In chemistry, a racemic mixture, or racemate, is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was "racemic acid," which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.
A racemate is optically inactive, meaning that there is no net rotation of plane-polarized light. Although the two enantiomers rotate plane-polarized light in opposite directions, because they are present in equal amounts, the rotations cancel.
In contrast to the two pure enantiomers, which have identical physical properties except for the direction of rotation of plane-polarized light, a racemate often has different properties from either 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 the pure enantiomer, which might have different potencies.
There are four ways in which a racemate can crystallize, three of which H. W. B. Roozeboom had distinguished by 1899:
The separation of a racemate into its components, the pure enantiomers, is called a chiral 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 of how biological homochirality evolved on a presumably racemic primordial earth.
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. Adderall is a mixture of several different amphetamine 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. The prescription analgesic tramadol is also a racemate.
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. However sometimes samples containing pure enantiomers may be made and sold at a higher cost in cases where the use specifically requires one isomer (e.g for a stereospecific reagent).
Methamphetamine is available by prescription under the brand name Desoxyn. The active component of Desoxyn is dextromethamphetamine hydrochloride. This is the right-hand isomer of methamphetamine. The left-handed isomer of methamphetamine, levomethamphetamine, is an OTC drug that is less centrally-acting and more peripherally-acting.
In the chemical name, a D/L- prefix indicates that both the levo & dextro isomers are contained in the product to some unspecified degree. Whereas a DL- prefix with no slash mark indicates that both levo & dextro isomers of the molecule are present in an equal 1:1 ratio, with 50% of each.