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Isomer: Difference between revisions
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===Stereoisomers=== | ===Stereoisomers=== | ||
In stereoisomers, the bond structure is the same, but the geometrical positioning of atoms and functional groups in space differs. This class includes enantiomers which are non-superimposable mirror images of each other, and diastereomers which are not. Enantiomers always contain chiral centres and diastereomers often do, but there are some diastereomers which are neither chiral nor contain chiral centers. Another type of isomer, conformational isomers (conformers), may be rotamers, diastereomers or enantiomers depending on the exact compound. | In stereoisomers, the bond structure is the same, but the geometrical positioning of atoms and functional groups in space differs. This class includes enantiomers which are non-superimposable mirror images of each other, and diastereomers which are not. Enantiomers always contain chiral centres and diastereomers often do, but there are some diastereomers which are neither chiral nor contain chiral centers. Another type of isomer, conformational isomers (conformers), may be rotamers, diastereomers or enantiomers depending on the exact compound. | ||
===Diastereomers=== | |||
Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent stereocenters and are not mirror images (enantiomers) of each other. When two diastereoisomers differ from each other at only one stereocenter, they are epimers. Each stereocenter gives rise to two different configurations and thus increases the number of stereoisomers by a factor of two. | Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent stereocenters and are not mirror images (enantiomers) of each other. When two diastereoisomers differ from each other at only one stereocenter, they are epimers. Each stereocenter gives rise to two different configurations and thus increases the number of stereoisomers by a factor of two. | ||
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E/Z isomers which have restricted rotation within the molecule, specifically isomers containing a double bond, are configurational isomers. For alkenes with more than two substituents, E-Z notation is used instead of cis and trans. If possible, E and Z is also preferred in compounds with two substituents. | E/Z isomers which have restricted rotation within the molecule, specifically isomers containing a double bond, are configurational isomers. For alkenes with more than two substituents, E-Z notation is used instead of cis and trans. If possible, E and Z is also preferred in compounds with two substituents. | ||
===Enantiomers=== | |||
An enantiomer is one of two stereoisomers that are mirror images of each other that are non-superimposable (not identical). The model used to convey the idea of isomers being non-superimposable is a person's hands. One's left and right hands are the same except for opposite orientation. Organic compounds that contain a chiral carbon usually have two non-superimposable structures. A chiral carbon is a carbon attached to four completely different functional groups. The enantiomers will have a "left-hand" and a "right-hand" molecule. This property is used in naming the isomers: the "right-hand" is given the prefix (R) and the "left-hand" is given the prefix (S). Enantiomers rotate in plane-polarised light, and this property is also used in naming the enantiomers as either clockwise (prefix of dextro- or +) or anti-clockwise (prefix of levo or -). | An enantiomer is one of two stereoisomers that are mirror images of each other that are non-superimposable (not identical). The model used to convey the idea of isomers being non-superimposable is a person's hands. One's left and right hands are the same except for opposite orientation. Organic compounds that contain a chiral carbon usually have two non-superimposable structures. A chiral carbon is a carbon attached to four completely different functional groups. The enantiomers will have a "left-hand" and a "right-hand" molecule. This property is used in naming the isomers: the "right-hand" is given the prefix (R) and the "left-hand" is given the prefix (S). Enantiomers rotate in plane-polarised light, and this property is also used in naming the enantiomers as either clockwise (prefix of dextro- or +) or anti-clockwise (prefix of levo or -). | ||
[[File:Enantiomer.png|298px|thumb|center|Example of enantiomerism in lactic acid, showing left-hand and right-hand sides]] | [[File:Enantiomer.png|298px|thumb|center|Example of enantiomerism in lactic acid, showing left-hand and right-hand sides]] | ||
==Chirality== | == Chirality == | ||
[[File:File_Chirality_with_hands.png|thumb|right|Two enantiomers of a generic amino acid that is chiral]] | [[File:File_Chirality_with_hands.png|thumb|right|Two enantiomers of a generic amino acid that is chiral]] | ||
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One example of this is with the [[dissociative]] drug [[DXM]]. DXM, or dextromethorphan, is the isomer that rotates clockwise in plane-polarised light. It has antitussive (cough suppressant) and dissociative effects. Levomethorphan is the isomer that rotates anti-clockwise in plane-polarised light. This difference in rotation can radically alter the effects and character of the drug; for example, levomethorphan is an [[opioid]] painkiller. | One example of this is with the [[dissociative]] drug [[DXM]]. DXM, or dextromethorphan, is the isomer that rotates clockwise in plane-polarised light. It has antitussive (cough suppressant) and dissociative effects. Levomethorphan is the isomer that rotates anti-clockwise in plane-polarised light. This difference in rotation can radically alter the effects and character of the drug; for example, levomethorphan is an [[opioid]] painkiller. | ||
[[File:DXM_chirality.png|thumb|center|Two different chiral forms of DXM can give rise to radically different effects]] | |||
=== Racemic mixture === | |||
In chemistry, a racemic mixture, or racemate, is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. A racemic mixture is the most common forms chemicals (psychoactive or not) end up synthesized as. | |||
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, the rotations cancel because they are present in equal amounts. | |||
In contrast to the two pure enantiomers, which have identical physical properties except for the direction of rotation of plane-polarized light, a racemate sometimes has different properties from either of the pure enantiomers. Different melting points are most common, but different solubilities and boiling points are also possible.<ref>https://en.wikipedia.org/wiki/Racemic_mixture | Racemic mixture {Wikipedia)</ref> | |||
Pharmaceuticals may be available as a racemate or as the pure enantiomer (enantiopure), which might have different potencies and effects. Because biological systems have many chiral asymmetries, pure enantiomers frequently have very different biological effects. Examples include glucose and [[methamphetamine]].<ref>https://en.wikipedia.org/wiki/Racemic_mixture | Racemic mixture {Wikipedia)</ref> | |||
=== Enantiopure === | |||
A sample with only a single enantiomer is an enantiomerically pure, enantiopure or homochiral compound. | |||
==See also== | ==See also== | ||
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==References== | ==References== | ||
{{ | {{reflist|2}} | ||