configurational isomers

Meaning: Configurational isomers are stereoisomers: same atoms, same bonding order, different spatial arrangement that is locked in. Changing one into another requires breaking and re-forming a bond. The group includes enantiomers (non-superimposable mirror images) and diastereomers (stereoisomers that are not mirror images). Cis/trans (E/Z) alkene isomers and ring isomers are diastereomers. Conformational isomers differ only by rotation about single bonds.

Equation: Maximum number of stereoisomers = 2ⁿ, where n is the number of stereocenters (a pure count, no unit); meso compounds reduce the actual number.

On the MCAT: You are given two structures and asked to classify the pair. Assign R or S at every stereocenter. If all centers are opposite, the pair are enantiomers. If some but not all are opposite, they are diastereomers. If all match, they are the same compound. Enantiomers share melting point, boiling point and solubility in achiral solvents, but rotate plane-polarized light in opposite directions. Diastereomers differ in physical properties, so they can be separated by crystallization, distillation or chromatography. A passage may also ask what happens to configuration in a reaction, such as inversion in SN2.

Common confusion: Students call chair flips or Newman-projection rotations configurational changes. They are conformational, because no bond breaks. Configurational isomers are also not constitutional isomers, which differ in connectivity. The test is whether a bond must break to interconvert the two.