Chiral molecules have no plane of symmetry and their molecular mirror images are not related by rotation. These enantiomers have mirror images that are not superimposable on themselves.
If a molecule does not have a plane of symmetry and its isomers cannot be rotated or reflected to match, it is called a chiral molecule. Chirality is a form of spatial isomerism. Two molecules that have the same molecular formula and bond structures but are not superimposable are enantiomers. An enantiomer is a stereoisomer that has a mirror image that is not superimposable on itself. Carbon atoms that contain sp3 hybridization and four unique substituents are likely to be chiral. CH2 and CH3 are never chiral because they have identical groups (H). C≡C can never be chiral because it isn't sp3. A carbon atom that has four unique substituents is a stereogenic center, also called a chiral center.
When describing enantiomers, they are differentiated by the prefixes R- and S-, based on a system of prioritization. For a given chiral center, identify the four atoms directly attached to the chiral center and assign them ranks according to their molecular weight with the heaviest atom ranked first and the lightest atom ranked last. Since the asymmetry of a chiral center sometimes occurs a number of atoms away from the chiral center, apply the rule of ranking to the second atom from the chiral center, then the third atom, and so forth, until a ranking is complete. Treat multiple bonds as two (or three) single bonds to the same atom.
Once the ranking is made, orient the molecule so that the lowest ranking atom (the lightest) is pointing away from the observer. Then draw an arrow from the highest-ranking atom (the heaviest) to the second-highest and then to the third-highest. If this arrow points clockwise, the enantiomer is R- (for the Latin rectus, which means "right"). If the arrow points counterclockwise, the enantiomer is S- (for the Latin sinister, which means "left").
Enantiomers were discovered because of their ability to rotate, or polarize, light. It was discovered that some isomers of a compound rotate light to the left; these are called levorotatory, or L-. Other isomers were found to rotate light to the right; these are called dextrorotatory, or D-. There is no correlation between R- and S-conformations and the direction that a molecule will rotate light.
Chirality and the ability to polarize light is important because many biochemical molecules are enantiomers. The amino acids used to build proteins, for example, are chiral, and all the amino acids found in nature are L-. Proteins have many functions that rely on their physical shapes, so D-amino acids would make proteins unable to fold into the correct shape and, therefore, unable to perform their functions.