Diastereomers are different stereoisomers that are not enantiomers. The meso compound form of tartaric acid is a diastereomer of either member of its enantiomer pair.
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Diastereomers are a type of stereoisomer that are not mirror images of each other. They occur when a molecule has multiple stereocenters (chiral centers) and varies at one or more, but not all, of those centers. This leads to different spatial arrangements of the atoms in the molecule, resulting in distinct compounds with different physical and chemical properties. For example, consider a molecule with two chiral centers.