A non-superimposable set of two molecules that are mirror images of one another. The existence of these molecules is determined by concept known as chirality. The word “chiral” was derived from the Greek word for hand, because our hands are good example of chirality since they are non-superimposable mirror images of each other. Chiral objects do not have a plane of symmetry. CHFClBr is therefore a chiral molecule that exists in the form of a pair of stereoisomers that are mirror images of each other. Compounds that are optically active contain molecules that are chiral. Optical activity is a macroscopic property of a collection of these molecules that arises from the way they interact with light. Lactic acid, Tartaric acid,
A molecule is achiral if it is super imposable on its mirror image. Most achiral molecules do have a plane of symmetry or a centre of symmetry. Achiral molecules that contain a stereocenter are called meso. Examples are trans-1,2-dichloro-1,2-ethanediol, (meso)-2,3-dibromobutane.