Stereoisomerism - Wikipedia, the free encyclopedia

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Stereoisomerism
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Stereoisomers areisomeric molecules whose atomic connectivity is the same but whose atomic arrangement in space is different.
Contents
[hide]
1 Enantiomers
2 Diastereomers
3 Cis-trans and E-Z isomerism
4 Conformers
5 References
6 External links
[edit] Enantiomers
Main article:enantiomer
Enantiomers are two stereoisomers that are related to each other by a reflection: they aremirror images of each other, which are non-superimposable. Everystereocenterin one has the opposite configuration in the other. Two compounds thatare enantiomers of each other have the same physical properties, exceptfor the direction in which they rotatepolarized light and how they interact with differentoptical isomers of other compounds. For this reason, pure enantiomers exhibit the phenomenon ofoptical activity and can only be separated with the use of achiral agent. In nature, only one enantiomer of most chiral biological compounds, such asamino acids, is present. As a result, different enantiomers of a compound may have substantially different biological effects.
[edit] Diastereomers
Diastereomers are stereoisomers not related through a reflection operation. They are not mirror images of each other. These includemeso compounds, cis-trans (E-Z) isomers, and non-enantiomericoptical isomers.Diastereomers seldom have the same physical properties. In the exampleshown below, the meso form of tartaric acid forms a diastereomeric pairwith both levo and dextro tartaric acids, which form an enantiomericpair.


(natural) tartaric acid
L-(+)-tartaric acid
dextrotartaric acid
D-(-)-tartaric acid
levotartaric acid
mesotartaric acid
(1:1)
DL-tartaric acid
"racemic acid"
[edit] Cis-trans and E-Z isomerism
Main article:Cis-trans isomerism
Stereoisomerism about double bonds arises because rotation about thedouble bond is restricted, keeping the substituents fixed relative toeach other. If the substituents on either end of a double bond are thesame, it is not considered a stereo bond.
Traditionally, double bond stereochemistry was described as eithercis (Latin, on this side) or trans (Latin, across). (The terms cis andtrans are also used to describe the relative position of twosubstituents on a ring; cis if on the same side, otherwise trans.) Dueto occasional ambiguity, IUPAC adopted a more rigorous system whereinthe substituents at each end of the double bond are assigned prioritynumbers. If the high priority substituents are on the same side of thebond it is assigned Z (Ger. zusammen, together). If they are on opposite sides it is E (Ger. entgegen, opposite).

An example of double bond stereoisomerism is1,2-dichloroethene, C2H2Cl2. Molecule I is Z-1,2-dichloroethene (chlorines on same side - the top) and molecule II (chlorines on opposite sides) is E-1,2-dichloroethene.There is no way of "superimposing" the structures on each other throughbond rotation, because of the central double bond of C=C (composed of asigma bond and api bond),through which rotation is not allowed. If rotation were allowed, suchas a single bond would allow, these two molecules would be the same.
In contrast, for1,2-dichloroethane, C2H4Cl2, which is similar except that it has an extra H attached to each C and a single bond, the E- and Z-forms do not exist. Since the carbon atoms can rotate around the singlebond, in a flat projection of the molecule, all three atoms attached toone carbon could swap places and still represent the same structure.
Configurational isomers are diastereomers and can possess different physical, biological and chemical properties.
[edit] Conformers
Main article:Conformational isomerism
Conformational isomerism is a form of isomerism that describes thephenomenon of molecules with the same structural formula havingdifferent shapes due to rotations about one or more bonds. Differentconformations can have different energies, can usually interconvert,and are very rarely isolatable. For example,cyclohexanecan exist in a variety of different conformations including a chairconformation and a boat conformation, but for cyclohexane itself, thesecan never be separated. The boat conformation represents an energyminimum (and not a transition state) on the conformational itinerarybetween the two equivalent chair forms. There are some molecules thatcan be isolated in several conformations, due to the large energybarriers between different conformations. 2,2,2‘,2‘-Tetrasubstitutedbiphenyls can fit into this latter category.
[edit] References
Compendium of Chemical Terminology,stereoisomerism
Compendium of Chemical Terminology,geometric isomerism
Columbia Encyclopedia. "Stereoisomers" in Encyclopedia.com, n.l., 2005,Link, December 7th 2005.
[edit] External links
Chin. Chem. Soc., Vol. 46(3) 283 (1999) On the Assignment of Anomeric Configuration (pdf)
[hide]v • d • e
Concepts inasymmetric synthesis
NomenclatureChirality,Stereocenter, Stereoisomer,Enantiomer,Diastereomer,Meso compound,Planar chirality,Chiral ligand,Axial chirality
AnalysisOptical rotation,Enantiomeric excess,Diastereomeric excess,Chiral derivitizing agents
Chiral resolutionCrystallization,Kinetic resolution,Chiral column chromatography
ReactionsAsymmetric induction,Chiral pool synthesis,Chiral auxiliaries,Asymmetric catalytic reduction,Asymmetric catalytic oxidation,Organocatalysis,Biocatalysis
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Category:Stereochemistry