Chemistry An enantiomer is one of the two non-superposable forms (mirror images of each other) of certain molecules, called chiral molecules.
Enantiomerism is a form of optical isomerism where a molecule can exist in two structures that are mirror images of each other, in a plane of symmetry, and of which the physical properties can differ (specific rotation, a characteristic of their optical activity).
There are levorotatory compounds which rotate the plane of polarisation of light to the left, and dextrorotatory ones that rotate it to the right. The amount of rotation defines the optical activity.
Some medication is active in only one of the two forms, the other form even possibly being harmful. In that case, only one of the enantiomers must be synthesised. The processes used are asymmetric synthesis or the separation of a racemic mixture into the two enantiomers.