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A racemic mixture or racemate in chemistry is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture, or racemate, was 'racemic acid', which Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.
In contrast to the two separate enantiomers, which generally have identical physical properties, a racemate often has different properties compared to either one of the pure enantiomers. Different melting points and solubilities are very common, but different boiling points are also possible.
Pharmaceuticals may be available as a racemate or as pure enantiomer, which might have different potencies. It is nowadays regulated by law that both enantiomers have to be tested separately for their pharmacological effects (see Enantiomer).
There are three ways a racemate can crystallize, this is important for the resolution of a racemate by crystallisation. Precise ways to distinguish between these crystal forms were summarised already in 1899 by H. W. B. Roozeboom.
The separation of a racemate into its components, the pure enantiomers, is called a resolution. There are various methods, including crystallization, chromatography and the use of enzymes. The first successful resolution of a racemate was performed by Louis Pasteur, who manually separated the crystals of a conglomerate.
Without a chiral influence (for example a chiral catalyst, solvent or starting material), a chemical reaction that makes a chiral product will always yield a racemate. That can make the synthesis of a racemate cheaper and easier than making the pure enantiomer, because it does not require special conditions. This fact also leads to the question how biological homochirality evolved on a presumably racemic primordial earth.
The reagents of, and the reactions that produce, racemic mixtures are said to be "not stereospecific" or "not stereoselective", for their indecision in a particular stereoisomerism.
Some drug molecules are chiral and the enantiomers have different effects on biological entities. They can be sold as one enantiomer or as a racemic mixture. Examples include Thalidomide, Ibuprofen, and Salbutamol. Ritalin is a mixture of several different enantiomers. A single amphetamine dose combines the neutral sulfate salts of dextroamphetamine and amphetamine, with the dextro isomer of amphetamine saccharate and d, l-amphetamine aspartate monohydrate.
In some cases (e.g. Ibuprofen and Thalidomide) the enantiomers are interconverted in vivo. This means that preparing a pure enantiomer for medication is largely pointless.
In cases like Salbutamol and Thalidomide the inactive isomer may be harmful.
Methamphetamine is available by prescription under the brand name Desoxyn. The active component of Desoxyn is dextro-methamphetamine hydrochloride. This is the right-hand isomer of methamphetamine. The left-handed isomer of methamphetamine, levo-methamphetamine, is less centrally acting and more peripherally acting; therefore a racemic mixture of dextro/levo-methamphetamine isn't used in current medical practice. In the past, due to different levels of restrictions on precursor chemicals and lack of knowledge by those preparing the final product, racemic methamphetamine was produced and sold on the black market. Newer techniques typically use asymmetric synthesis methods and yield a majority of d-methamphetamine and relatively little l-methamphetamine.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Racemic_mixture". A list of authors is available in Wikipedia.|