Isomer
This article needs additional citations for verification. (September 2021) |
In
Isomers do not necessarily share similar
Isomeric relationships form a hierarchy. Two chemicals might be the same constitutional isomer, but upon deeper analysis be stereoisomers of each other. Two molecules that are the same stereoisomer as each other might be in different conformational forms or be different isotopologues. The depth of analysis depends on the field of study or the chemical and physical properties of interest.
The English word "isomer" (/ˈaɪsəmər/) is a back-formation from "isomeric",[2] which was borrowed through German isomerisch[3] from Swedish isomerisk; which in turn was coined from Greek ἰσόμερoς isómeros, with roots isos = "equal", méros = "part".[4]
Structural isomers
Structural isomers have the same number of atoms of each element (hence the same
Example: C
3H
8O
For example, there are three distinct compounds with the molecular formula :
The first two isomers shown of are
The third isomer of is the ether methoxyethane (ethyl-methyl-ether; III). Unlike the other two, it has the oxygen atom connected to two carbons, and all eight hydrogens bonded directly to carbons. It can be described by the condensed formula .
The alcohol "3-propanol" is not another isomer, since the difference between it and 1-propanol is not real; it is only the result of an arbitrary choice in the direction of numbering the carbons along the chain. For the same reason, "ethoxymethane" is the same molecule as methoxyethane, not another isomer.
1-Propanol and 2-propanol are examples of positional isomers, which differ by the position at which certain features, such as double bonds or functional groups, occur on a "parent" molecule (propane, in that case).
Example: C
3H
4
There are also three structural isomers of the hydrocarbon :
I Propadiene | II Propyne | III Cyclopropene |
In two of the isomers, the three carbon atoms are connected in an open chain, but in one of them (propadiene or allene; I) the carbons are connected by two double bonds, while in the other (propyne or methylacetylene; II) they are connected by a single bond and a triple bond. In the third isomer (cyclopropene; III) the three carbons are connected into a ring by two single bonds and a double bond. In all three, the remaining valences of the carbon atoms are satisfied by the four hydrogens.
Again, note that there is only one structural isomer with a triple bond, because the other possible placement of that bond is just drawing the three carbons in a different order. For the same reason, there is only one cyclopropene, not three.
Tautomers
Tautomers are structural isomers which readily interconvert, so that two or more species co-exist in equilibrium such as
.[6]
Important examples are
Resonance forms
The structure of some molecules is sometimes described as a resonance between several apparently different structural isomers. The classical example is 1,2-dimethylbenzene (o-xylene), which is often described as a mix of the two apparently distinct structural isomers:
However, neither of these two structures describes a real compound; they are fictions devised as a way to describe (by their "averaging" or "resonance") the actual delocalized bonding of o-xylene, which is the single isomer of with a benzene core and two methyl groups in adjacent positions.
Stereoisomers
Stereoisomers have the same atoms or isotopes connected by bonds of the same type, but differ in their shapes – the relative positions of those atoms in space – apart from
In theory, one can imagine any arrangement in space of the atoms of a molecule or ion to be gradually changed to any other arrangement in infinitely many ways, by moving each atom along an appropriate path. However, changes in the positions of atoms will generally change the internal energy of a molecule, which is determined by the angles between bonds in each atom and by the distances between atoms (whether they are bonded or not).
A
A classic example of conformational isomerism is
If the energy barrier between two conformational isomers is low enough, it may be overcome by the random inputs of thermal energy that the molecule gets from interactions with the environment or from its own vibrations. In that case, the two isomers may as well be considered a single isomer, depending on the temperature and the context. For example, the two conformations of cyclohexane convert to each other quite rapidly at room temperature (in the liquid state), so that they are usually treated as a single isomer in chemistry.[7]
In some cases, the barrier can be crossed by
Conversely, the energy barrier may be so high that the easiest way to overcome it would require temporarily breaking and then reforming one or more bonds of the molecule. In that case, the two isomers usually are stable enough to be isolated and treated as distinct substances. These isomers are then said to be different
Interactions with other molecules of the same or different compounds (for example, through
Enantiomers
Two compounds are said to be enantiomers if their molecules are mirror images of each other, that cannot be made to coincide only by rotations or translations – like a left hand and a right hand. The two shapes are said to be chiral.
A classical example is bromochlorofluoromethane (). The two enantiomers can be distinguished, for example, by whether the path turns clockwise or counterclockwise as seen from the hydrogen atom. In order to change one conformation to the other, at some point those four atoms would have to lie on the same plane – which would require severely straining or breaking their bonds to the carbon atom. The corresponding energy barrier between the two conformations is so high that there is practically no conversion between them at room temperature, and they can be regarded as different configurations.
The compound chlorofluoromethane , in contrast, is not chiral: the mirror image of its molecule is also obtained by a half-turn about a suitable axis.
Another example of a chiral compound is 2,3-pentadiene a hydrocarbon that contains two overlapping double bonds. The double bonds are such that the three middle carbons are in a straight line, while the first three and last three lie on perpendicular planes. The molecule and its mirror image are not superimposable, even though the molecule has an axis of symmetry. The two enantiomers can be distinguished, for example, by the right-hand rule. This type of isomerism is called axial isomerism.
Enantiomers behave identically in chemical reactions, except when reacted with chiral compounds or in the presence of chiral
Each enantiomer of a chiral compound typically rotates the plane of
Stereoisomers that are not enantiomers are called
Some enantiomer pairs (such as those of trans-cyclooctene) can be interconverted by internal motions that change bond lengths and angles only slightly. Other pairs (such as CHFClBr) cannot be interconverted without breaking bonds, and therefore are different configurations.
Cis-trans isomerism
A double bond between two carbon atoms forces the remaining four bonds (if they are single) to lie on the same plane, perpendicular to the plane of the bond as defined by its
The classical example is dichloroethene , specifically the structural isomer that has one chlorine bonded to each carbon. It has two conformational isomers, with the two chlorines on the same side or on opposite sides of the double bond's plane. They are traditionally called cis (from Latin meaning "on this side of") and trans ("on the other side of"), respectively; or Z and E in the
More generally, cis–trans isomerism (formerly called "geometric isomerism") occurs in molecules where the relative orientation of two distinguishable functional groups is restricted by a somewhat rigid framework of other atoms.[15]
For example, in the cyclic alcohol inositol (a six-fold alcohol of cyclohexane), the six-carbon cyclic backbone largely prevents the hydroxyl and the hydrogen on each carbon from switching places. Therefore, one has different configurational isomers depending on whether each hydroxyl is on "this side" or "the other side" of the ring's mean plane. Discounting isomers that are equivalent under rotations, there are nine isomers that differ by this criterion, and behave as different stable substances (two of them being enantiomers of each other). The most common one in nature (myo-inositol) has the hydroxyls on carbons 1, 2, 3 and 5 on the same side of that plane, and can therefore be called cis-1,2,3,5-trans-4,6-cyclohexanehexol. And each of these cis-trans isomers can possibly have stable "chair" or "boat" conformations (although the barriers between these are significantly lower than those between different cis-trans isomers).
For more complex organic molecules, the cis and trans labels are ambiguous. The IUPAC recommends a more precise labeling scheme, based on the
Centers with non-equivalent bonds
More generally, atoms or atom groups that can form three or more non-equivalent single bonds (such as the transition metals in coordination compounds) may give rise to multiple stereoisomers when different atoms or groups are attached at those positions. The same is true if a center with six or more equivalent bonds has two or more substituents.
For instance, in the compound , the bonds from the
For the compound , three isomers are possible, with zero, one, or two chlorines in the axial positions.
As another example, a complex with a formula like , where the central atom M forms six bonds with
Rotamers and atropisomers
Two parts of a molecule that are connected by just one single bond can rotate about that bond. While the bond itself is indifferent to that rotation, attractions and repulsions between the atoms in the two parts normally cause the energy of the whole molecule to vary (and possibly also the two parts to deform) depending on the relative angle of rotation φ between the two parts. Then there will be one or more special values of φ for which the energy is at a local minimum. The corresponding conformations of the molecule are called rotational isomers or
Thus, for example, in an ethane molecule , all the bond angles and length are narrowly constrained, except that the two
Rotation between the two halves of the molecule
If the two parts of the molecule connected by a single bond are bulky or charged, the energy barriers may be much higher. For example, in the compound
Topoisomers
Large molecules may have isomers that differ by the topology of their overall arrangement in space, even if there is no specific geometric constraint that separate them. For example, long chains may be twisted to form topologically distinct knots, with interconversion prevented by bulky substituents or cycle closing (as in circular DNA and RNA plasmids). Some knots may come in mirror-image enantiomer pairs. Such forms are called topological isomers or topoisomers.
Also, two or more such molecules may be bound together in a
20H
20) and carbon peapods, are a similar type of topological isomerism involving molecules with large internal voids with restricted or no openings.[19]
Isotopes and spin
Isotopomers
Different isotopes of the same element can be considered as different kinds of atoms when enumerating isomers of a molecule or ion. The replacement of one or more atoms by their isotopes can create multiple structural isomers and/or stereoisomers from a single isomer.
For example, replacing two atoms of common hydrogen () by deuterium (, or ) on an ethane molecule yields two distinct structural isomers, depending on whether the substitutions are both on the same carbon (1,1-dideuteroethane, ) or one on each carbon (1,2-dideuteroethane, ); as if the substituent was chlorine instead of deuterium. The two molecules do not interconvert easily and have different properties, such as their microwave spectrum.[20]
Another example would be substituting one atom of deuterium for one of the hydrogens in chlorofluoromethane (). While the original molecule is not chiral and has a single isomer, the substitution creates a pair of chiral enantiomers of , which could be distinguished (at least in theory) by their optical activity.[21]
When two isomers would be identical if all isotopes of each element were replaced by a single isotope, they are described as isotopomers or isotopic isomers.[22] In the above two examples if all were replaced by , the two dideuteroethanes would both become ethane and the two deuterochlorofluoromethanes would both become .
The concept of isotopomers is different from isotopologs or isotopic homologs, which differ in their isotopic composition.[22] For example, and are isotopologues and not isotopomers, and are therefore not isomers of each other.
Spin isomers
Another type of isomerism based on nuclear properties is spin isomerism, where molecules differ only in the relative spin magnetic quantum numbers ms of the constituent atomic nuclei. This phenomenon is significant for molecular hydrogen, which can be partially separated into two long-lived states described as spin isomers[23] or nuclear spin isomers:[24] parahydrogen, with the spins of the two nuclei pointing in opposite directions, and orthohydrogen, where the spins point in the same direction.
Isomerization
Isomerization is the process by which one molecule is transformed into another molecule that has exactly the same atoms, but the atoms are rearranged.[25] In some molecules and under some conditions, isomerization occurs spontaneously. Many isomers are equal or roughly equal in bond energy, and so exist in roughly equal amounts, provided that they can interconvert relatively freely, that is the energy barrier between the two isomers is not too high. When the isomerization occurs intramolecularly, it is considered a rearrangement reaction.
An example of an
- Synthesis of fumaric acid
Industrial synthesis of fumaric acid proceeds via the cis-trans isomerization of maleic acid:
Topoisomerases are enzymes that can cut and reform circular DNA and thus change its topology.
Medicinal chemistry
Isomers having distinct biological properties are common; for example, the placement of
In
As an inorganic example, cisplatin (see structure above) is an important drug used in cancer chemotherapy, whereas the trans isomer (transplatin) has no useful pharmacological activity.
History
Isomerism was first observed in 1827, when Friedrich Wöhler prepared silver cyanate and discovered that, although its elemental composition of was identical to silver fulminate (prepared by Justus von Liebig the previous year),[28] its properties were distinct. This finding challenged the prevailing chemical understanding of the time, which held that chemical compounds could be distinct only when their elemental compositions differ. (We now know that the bonding structures of fulminate and cyanate can be approximately described as ≡ and , respectively.)
Additional examples were found in succeeding years, such as Wöhler's 1828 discovery that urea has the same atomic composition () as the chemically distinct ammonium cyanate. (Their structures are now known to be and , respectively.) In 1830 Jöns Jacob Berzelius introduced the term isomerism to describe the phenomenon.[4][29][30][31]
In 1848, Louis Pasteur observed that tartaric acid crystals came into two kinds of shapes that were mirror images of each other. Separating the crystals by hand, he obtained two version of tartaric acid, each of which would crystallize in only one of the two shapes, and rotated the plane of polarized light to the same degree but in opposite directions.[32][33] In 1860, Pasteur explicitly hypothesized that the molecules of isomers might have the same composition but different arrangements of their atoms.[34]
See also
- Chirality (chemistry)
- Cis-trans isomerism
- Cyclohexane conformation
- Descriptor (chemistry)
- Electromerism
- Isomery (botany)
- Ligand isomerism
- Nuclear isomer
- Stereocenter
- Structural isomerism
- Tautomer
- Vitamer
References
- OCLC 46872308.
- ^ Merriam-Webster: "isomer" online dictionary entry. Accessed on 2020-08-26
- ^ Merriam-Webster: "isomeric" online dictionary entry. Accessed on 2020-08-26
- ^ a b Jac. Berzelius (1830): "Om sammansättningen af vinsyra och drufsyra (John's säure aus den Voghesen), om blyoxidens atomvigt, samt allmänna anmärkningar om sådana kroppar som hafva lika sammansättning, men skiljaktiga egenskaper" ("On the composition of tartaric acid and racemic acid (John's acid of the Vosges), on the molecular weight of lead oxide, together with general observations on those bodies that have the same composition but distinct properties"). Kongliga Svenska Vetenskaps Academiens Handling (Transactions of the Royal Swedish Science Academy), volume 49, pages 49–80
- ISBN 978-0-07-302657-2.
- . Retrieved 21 April 2019.
- ^ ISBN 9789814730235.
- ^ Rowena Ball and John Brindley (2016): "The life story of hydrogen peroxide III: Chirality and physical effects at the dawn of life". Origins of Life and Evolution of Biospheres, volume 46, pages 81–93
- ISBN 9789057023156
- ^ Petrucci, Harwood & Herring 2002, pp. 996–997.
- ISBN 978-0-03-072373-5
- ^ Ernest L. Eliel and Samuel H. Wilen (1994). Stereochemistry of Organic Compounds. Wiley Interscience. p. 1203.
- ^ a b Ernest L. Eliel and Samuel H. Wilen (1994). Stereochemistry of Organic Compounds. Wiley Interscience. pp. 52–53.
- ^
- ISBN 9780323149327
- ISSN 2469-9926.
- ^ .
- . Retrieved 1 May 2022.
- S2CID 20443766.
- .
- .
- doi:10.1021/ed077p851. Archived from the originalon 18 February 2009. Retrieved 27 July 2012.
- ^ J. J. Berzelius (1831): "Über die Zusammensetzung der Weinsäure und Traubensäure (John's säure aus den Voghesen), über das Atomengewicht des Bleioxyds, nebst allgemeinen Bemerkungen über solche Körper, die gleiche Zusammensetzung, aber ungleiche Eigenschaften besitzen". Annalen der Physik und Chemie, volume 19, pages 305–335
- ^ J. J. Berzelius (1831): "Composition de l'acide tartarique et de l'acide racémique (traubensäure); poids atomique de l'oxide de plomb, et remarques générals sur les corps qui ont la même composition, et possèdent des proprietés différentes". Annales de Chimie et de Physique, volume 46, pages 113–147.
- .
- ^ L. Pasteur (1848) "Mémoire sur la relation qui peut exister entre la forme cristalline et la composition chimique, et sur la cause de la polarisation rotatoire" (Memoir on the relationship which can exist between crystalline form and chemical composition, and on the cause of rotary polarization)," Comptes rendus de l'Académie des sciences (Paris), vol. 26, pages 535–538.
- ^ L. Pasteur (1848) "Sur les relations qui peuvent exister entre la forme cristalline, la composition chimique et le sens de la polarisation rotatoire" ("On the relations that can exist between crystalline form, chemical composition, and the sense of rotary polarization"), Annales de Chimie et de Physique, 3rd series, volume 24, issue 6, pages 442–459.
- ^ Pullman (1998). The Atom in the History of Human Thought, p. 230