Enantiomer

Enantiomer

Overview
In chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, an enantiomer (icon ; from the Greek ἐνάντιος, opposite, and μέρος, part or portion) is one of two stereoisomers that are mirror image
Mirror image
A mirror image is a reflected duplication of an object that appears identical but reversed. As an optical effect it results from reflection off of substances such as a mirror or water. It is also a concept in geometry and can be used as a conceptualization process for 3-D structures...

s of each other that are non-superposable (not identical), much as one's left and right hands
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

 are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without touching the back or palm of the left to the same of the right.
Discussion
Ask a question about 'Enantiomer'
Start a new discussion about 'Enantiomer'
Answer questions from other users
Full Discussion Forum
 
Unanswered Questions
Encyclopedia
In chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, an enantiomer (icon ; from the Greek ἐνάντιος, opposite, and μέρος, part or portion) is one of two stereoisomers that are mirror image
Mirror image
A mirror image is a reflected duplication of an object that appears identical but reversed. As an optical effect it results from reflection off of substances such as a mirror or water. It is also a concept in geometry and can be used as a conceptualization process for 3-D structures...

s of each other that are non-superposable (not identical), much as one's left and right hands
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

 are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without touching the back or palm of the left to the same of the right. You observe that the thumb of one is always over the little-finger of the other, thus explaining the non-superimposable or non-coincident property known as chirality.

Organic compounds that contain an asymmetric (chiral) Carbon usually have two non-superimposable structures. These two structures are mirror images of each other and are, thus, commonly called enantiomorphs (enantio = opposite ; morph = form) Hence, optical isomerism (which occurs due to these same mirror-image properties) is now commonly referred to as enantiomerism

Enantiopure compounds refer to samples having, within the limits of detection, molecules of only one chirality
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

.

Enantiomers have, when present in a symmetric environment, identical chemical and physical properties except for their ability to rotate plane
Plane (mathematics)
In mathematics, a plane is a flat, two-dimensional surface. A plane is the two dimensional analogue of a point , a line and a space...

-polarized light (+/−) by equal amounts but in opposite directions (although the polarized light can be considered an asymmetric medium). A mixture of equal parts of an optically active isomer and its enantiomer is termed racemic
Racemic
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.- Nomenclature :A...

 and has zero (± 0) net rotation of plane-polarized light.

Enantiomers of each other often show different chemical reactions with other substances that are also enantiomers. Since many molecules in the body of living beings are enantiomers themselves, there is often a marked difference in the effects of two enantiomers on living beings. In drugs, for example, often only one of a drug's enantiomers is responsible for the desired physiologic effects, while the other enantiomer is less active, inactive, or sometimes even responsible for adverse effects (unwanted side-effects).

Owing to this discovery, drugs composed of only one enantiomer ("enantiopure") can be developed to enhance the pharmacological efficacy and sometimes do away with some side effects. An example of this kind of drug is eszopiclone
Eszopiclone
Eszopiclone, marketed by Sepracor under the brand-name Lunesta, is a nonbenzodiazepine hypnotic used as a treatment for insomnia. Eszopiclone is the active dextrorotatory stereoisomer of zopiclone, and belongs to the class of drugs known as cyclopyrrolones.Eszopiclone is a short acting...

 (Lunesta), which is enantiopure and therefore is given in doses that are exactly 1/2 of the older, racemic
Racemic
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.- Nomenclature :A...

 mixture called zopiclone
Zopiclone
Zopiclone is a non-benzodiazepine hypnotic agent used in the treatment of insomnia. In the United States, zopiclone is not commercially available, although its active stereoisomer, eszopiclone, is sold under the name Lunesta...

. In the case of eszopiclone, the S enantiomer is responsible for all the desired effects, though the other enantiomer seems to be inactive; while an individual must take 2 mg of zopiclone to get the same therapeutic benefit as they would receive from 1 mg of eszopiclone, that appears to be the only difference between the two drugs.

Criterion of Enantiomerism


Most compounds that contain one or more asymmetric Carbon atoms show enantiomerism. But this is not always true.

There are a few known compounds that do have asymmetric Carbons but being non-dissymetric do not show enantiomerism. Thus, meso tartaric acid has two asymmetric Carbons but is still optically inactive. Likewise, trans-cyclohexane - 1,4 - dicarboxylic acid has asymmetric Carbons but also has a centre of symmetry (thereby making it a non-dissymmetric compound) and, therefore, exhibits no enantiomerism.

Examples




An example of such an enantiomer is the sedative
Sedative
A sedative or tranquilizer is a substance that induces sedation by reducing irritability or excitement....

 thalidomide
Thalidomide
Thalidomide was introduced as a sedative drug in the late 1950s that was typically used to cure morning sickness. In 1961, it was withdrawn due to teratogenicity and neuropathy. There is now a growing clinical interest in thalidomide, and it is introduced as an immunomodulatory agent used...

. It was sold in a number of countries across the world from 1957 until 1961, when it was withdrawn from the market after being found to be a cause of birth defects.

In the herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...

 mecoprop
Mecoprop
Mecoprop, or methylchlorophenoxypropionic acid , is a common general use herbicide found in many household weed killers and "weed-and-feed" type lawn fertilizers. It is primarily used to control broadleaf weeds...

, the carboxyl group and the hydrogen atom on the central C-atom are exchanged (with the screen as plane of symmetry). After rotating one of the isomers 180 degrees (in the same plane), the two are still mirror images of each other. The mirror image of each enantiomer is superposable on the other enantiomer.

Another example is the antidepressant drugs escitalopram
Escitalopram
Escitalopram is an antidepressant of the selective serotonin reuptake inhibitor class. It is approved by the U.S...

 and citalopram
Citalopram
Citalopram brand names: Celexa, Cipramil) is an antidepressant drug of the selective serotonin reuptake inhibitor class. It has U.S...

. Citalopram is a racemate [1:1 mixture of (S)-citalopram and (R)-citalopram]; escitalopram [(S)-citalopram] is a pure enantiomer. The dosages for escitalopram are typically 1/2 of those for citalopram.

Enantioselective preparations



There are two main strategies for the preparation of enantiopure compounds. The first is known as chiral resolution. This method involves preparing the compound in racemic
Racemic
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.- Nomenclature :A...

 form, and separating it into its isomers. In his pioneering work, Louis Pasteur
Louis Pasteur
Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

 was able to isolate the isomers of tartaric acid
Tartaric acid
Tartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds; is commonly combined with baking soda to function as a leavening agent in recipes, and is one of the main acids found in wine. It is added to other foods to...

 because they crystallize from solution as crystals each with a different symmetry. A less common method is by enantiomer self-disproportionation
Enantiomer self-disproportionation
Enantiomer self-disproportionation is a process in stereochemistry describing the separation of a non-racemic mixture of enantiomers in an enantioenriched fraction and a more racemic fraction as a result of the formation of heterochiral or homochiral aggregates...

.

The second strategy is asymmetric synthesis: the use of various techniques to prepare the desired compound in high enantiomeric excess
Enantiomeric excess
The enantiomeric excess of a substance is a measure of how pure it is. In this case, the impurity is the undesired enantiomer .-Definition:...

. Techniques encompassed include the use of chiral starting materials (chiral pool synthesis
Chiral pool synthesis
Chiral pool synthesis is a strategy that aims to improve the efficiency of chiral synthesis. It starts the organic synthesis of a complex enantiopure chemical compound from a stock of readily available enantiopure substances. Common chiral starting materials include monosaccharides and amino acids...

), the use of chiral auxiliaries and chiral catalysts, and the application of asymmetric induction
Asymmetric induction
Asymmetric induction in stereochemistry describes the preferential formation in a chemical reaction of one enantiomer or diastereoisomer over the other as a result of the influence of a chiral feature present in the substrate, reagent, catalyst or environment...

. The use of enzymes (biocatalysis
Biocatalysis
Biocatalysis is the use of natural catalysts, such as protein enzymes, to perform chemical transformations on organic compounds. Both enzymes that have been more or less isolated and enzymes still residing inside living cells are employed for this task....

) may also produce the desired compound.

Enantioconvergent synthesis is the synthesis of one enantiomer from a racemic precursor molecule utilizing both enantiomers. Thus, the two enantiomers of the reactant produce a single enantiomer of product.

Enantiopure medications



Advances in industrial chemical processes have made it economical for pharmaceutical manufacturers to take drugs that were originally marketed as a racemic mixture and market the individual enantiomers. In some cases, the enantiomers have genuinely different effects. In other cases, there may be no clinical benefit to the patient. In some jurisdictions, single-enantiomer drugs are separately patentable from the racemic mixture. It is possible that both enantiomers are active. Or, it may be that only one is active, in which case separating the mixture has no objective benefits, but extends the drug's patentability.

Quasi-enantiomers


These molecular species are not strictly enantiomers, but they behave as if they are. Quasi-enantiomers have applications in parallel kinetic resolution (see kinetic resolution
Kinetic resolution
In kinetic resolution, two enantiomers show different reaction rates in a chemical reaction, thereby creating an excess of the less reactive enantiomer. This excess goes through a maximum and disappears on full completion of the reaction. Kinetic resolution is a very old concept in organic...

).

See also

  • Enantiopure drug
    Enantiopure drug
    An enantiopure drug is a pharmaceutical that is available in one specific enantiomeric form. Most biological molecules are present in only one of many chiral forms, so different enantiomers of a chiral drug molecule bind differently to target receptors...

  • Stereochemistry
    Stereochemistry
    Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules....

  • Dynamic stereochemistry
    Dynamic stereochemistry
    In chemistry, dynamic stereochemistry studies the effect of stereochemistry on the reaction rate of a chemical reaction. Stereochemistry is involved in:* stereospecific reactions* stereoselective or asymmetric reactions* racemisation processes-References:...

  • Chirality (chemistry)
    Chirality (chemistry)
    A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

  • Diastereomers
  • Stereogenic
  • Atropisomerism
  • Antipode (chemistry)
    Antipode (chemistry)
    In chemistry, antipode is synonymous with optical antipode. An optical antipode is the same as an enantiomer. A solution of enantiomer rotates plane polarized light in a certain direction. A solution of the opposite enantiomer, the antipode, rotates plane polarized light in the opposite...


External Links


chemwiki:stereoisomerism