diastereomers

Meaning: Diastereomers are stereoisomers that are not mirror images. They share connectivity but differ in spatial arrangement. They arise when a molecule has two or more stereocenters and some, but not all, of them are inverted. Cis and trans alkene isomers are also diastereomers. Epimers, which differ at exactly one stereocenter, are a subset.

Equation: maximum number of stereoisomers = 2ⁿ, where n is the number of stereocenters (a pure count, so no SI unit). Meso forms reduce the total. Any one stereoisomer has one enantiomer, and the rest are its diastereomers.

On the MCAT: A question shows structures with two or more stereocenters. Assign R or S at each center. All centers opposite means enantiomers, all the same means identical, and a mix means diastereomers. You may be asked to predict that diastereomers differ in melting point, boiling point and solubility, so crystallization, distillation or chromatography can separate them. Resolving a racemic mixture works by reacting it with a pure chiral reagent to form diastereomers, which are then separated.

Common confusion: Inverting one stereocenter in a molecule with several does not give the enantiomer. The enantiomer requires every center to be inverted. Also, diastereomers do not have equal and opposite optical rotations. That relationship holds for enantiomers.