Classification of the isomeric relationships between molecules tells us about their relative chemistries. A particularly important distinction is between the two kinds of stereoisomers, enantiomers and diastereomers.

• Enantiomers are stereoisomers that are related as non-superposable mirror images

• Diastereomers are stereoisomers that are not enantiomers

The distinction is important because enantiomers must have identical chemical and physical properties, except in chiral environments, whereas diastereomers must have different properties in all environments.

This distinction includes energies; enantiomers have identical DH_f, but diastereomers must have different DH_f.

Individual members of a pair of enantiomers have the property of chirality, which is simply the name we give to non-superposability on a mirror image.

• The name comes from the Greek word chiros, which means "hand"; it is chosen since our hands are in fact enantiomers

• Members of a pair of diastereomers need not necessarily be chiral, but individual enantiomers always are.

The fundamental test for chirality is the absence of a plane of symmetry.

• Any object, molecule or otherwise, that lacks a plane of symmetry is chiral, and will be non-superposable on its mirror image.

• All of the common amino acids except glycine are chiral, owing to the presence of a tetrahedral atom with four different ligands.

• Such a stereogenic center is a sufficient condition for chirality, it is not a necessary one:
  • The only necessary condition is the absence of the plane of symmetry.

A pair of enantiomers; each is chiral	A pair of diastereomers; only the S,S one is chiral
A pair of diastereomers; neither one is chiral