Mesocompounds

Mesocompounds, commonly called meso compounds, are stereoisomeric molecules that contain at least two stereogenic centers but are overall achiral because they possess an internal plane of symmetry (or, less commonly, a center of symmetry). Each stereocenter in such a molecule would suggest chirality, yet the symmetry makes the mirror image superimposable on the original structure.

In a compound with n stereocenters, up to 2^n stereoisomers can exist. Among these, enantiomeric pairs account for some forms, while one or more meso forms can arise when the molecule has a symmetry element that links one half to the other. Mes o compounds are optically inactive as single molecules, unlike most chiral compounds that rotate plane-polarized light. This optical inactivity is distinct from a racemic mixture, which is a mixture of enantiomers rather than a single meso molecule.

Classic examples include meso-2,3-butanediol (CH3-CHOH-CHOH-CH3), meso-tartaric acid (HOOC-CHOH-CHOH-COOH), and meso-2,3-dibromobutane (CH3-CH(Br)-CH(Br)-CH3). These compounds each contain two stereocenters arranged such that an internal symmetry plane renders the molecule achiral.

Identification often relies on symmetry analysis or examining Fischer projections for an internal mirror plane. If symmetry exists and the molecule is superimposable on its mirror image, the substance is meso. Mesocosms play a key role in understanding stereochemistry, illustrating how multiple stereocenters can yield optically inactive yet structurally complex compounds.