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Concept
Racémique
Résumé
In chemistry, a racemic mixture or racemate (reɪˈsiːmeɪt,_rə-,_ˈræsɪmeɪt), is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as racemates.
The first known racemic mixture was racemic acid, which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid. He manually separated the crystals of a mixture by hand, starting from an aqueous solution of the sodium ammonium salt of racemate tartaric acid. Pasteur benefited from the fact that ammonium tartrate salt gives enantiomeric crystals with distinct crystal forms (at 77 °F). Reasoning from the macroscopic scale down to the molecular, he reckoned that the molecules had to have non-superimposable mirror images.
A sample with only a single enantiomer is an enantiomerically pure or enantiopure compound.
From racemic acid found in grapes; from Latin racemus, meaning a bunch of grapes. This acid, when naturally produced in grapes, is only the right-handed version of the molecule, better known as tartaric acid. In many Germanic languages racemic acid is called “grape acid” e.g. German traubensäure and Swedish druvsyra. Carl von Linné gave red elderberry the scientific name Sambucus racemosa as the Swedish name, druvfläder, means 'grape elder', so called because its berries grow in a grape-like cluster.
A racemic mixture is denoted by the prefix (±)- or dl- (for sugars the prefix - may be used), indicating an equal (1:1) mixture of dextro and levo isomers. Also the prefix rac- (or racem-) or the symbols RS and SR (all in italic letters) are used.
If the ratio is not 1:1 (or is not known), the prefix (+)/(−), - or d/l- (with a slash) is used instead.
The usage of d and l is discouraged by IUPAC.
A racemate is optically inactive (achiral), meaning that such materials do not rotate the polarization of plane-polarized light.
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