![Isomerism](https://upload.wikimedia.org/wikipedia/commons/0/04/Isomerism.svg)
In
stereochemistry, stereoisomerism, or spatial isomerism, is a form of
isomerism in which molecules have the same
molecular formula
In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. 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.
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
Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object.
An object or a system is ''chiral'' if it is distinguishable from i ...
agent. In nature, only one enantiomer of most chiral biological compounds, such as
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
s (except
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid ( carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinog ...
, which is achiral), is present. An optically active compound shows two forms:
D-(+) form and
L-(−) form.
Diastereomers
Diastereomers are stereoisomers not related through a reflection operation. They are not mirror images of each other. These include
meso compound
A meso compound or meso isomer is a non-optically active member of a set of stereoisomers, at least two of which are optically active. This means that despite containing two or more stereocenters, the molecule is not chiral. A meso compound is "su ...
s,
''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.
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
In chemistry, the Fischer projection, devised by Emil Fischer in 1891, is a two-dimensional representation of a three-dimensional organic molecule by projection. Fischer projections were originally proposed for the depiction of carbohydrates ...
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. 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, CH
3CH=CH
2 where the two substituents at one end are both H.
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 (C
2H
2Cl
2) isomers shown below.
![Dichloroethene](https://upload.wikimedia.org/wikipedia/commons/a/a5/Dichloroethene.png)
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
The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...
. 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). Since chlorine has a larger atomic number than hydrogen, it is the highest-priority group. 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](https://upload.wikimedia.org/wikipedia/commons/b/be/Fluoromethylpentene.png)
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''.
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. Different conformations can have different energies, can usually interconvert, and are very rarely isolatable. For example, there exists a variety of
Cyclohexane conformation
In organic chemistry, cyclohexane conformations are any of several three-dimensional shapes adopted by molecules of cyclohexane. Because many compounds feature structurally similar six-membered rings, the structure and dynamics of cyclohexane ...
s (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
In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked ...
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.
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. 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. 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.
Atropisomers
Atropisomers are stereoisomers resulting from hindered rotation about
single bonds where the
steric strain Van der Waals strain is strain resulting from Van der Waals repulsion when two substituents in a molecule approach each other with a distance less than the sum of their Van der Waals radii.
Van der Waals strain is also called Van der Waals repul ...
barrier to rotation is high enough to allow for the isolation of the conformers.
More definitions
* 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
stereocenter
In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups c ...
s 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
Le Bel-van't Hoff rule states that for a structure with ''n''
asymmetric carbon An asymmetric carbon atom (chiral carbon) is a carbon atom that is attached to four different types of atoms or groups of atoms. Le Bel-van't Hoff rule states that the number of stereoisomers of an organic compound is 2n, where n represents the num ...
atoms, there is a maximum of 2
''n'' different stereoisomers possible. As an example,
D-glucose is an
aldohexose and has the formula C
6H
12O
6. Four of its six carbon atoms are stereogenic, which means
D-glucose is one of 2
4=16 possible stereoisomers.
See also
*
Descriptor (chemistry)
A descriptor is in chemical nomenclature a prefix placed before the systematic substance name, which describes the configuration or the stereochemistry of the molecule. Some listed descriptors are only of historical interest and should not be us ...
*
Backbone-dependent rotamer library
In biochemistry, a backbone-dependent rotamer library provides the frequencies, mean dihedral angles, and standard deviations of the discrete conformations (known as rotamers) of the amino acid side chains in proteins as a function of the backb ...
References
{{Chiral synthesis
Stereochemistry
Isomerism
Jacobus Henricus van 't Hoff