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Stereoisomerism

February 15, 2024

In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space.[1][2] This contrasts with structural isomers, which share the same molecular formula, but the bond connections or their order differs. By definition, molecules that are stereoisomers of each other represent the same structural isomer.[3]

The different types of isomers. Stereochemistry focuses on stereoisomers, red boxes in the picture.

Enantiomers edit

Main articles: Chirality (chemistry) and Enantiomers

Enantiomers, also known as optical isomers, are two stereoisomers that are related to each other by a reflection: they are mirror images of each other that are non-superposable. Human hands are a macroscopic analog of this. Every stereogenic center in one has the opposite configuration in the other. Two compounds that are enantiomers of each other have the same physical properties, except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds. As a result, different enantiomers of a compound may have substantially different biological effects. Pure enantiomers also exhibit the phenomenon of optical activity and can be separated only with the use of a chiral agent. In nature, only one enantiomer of most chiral biological compounds, such as amino acids (except glycine, which is achiral), is present. An optically active compound shows two forms: D-(+) form and L-(−) form.

Diastereomers edit

Main article: Diastereomer

Diastereomers are stereoisomers not related through a reflection operation.[4] They are not mirror images of each other. These include meso compounds, cis–trans isomers, E-Z isomers, and non-enantiomeric optical isomers. Diastereomers seldom have the same physical properties. In the example shown below, the meso form of tartaric acid forms a diastereomeric pair with both levo- and dextro-tartaric acids, which form an enantiomeric pair.

 

   

(natural) tartaric acid
L-tartaric acid
L-(+)-tartaric acid
levo-tartaric acid

D-tartaric acid
D-(-)-tartaric acid
dextro-tartaric acid

meso-tartaric acid

(1:1)
DL-tartaric acid
"racemic acid"

The D- and L- labeling of the isomers above is not the same as the d- and l- labeling more commonly seen, explaining why these may appear reversed to those familiar with only the latter naming convention.

A Fischer projection can be used to differentiate between L- and D- molecules Chirality (chemistry). For instance, by definition, in a Fischer projection the penultimate carbon of D-sugars are depicted with hydrogen on the left and hydroxyl on the right. L-sugars will be shown with the hydrogen on the right and the hydroxyl on the left.

The other refers to Optical rotation, when looking at the source of light, the rotation of the plane of polarization may be either to the right (dextrorotary — d-rotary, represented by (+), clockwise), or to the left (levorotary — l-rotary, represented by (−), counter-clockwise) depending on which stereoisomer is dominant. For instance, sucrose and camphor are d-rotary whereas cholesterol is l-rotary.

Cis–trans and E-Z isomerism edit

Main articles: Cis–trans isomerism and E-Z notation

Stereoisomerism about double bonds arises because rotation about the double bond is restricted, keeping the substituents fixed relative to each other.[5] If the two substituents on at least one end of a double bond are the same, then there is no stereoisomer and the double bond is not a stereocenter, e.g. propene, CH3CH=CH2 where the two substituents at one end are both H.[6]

Traditionally, double bond stereochemistry was described as either cis (Latin, on this side) or trans (Latin, across), in reference to the relative position of substituents on either side of a double bond. A simple example of cis-trans isomerism is the 1,2-disubstituted ethenes, like the dichloroethene (C2H2Cl2) isomers shown below.[7]

 
Dichloroethene isomers

Molecule I is cis-1,2-dichloroethene and molecule II is trans-1,2-dichloroethene. Due to occasional ambiguity, IUPAC adopted a more rigorous system wherein the substituents at each end of the double bond are assigned priority based on their atomic number. If the high-priority substituents are on the same side of the bond, it is assigned Z (Ger. zusammen, together). If they are on opposite sides, it is E (Ger. entgegen, opposite).[8] Since chlorine has a larger atomic number than hydrogen, it is the highest-priority group.[9] Using this notation to name the above pictured molecules, molecule I is (Z)-1,2-dichloroethene and molecule II is (E)-1,2-dichloroethene. It is not the case that Z and cis or E and trans are always interchangeable. Consider the following fluoromethylpentene:

 
Fluoromethylpentene

The proper name for this molecule is either trans-2-fluoro-3-methylpent-2-ene because the alkyl groups that form the backbone chain (i.e., methyl and ethyl) reside across the double bond from each other, or (Z)-2-fluoro-3-methylpent-2-ene because the highest-priority groups on each side of the double bond are on the same side of the double bond. Fluoro is the highest-priority group on the left side of the double bond, and ethyl is the highest-priority group on the right side of the molecule.

The terms cis and trans are also used to describe the relative position of two substituents on a ring; cis if on the same side, otherwise trans.[10][11][12]

Conformers edit

Main article: Conformational isomerism

Conformational isomerism is a form of isomerism that describes the phenomenon of molecules with the same structural formula but with different shapes due to rotations about one or more bonds.[13][14][15] Different conformations can have different energies, can usually interconvert, and are very rarely isolatable. For example, there exists a variety of Cyclohexane conformations (which cyclohexane is an essential intermediate for the synthesis of nylon–6,6) including a chair conformation where four of the carbon atoms form the "seat" of the chair, one carbon atom is the "back" of the chair, and one carbon atom is the "foot rest"; and a boat conformation, the boat conformation represents the energy maximum on a conformational itinerary between the two equivalent chair forms; however, it does not represent the transition state for this process, because there are lower-energy pathways. The conformational inversion of substituted cyclohexanes is a very rapid process at room temperature, with a half-life of 0.00001 seconds.[16][17]

There are some molecules that can be isolated in several conformations, due to the large energy barriers between different conformations. 2,2',6,6'-Tetrasubstituted biphenyls can fit into this latter category.

Anomers edit

Anomerism is an identity for single bonded ring structures where "cis" or "Z" and "trans" or "E" (geometric isomerism) needs to name the substitutions on a carbon atom that also displays the identity of chirality; so anomers have carbon atoms that have geometric isomerism and optical isomerism (enantiomerism) on one or more of the carbons of the ring.[18][19][20][21] Anomers are named "alpha" or "axial" and "beta" or "equatorial" when substituting a cyclic ring structure that has single bonds between the carbon atoms of the ring for example, a hydroxyl group, a methyl hydroxyl group, a methoxy group or another pyranose or furanose group which are typical single bond substitutions but not limited to these.[22] Axial geometric isomerism will be perpendicular (90 degrees) to a reference plane and equatorial will be 120 degrees away from the axial bond or deviate 30 degrees from the reference plane.[23]

Atropisomers edit

Main article: Atropisomer

Atropisomers are stereoisomers resulting from hindered rotation about single bonds where the steric strain barrier to rotation is high enough to allow for the isolation of the conformers.[24][25]

More definitions edit

  • A configurational stereoisomer is a stereoisomer of a reference molecule that has the opposite configuration at a stereocenter (e.g., R- vs S- or E- vs Z-). This means that configurational isomers can be interconverted only by breaking covalent bonds to the stereocenter, for example, by inverting the configurations of some or all of the stereocenters in a compound.
  • An epimer is a diastereoisomer that has the opposite configuration at only one of the stereocenters.

Le Bel-van't Hoff rule edit

Le Bel-van't Hoff rule states that for a structure with n asymmetric carbon atoms, there is a maximum of 2n different stereoisomers possible. As an example, D-glucose is an aldohexose and has the formula C6H12O6. Four of its six carbon atoms are stereogenic, which means D-glucose is one of 24=16 possible stereoisomers.[26][27]

See also edit

References edit

  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "stereoisomerism". doi:10.1351/goldbook.S05983
  2. ^ Columbia Encyclopedia. "Stereoisomers" in Encyclopedia.com, n.l., 2005, Link
  3. ^ Clark, Jim (November 2012). "Optical isomerism". chemguide.co.uk. Retrieved 2022-08-09.
  4. ^ "Diastereomer - an overview | ScienceDirect Topics".
  5. ^ "Geometric Isomers Definition And Examples | Chemistry Dictionary". Retrieved 2022-06-20.
  6. ^ Clark, Jim (February 2020). "geometric (cis / trans) isomerism". Chemguide.uk. Retrieved 2022-08-09.
  7. ^ Helmenstine, Anne Marie. "Geometric Isomer Definition (Cis-Trans Isomers)". ThoughtCo.
  8. ^ Roberts, John D.; Caserio, Marjorie C. (1977). Basic Principles of Organic Chemistry (second ed.). W. A. Benjamin, Inc. p. 19.7. ISBN 0805383298.
  9. ^ Roberts, John D.; Caserio, Marjorie C. (1977). Basic Principles of Organic Chemistry (second ed.). W. A. Benjamin, Inc. p. 19.6. ISBN 0805383298.
  10. ^ "Cis-trans isomerism | NAL Agricultural Thesaurus".
  11. ^ Clark, Jim (November 2012). "E-Z notation for geometric isomerism". chemguide.co.uk. Retrieved 2022-08-09.
  12. ^ "E and Z Notation for Alkenes (+ Cis/Trans)". 3 November 2016.
  13. ^ Hunt, Ian. "What are Conformational Isomers?". chem.ucalgary.ca. Retrieved 2022-08-09.
  14. ^ "Conformational Isomer - an overview | ScienceDirect Topics".
  15. ^ "Isomerism - Conformational isomers | Britannica".
  16. ^ "Cyclohexane - an overview | ScienceDirect Topics".
  17. ^ Reusch, William (2013-05-05). "Stereoisomers". chemistry.msu.edu. Retrieved 2022-08-09.
  18. ^ Hunt, Ian. "What do the α- and β- forms look like?". chem.ucalgary.ca. Retrieved 2022-08-09.
  19. ^ "Anomer - an overview | ScienceDirect Topics".
  20. ^ "Anomeric Effect - an overview | ScienceDirect Topics".
  21. ^ Ashenhurst, James (2022-08-03). "The Big Damn Post Of Carbohydrate-Related Chemistry Definitions". masterorganicchemistry.com. Retrieved 2022-08-09.
  22. ^ Reusch, William. "Stereoisomers". chemistry.msu.edu. Retrieved 2022-08-09.
  23. ^ Morrison and Boyd Organic Chemistry Sixth ed. pgs. 1170-1171 ISBN 0-13-643669-2
  24. ^ "Atropisomer - an overview | ScienceDirect Topics".
  25. ^ Metrano, Anthony J. (2018-06-09). "Atropisomers" (PDF). knowleslab.princeton.edu. Retrieved 2022-08-09.
  26. ^ Blackmond, D. G. (2019). "The Origin of Biological Homochirality". Cold Spring Harbor Perspectives in Biology. 11 (3): a032540. doi:10.1101/cshperspect.a032540. PMC 6396334. PMID 30824575.
  27. ^ Brooks, Benjamin T. (1918). "The German Chemical Myth". The North American Review. 208 (756): 729–735. JSTOR 25151064.

February 15, 2024
stereoisomerism, stereochemistry, stereoisomerism, spatial, isomerism, form, isomerism, which, molecules, have, same, molecular, formula, sequence, bonded, atoms, constitution, differ, three, dimensional, orientations, their, atoms, space, this, contrasts, wit. In stereochemistry stereoisomerism or spatial isomerism is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms constitution but differ in the three dimensional orientations of their atoms in space 1 2 This contrasts with structural isomers which share the same molecular formula but the bond connections or their order differs By definition molecules that are stereoisomers of each other represent the same structural isomer 3 The different types of isomers Stereochemistry focuses on stereoisomers red boxes in the picture Contents 1 Enantiomers 2 Diastereomers 3 Cis trans and E Z isomerism 4 Conformers 5 Anomers 6 Atropisomers 7 More definitions 8 Le Bel van t Hoff rule 9 See also 10 ReferencesEnantiomers editMain articles Chirality chemistry and Enantiomers Enantiomers also known as optical isomers are two stereoisomers that are related to each other by a reflection they are mirror images of each other that are non superposable Human hands are a macroscopic analog of this Every stereogenic center in one has the opposite configuration in the other Two compounds that are enantiomers of each other have the same physical properties except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds As a result different enantiomers of a compound may have substantially different biological effects Pure enantiomers also exhibit the phenomenon of optical activity and can be separated only with the use of a chiral agent In nature only one enantiomer of most chiral biological compounds such as amino acids except glycine which is achiral is present An optically active compound shows two forms D form and L form Diastereomers editMain article Diastereomer Diastereomers are stereoisomers not related through a reflection operation 4 They are not mirror images of each other These include meso compounds cis trans isomers E Z isomers and non enantiomeric optical isomers Diastereomers seldom have the same physical properties In the example shown below the meso form of tartaric acid forms a diastereomeric pair with both levo and dextro tartaric acids which form an enantiomeric pair nbsp nbsp nbsp natural tartaric acidL tartaric acidL tartaric acidlevo tartaric acid D tartaric acidD tartaric aciddextro tartaric acid meso tartaric acid 1 1 DL tartaric acid racemic acid The D and L labeling of the isomers above is not the same as the d and l labeling more commonly seen explaining why these may appear reversed to those familiar with only the latter naming convention A Fischer projection can be used to differentiate between L and D molecules Chirality chemistry For instance by definition in a Fischer projection the penultimate carbon of D sugars are depicted with hydrogen on the left and hydroxyl on the right L sugars will be shown with the hydrogen on the right and the hydroxyl on the left The other refers to Optical rotation when looking at the source of light the rotation of the plane of polarization may be either to the right dextrorotary d rotary represented by clockwise or to the left levorotary l rotary represented by counter clockwise depending on which stereoisomer is dominant For instance sucrose and camphor are d rotary whereas cholesterol is l rotary Cis trans and E Z isomerism editMain articles Cis trans isomerism and E Z notation Stereoisomerism about double bonds arises because rotation about the double bond is restricted keeping the substituents fixed relative to each other 5 If the two substituents on at least one end of a double bond are the same then there is no stereoisomer and the double bond is not a stereocenter e g propene CH3CH CH2 where the two substituents at one end are both H 6 Traditionally double bond stereochemistry was described as either cis Latin on this side or trans Latin across in reference to the relative position of substituents on either side of a double bond A simple example of cis trans isomerism is the 1 2 disubstituted ethenes like the dichloroethene C2H2Cl2 isomers shown below 7 nbsp Dichloroethene isomersMolecule I is cis 1 2 dichloroethene and molecule II is trans 1 2 dichloroethene Due to occasional ambiguity IUPAC adopted a more rigorous system wherein the substituents at each end of the double bond are assigned priority based on their atomic number If the high priority substituents are on the same side of the bond it is assigned Z Ger zusammen together If they are on opposite sides it is E Ger entgegen opposite 8 Since chlorine has a larger atomic number than hydrogen it is the highest priority group 9 Using this notation to name the above pictured molecules molecule I is Z 1 2 dichloroethene and molecule II is E 1 2 dichloroethene It is not the case that Z and cis or E and trans are always interchangeable Consider the following fluoromethylpentene nbsp FluoromethylpenteneThe proper name for this molecule is either trans 2 fluoro 3 methylpent 2 ene because the alkyl groups that form the backbone chain i e methyl and ethyl reside across the double bond from each other or Z 2 fluoro 3 methylpent 2 ene because the highest priority groups on each side of the double bond are on the same side of the double bond Fluoro is the highest priority group on the left side of the double bond and ethyl is the highest priority group on the right side of the molecule The terms cis and trans are also used to describe the relative position of two substituents on a ring cis if on the same side otherwise trans 10 11 12 Conformers editMain article Conformational isomerism Conformational isomerism is a form of isomerism that describes the phenomenon of molecules with the same structural formula but with different shapes due to rotations about one or more bonds 13 14 15 Different conformations can have different energies can usually interconvert and are very rarely isolatable For example there exists a variety of Cyclohexane conformations which cyclohexane is an essential intermediate for the synthesis of nylon 6 6 including a chair conformation where four of the carbon atoms form the seat of the chair one carbon atom is the back of the chair and one carbon atom is the foot rest and a boat conformation the boat conformation represents the energy maximum on a conformational itinerary between the two equivalent chair forms however it does not represent the transition state for this process because there are lower energy pathways The conformational inversion of substituted cyclohexanes is a very rapid process at room temperature with a half life of 0 00001 seconds 16 17 There are some molecules that can be isolated in several conformations due to the large energy barriers between different conformations 2 2 6 6 Tetrasubstituted biphenyls can fit into this latter category Anomers editAnomerism is an identity for single bonded ring structures where cis or Z and trans or E geometric isomerism needs to name the substitutions on a carbon atom that also displays the identity of chirality so anomers have carbon atoms that have geometric isomerism and optical isomerism enantiomerism on one or more of the carbons of the ring 18 19 20 21 Anomers are named alpha or axial and beta or equatorial when substituting a cyclic ring structure that has single bonds between the carbon atoms of the ring for example a hydroxyl group a methyl hydroxyl group a methoxy group or another pyranose or furanose group which are typical single bond substitutions but not limited to these 22 Axial geometric isomerism will be perpendicular 90 degrees to a reference plane and equatorial will be 120 degrees away from the axial bond or deviate 30 degrees from the reference plane 23 Atropisomers editMain article Atropisomer Atropisomers are stereoisomers resulting from hindered rotation about single bonds where the steric strain barrier to rotation is high enough to allow for the isolation of the conformers 24 25 More definitions editA configurational stereoisomer is a stereoisomer of a reference molecule that has the opposite configuration at a stereocenter e g R vs S or E vs Z This means that configurational isomers can be interconverted only by breaking covalent bonds to the stereocenter for example by inverting the configurations of some or all of the stereocenters in a compound An epimer is a diastereoisomer that has the opposite configuration at only one of the stereocenters Le Bel van t Hoff rule editLe Bel van t Hoff rule states that for a structure with n asymmetric carbon atoms there is a maximum of 2n different stereoisomers possible As an example D glucose is an aldohexose and has the formula C6H12O6 Four of its six carbon atoms are stereogenic which means D glucose is one of 24 16 possible stereoisomers 26 27 See also editDescriptor chemistry Backbone dependent rotamer libraryReferences edit IUPAC Compendium of Chemical Terminology 2nd ed the Gold Book 1997 Online corrected version 2006 stereoisomerism doi 10 1351 goldbook S05983 Columbia Encyclopedia Stereoisomers in Encyclopedia com n l 2005 Link Clark Jim November 2012 Optical isomerism chemguide co uk Retrieved 2022 08 09 Diastereomer an overview ScienceDirect Topics Geometric Isomers Definition And Examples Chemistry Dictionary Retrieved 2022 06 20 Clark Jim February 2020 geometric cis trans isomerism Chemguide uk Retrieved 2022 08 09 Helmenstine Anne Marie Geometric Isomer Definition Cis Trans Isomers ThoughtCo Roberts John D Caserio Marjorie C 1977 Basic Principles of Organic Chemistry second ed W A Benjamin Inc p 19 7 ISBN 0805383298 Roberts John D Caserio Marjorie C 1977 Basic Principles of Organic Chemistry second ed W A Benjamin Inc p 19 6 ISBN 0805383298 Cis trans isomerism NAL Agricultural Thesaurus Clark Jim November 2012 E Z notation for geometric isomerism chemguide co uk Retrieved 2022 08 09 E and Z Notation for Alkenes Cis Trans 3 November 2016 Hunt Ian What are Conformational Isomers chem ucalgary ca Retrieved 2022 08 09 Conformational Isomer an overview ScienceDirect Topics Isomerism Conformational isomers Britannica Cyclohexane an overview ScienceDirect Topics Reusch William 2013 05 05 Stereoisomers chemistry msu edu Retrieved 2022 08 09 Hunt Ian What do the a and b forms look like chem ucalgary ca Retrieved 2022 08 09 Anomer an overview ScienceDirect Topics Anomeric Effect an overview ScienceDirect Topics Ashenhurst James 2022 08 03 The Big Damn Post Of Carbohydrate Related Chemistry Definitions masterorganicchemistry com Retrieved 2022 08 09 Reusch William Stereoisomers chemistry msu edu Retrieved 2022 08 09 Morrison and Boyd Organic Chemistry Sixth ed pgs 1170 1171 ISBN 0 13 643669 2 Atropisomer an overview ScienceDirect Topics Metrano Anthony J 2018 06 09 Atropisomers PDF knowleslab princeton edu Retrieved 2022 08 09 Blackmond D G 2019 The Origin of Biological Homochirality Cold Spring Harbor Perspectives in Biology 11 3 a032540 doi 10 1101 cshperspect a032540 PMC 6396334 PMID 30824575 Brooks Benjamin T 1918 The German Chemical Myth The North American Review 208 756 729 735 JSTOR 25151064 Retrieved from https en wikipedia org w index php title Stereoisomerism amp oldid 1194263109, wikipedia, wiki, book, books, library,

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