Chiral

Chiral describes objects whose mirror image cannot be superimposed onto the original. The term derives from Greek cheir, meaning hand, reflecting the common example of left and right hands. In chemistry, chirality refers to a molecule or ion that is not superimposable on its mirror image, often because it contains a stereogenic center.

A stereogenic center is a atom, typically carbon, bonded to four different substituents. Molecules may be chiral even without a stereogenic center, such as certain helical or axially chiral compounds. If a molecule has a pair of non-superimposable mirror images, these two forms are enantiomers. Enantiomers have identical physical properties in achiral environments but interact differently with chiral environments (including biological systems).

In many cases, the enantiomer pair will rotate plane-polarized light in opposite directions; the extent of rotation is called the specific rotation. A mixture containing equal amounts of both enantiomers is called racemic and is optically inactive. Distinguishing and separating enantiomers is called enantioseparation or chiral resolution. Absolute configuration at a stereocenter can be assigned using the Cahn–Ingold–Prelog rules, denoted as R or S.

Chirality is widespread in biology and pharmacology. Most amino acids in proteins are L-enantiomers; sugars in nature are often D-enantiomers. Enantiomeric purity can influence drug activity and toxicity. Methods to determine chirality include polarimetry, circular dichroism, and X-ray crystallography.

In physics, chirality also appears in the sense of handedness of particles or fields, separate from spatial mirror asymmetry. Chiral symmetry and related anomalies are topics in quantum field theory and particle physics.