racemisch

Racemisch, or racemic, refers to a mixture that contains equal amounts of two enantiomers of a chiral molecule. Enantiomers are non-superimposable mirror images of each other. A racemate rotates plane-polarized light neither to the left nor to the right, because the optical activities of the two enantiomers cancel one another. In English, the standard term is racemic.

The concept arises from early studies of optically active compounds such as tartaric acid and related substances. A sample is described as racemic when its enantiomeric excess (ee) is zero, meaning there is no bias toward either enantiomer. Conversely, a nonzero ee indicates a predominance of one enantiomer over the other.

Racemic mixtures can have properties distinct from those of a pure enantiomer, including different biological activity and pharmacology. In many cases only one enantiomer is responsible for the desired therapeutic effect, while the other may be less active, inactive, or even counterproductive. Some substances can racemize under certain conditions, meaning they convert between enantiomers, which can alter activity or safety profiles in storage, processing, or in vivo.

Control and separation of racemates employ several methods. Resolution can be achieved by forming diastereomeric salts with a chiral counterion, enabling physical separation. Chiral chromatography and enantioselective synthesis are alternative approaches to obtain enriched or pure enantiomers. Analytical techniques such as polarimetry measure optical rotation, while chiral high-performance liquid chromatography (HPLC) determines enantiomeric composition and ee.