Stereoisomerism
In
Enantiomers
Enantiomers, also known as optical isomers, are two stereoisomers that are related to each other by a reflection: they are
Diastereomers
Diastereomers are stereoisomers not related through a reflection operation.
(natural) tartaric acid |
D-tartaric acid |
meso-tartaric acid |
(1:1) |
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
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]
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:
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
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
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
More definitions
- A configurational stereoisomer is a stereoisomer of a reference molecule that has the opposite configuration at a stereocenter (e.g., 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 stereocentersin a compound.
- An epimer is a diastereoisomer that has the opposite configuration at only one of the stereocenters.
Le Bel-van't Hoff rule
See also
References
- ^ 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.
- ISBN 0805383298.
- 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 α- and β- 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.
- 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.
- PMID 30824575.
- JSTOR 25151064.