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• Asymmetric Carbon Examples
• Asymmetric Carbon Atom In Amino Acid
• What Is An Asymmetric Carbon Atom

This Introduction to Biology video teaches how to determine if a carbon is asymmetric. Definition of asymmetric carbon atom.: a carbon atom in union with four atoms or groups no two of which are alike, compounds containing such a carbon atom being capable of existing in two optically active forms which are distinguished by being respectively levorotatory and dextrorotatory and also in some cases by having enantiomorphous crystal forms.

In chemistry, an enantiomer (/ɪˈnæntiəmər,ɛ-,-tioʊ-/^[1]ə-NAN-tee-ə-mər; from Greekἐνάντιος (enántios) 'opposite', and μέρος (méros) 'part') (also named optical isomer,^[2] antipode,^[3] or optical antipode^[4]) is one of two stereoisomers that are mirror images of each other that are non-superposable (not identical), much as one's left and right hands are mirror images of each other that cannot appear identical simply by reorientation.^[5] A single chiral atom or similar structural feature in a compound causes that compound to have two possible structures which are non-superposable, each a mirror image of the other. Each member of the pair is termed an enantiomorph (enantio = opposite; morph = form);^[6] the structural property is termed enantiomerism. The presence of multiple chiral features in a given compound increases the number of geometric forms possible, though there may still be some perfect-mirror-image pairs.

A: Given mass of Ethanol (C2H5OH) = 45.6g To determine the number of carbon,hydrogen and oxygen atoms i. Questionanswer Q: Gold has 79 electrons per atom and an atomic mass of 197 u. The carbon atom with the four different groups attached which causes this lack of symmetry is described as a chiral centreor as an asymmetric carbon atom. The molecule on the left above (with a plane of symmetry) is described as achiral. Only chiral molecules have. The LibreTexts libraries are Powered by MindTouch ® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot.

A sample of a chemical is considered enantiopure (also termed enantiomerically pure) when it has, within the limits of detection, molecules of only one chirality.^[7]

When present in a symmetric environment, enantiomers have identical chemical and physical properties except for their ability to rotate plane-polarized light (+/−) by equal amounts but in opposite directions (although the polarized light can be considered an asymmetric medium). Such compounds are therefore described as optically active, with specific terms for each enantiomer based on the direction: a dextrorotatory compound rotates light a clockwise (+) direction whereas a levorotatory compound rotates light in a counter-clockwise (–) direction. A mixture of equal number of both enantiomers is called a racemic mixture or a racemate.^[8] In a racemic mixture, the amount of positive rotation is exactly counteracted by the equal amount of negative rotation, so the net rotation is zero (the mixture is not optically active). For all intents and purposes, pairs of enantiomers have the same Gibbs free energy. However, theoretical physics predicts that due to parity violation of the weak nuclear force (the only force in nature that can 'tell left from right'), there is actually a minute difference in energy between enantiomers (on the order of 10⁻¹² eV or 10⁻¹⁰ kJ/mol or less) due to the weak neutral current mechanism. This difference in energy is far smaller than energy changes caused by even a trivial change in molecular conformation and far too small to measure by current technology, and is therefore chemically inconsequential.^[9][10][11]

Enantiomer members often have different chemical reactions with other enantiomer substances. Since many biological molecules are enantiomers, there is sometimes a marked difference in the effects of two enantiomers on biological organisms. In drugs, for example, often only one of a drug's enantiomers is responsible for the desired physiological effects, while the other enantiomer is less active, inactive, or sometimes even productive of adverse effects. Owing to this discovery, drugs composed of only one enantiomer ('enantiopure') can be developed to make the drug work better and sometimes eliminate some side effects. An example is eszopiclone (Lunesta), which is just a single enantiomer of an older racemic drug called zopiclone. One enantiomer is responsible for all the desired effects, while the other enantiomer seems to be inactive, so the dose of eszopiclone is half that of zopiclone.

In chemical synthesis of enantiomeric substances, non-enantiomeric precursors inevitably produce racemic mixtures. In the absence of an effective enantiomeric environment (precursor, chiral catalyst, or kinetic resolution), separation of a racemic mixture into its enantiomeric components is impossible, although certain racemic mixtures spontaneously crystallize in the form of a racemic conglomerate, in which crystals of the enantiomers are physically segregated and may be separated mechanically (e.g., the enantiomers of tartaric acid, whose crystallized enantiomers were separated with tweezers by Pasteur). However, most racemates will crystallize in crystals containing both enantiomers in a 1:1 ratio, arranged in a regular lattice.

Naming conventions[edit]

The R/S system is an important nomenclature system used to denote distinct enantiomers. Another system is based on prefix notation for optical activity: (+)- and (−)- or d- and l-. The Latin words for left are laevus and sinister, and the word for right is dexter (or rectus in the sense of correct or virtuous). The English word right is a cognate of rectus. This is the origin of the L/D and S/R notations, and the employment of prefixes levo- and dextro- in common names.

The prefix ent- to a chemical name can be used to refer to the chemical that is the enantiomer of the one indicated by the name.^[12]

Criterion of enantiomerism[edit]

An asymmetric carbon atom is one which has bonds with four different atoms or groups, so that these bonds can be arranged in two different ways which are not superposable. Most compounds that contain one or more asymmetric carbon (or other element with a tetrahedral geometry) atoms show enantiomerism, but this is not always true. Compounds that contain two or more asymmetric carbon atoms but have a plane of symmetry with respect to the whole molecule are known as meso compounds. A meso compound does not have a mirror image stereoisomer because it is its own mirror image (i.e., it and its mirror image are the same molecule). For instance, mesotartaric acid (shown on the right) has two asymmetric carbon atoms, but it does not exhibit enantiomerism because each of the two halves of the molecule is equal and opposite to the other and thus is superposable on its geometric mirror image. Conversely, there exist forms of chirality that do not require individual asymmetric atoms. In fact, there are four distinct types of chirality: central, axial, planar, and helical chirality. Having an enantiomer by virtue of an asymmetric carbon atom represents the most common type of central chirality. The other three types of chirality do not involve asymmetric carbon atoms, and even central chirality does not require the center of chirality to be located at a carbon or any other atom. Consequently, while the presence of an asymmetric carbon atom is a convenient characteristic to look for when determining whether a molecule will have an enantiomer, it is neither sufficient nor necessary as a criterion.

As a rigorous criterion, a molecule is chiral, and will therefore possess an enantiomer, if and only if it belongs to one of the chiral point groups: C_n, D_n, T, O, and I. However, as a caveat, enantiomers are not necessarily isolable if there is an accessible pathway for racemization at a given temperature and timescale. For example, amines with three distinct substituents are chiral, but with the exception of only a few atypical cases (e.g. substituted N-chloroaziridines), they rapidly planarize and invert ('umbrella inversion') at room temperature, leading to racemization. If the racemization is fast enough, the molecule can often be treated as an achiral, averaged structure.

Asymmetric Carbon Examples

Examples[edit]

An example of such an enantiomer is the sedativethalidomide, which was sold in a number of countries around the world from 1957 until 1961. It was withdrawn from the market when it was found to cause birth defects. One enantiomer caused the desirable sedative effects, while the other, unavoidably^[13] present in equal quantities, caused birth defects.^[14]

The herbicidemecoprop is a racemic mixture, with the (R)-(+)-enantiomer ('Mecoprop-P', 'Duplosan KV') possessing the herbicidal activity.^[15]

Another example is the antidepressant drugs escitalopram and citalopram. 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[edit]

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 form, and separating it into its isomers. In his pioneering work, Louis Pasteur was able to isolate the isomers of tartaric acid because they crystallize from solution as crystals each with a different symmetry. A less common method is by enantiomer self-disproportionation.

The second strategy is asymmetric synthesis: the use of various techniques to prepare the desired compound in high enantiomeric excess. Techniques encompassed include the use of chiral starting materials (chiral pool synthesis), the use of chiral auxiliaries and chiral catalysts, and the application of asymmetric induction. The use of enzymes (biocatalysis) 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[edit]

Advances in industrial chemical processes have made it economic 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.^[16] It is possible that only one of the enantiomers is active. Or, it may be that both are active, in which case separating the mixture has no objective benefits, but extends the drug's patentability.^[17]

Quasi-enantiomers[edit]

Quasi-enantiomers are molecular species that are not strictly enantiomers, but behave as if they are. Quasi-enantiomers have applications in parallel kinetic resolution.^[18]

See also[edit]

References[edit]

1. ^'enantiomer - definition of enantiomer in English from the Oxford dictionary'. OxfordDictionaries.com. Retrieved 2016-01-20.
2. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'optical isomers'. doi:10.1351/goldbook.O04308
3. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'antipode'. doi:10.1351/goldbook.A00403
4. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'optical antipodes'. doi:10.1351/goldbook.O04304
5. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'enantiomer'. doi:10.1351/goldbook.E02069
6. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'enantiomorph'. doi:10.1351/goldbook.E02079
7. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'enantiomerically pure (enantiopure)'. doi:10.1351/goldbook.E02072
8. ^IUPAC, Compendium of Chemical Terminology, 2nd ed. (the 'Gold Book') (1997). Online corrected version: (2006–) 'racemate'. doi:10.1351/goldbook.R05025
9. ^1921-2008., Eliel, Ernest L. (Ernest Ludwig) (1994). Stereochemistry of organic compounds. Wilen, Samuel H., Mander, Lewis N. New York: Wiley. ISBN0471016705. OCLC27642721.CS1 maint: numeric names: authors list (link)
10. ^Albert, Guijarro (2008). The origin of chirality in the molecules of life: a revision from awareness to the current theories and perspectives of this unsolved problem. Yus, Miguel. Cambridge, UK: Royal Society of Chemistry. ISBN9781847558756. OCLC319518566.
11. ^In the sense used by particle physicists (see ref. 6), the 'true' enantiomer of a molecule, which has exactly the same mass-energy content as the original molecule, is a mirror-image, but also built from antimatter (antiprotons, antineutrons, and positrons). Nevertheless, in this article, 'enantiomer' will be used in the chemical sense and continue to refer to a nonidentical, mirror-image isomeric compound, consisting of ordinary matter elementary particles, in particular.
12. ^'P-101.8.1 Inversion of configuration'(PDF), IUPAC Provisional Recommendations—Preferred IUPAC Names, p. 48
13. ^Knoche, B; Blaschke, G (1994). 'Investigations on the in vitro racemization of thalidomide by high-performance liquid chromatography'. Journal of Chromatography A. Elsevier. 666 (1–2): 235–240. doi:10.1016/0021-9673(94)80385-4.
14. ^Voet, Donald; Voet, Judith G.; Pratt, Charlotte W. (2006). Fundamentals of Biochemistry. p. 89. ISBN0-471-21495-7.
15. ^G. Smith; C. H. L. Kennard; A. H. White; P. G. Hodgson (April 1980). '(±)-2-(4-Chloro-2-methylphenoxy)propionic acid (mecoprop)'. Acta Crystallogr. B. 36 (4): 992–994. doi:10.1107/S0567740880005134.
16. ^'European Medicines Agency - - Sepracor Pharmaceuticals Ltd withdraws its marketing authorisation application for Lunivia (eszopiclone)'. www.ema.europa.eu.
17. ^Merrill Goozner (2004). The $800 Million Pill: The Truth Behind the Cost of New Drugs(excerpt). University of California Press. ISBN0-520-23945-8.
18. ^G.S. Coumbarides, M. Dingjan, J. Eames, A. Flinn, J. Northen and Y. Yohannes, Tetrahedron Lett. 46 (2005), p. 2897er

External links[edit]

Asymmetric Carbon Atom In Amino Acid

• Media related to Enantiomers at Wikimedia Commons

An asymmetric carbon atom (chiral carbon) is a carbonatom that is attached to four different types of atoms or groups of atoms.^[1][2]Le Bel-van't Hoff rule states that the number of stereoisomers of an organic compound is 2ⁿ, where n represents the number of asymmetric carbon atoms (unless there is an internal plane of symmetry); a corollary of Le Bel and van't Hoff's simultaneously announced conclusions, in 1874, that the most probable orientation of the bonds of a carbon atom linked to four groups or atoms is toward the apexes of a tetrahedron, and that this accounted for all then-known phenomena of molecular asymmetry (which involved a carbon atom bearing four different atoms or groups).^[3] Knowing the number of asymmetric carbon atoms, one can calculate the maximum possible number of stereoisomers for any given molecule as follows:

Carbon is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Three isotopes occur naturally, ¹²C and ¹³C being stable, while ¹⁴C is a radionuclide, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity.

An atom is the smallest constituent unit of ordinary matter that constitutes a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are extremely small; typical sizes are around 100 picometers. They are so small that accurately predicting their behavior using classical physics – as if they were billiard balls, for example – is not possible. This is due to quantum effects. Current atomic models now use quantum principles to better explain and predict this behavior.

Joseph Achille Le Bel was a French chemist. He is best known for his work in stereochemistry. Le Bel was educated at the École Polytechnique in Paris. In 1874 he announced his theory outlining the relationship between molecular structure and optical activity. This discovery laid the foundation of the science of stereochemistry, which deals with the spatial arrangement of atoms in molecules. This hypothesis was put forward in the same year by the Dutch physical chemist Jacobus Henricus van 't Hoff and is currently known as Le Bel-van't Hoff rule. Le Bel wrote Cosmologie Rationelle in 1929.

If n is the number of asymmetric carbon atoms then the maximum number of isomers = 2ⁿ (Le Bel-van't Hoff rule)

As an example, malic acid has 4 carbon atoms but just one of them is asymmetric. The asymmetric carbon atom is the one attached to two carbon atoms, an oxygen atom, and a hydrogen atom. One may initially be inclined to think this atom is not asymmetric because it is attached to two carbon atoms, but because those two carbon atoms are not attached to exactly the same things, there are two different groups of atoms that the carbon atom in question is attached to, therefore making it an asymmetric carbon atom:

Malic acid is an organic compound with the molecular formula C₄H₆O_{5. It is a dicarboxylic acid that is made by all living organisms, contributes to the sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms (L- and D-enantiomers), though only the L-isomer exists naturally. The salts and esters of malic acid are known as malates. The malate anion is an intermediate in the citric acid cycle.}

A tetrose with 2 asymmetric carbon atoms has 2² = 4 stereoisomers:

A tetrose is a monosaccharide with 4 carbon atoms. They have either an aldehyde functional group in position 1 (aldotetroses) or a ketone functional group in position 2 (ketotetroses).

An aldopentose with 3 asymmetric carbon atoms has 2³ = 8 stereoisomers:

An aldohexose with 4 asymmetric carbon atoms has 2⁴ = 16 stereoisomers:

What Is An Asymmetric Carbon Atom

An aldohexose is a hexose with an aldehyde group on one end.

The four groups of atoms attached to the carbon atom can be arranged in space in two different ways that are mirror images of each other, and which lead to so-called left-handed and right-handed versions of the same molecule. Molecules that cannot be superimposed on their own mirror image are said to be chiral like mirror image.

Chirality is a geometric property of some molecules and ions. A chiral molecule/ion is non-superposable on its mirror image. The presence of an asymmetric carbon center is one of several structural features that induce chirality in organic and inorganic molecules. The term chirality is derived from the Ancient Greek word for hand, χείρ (cheir).