operations that leave a molecule indistinguishable from its original arrangement. A symmetry operation can map one part of a meso molecule onto another.

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