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In chemistry, isomers are or with an identical molecular formula – that is, the same number of of each element – but distinct arrangements of atoms in space. Isomerism refers to the existence or possibility of isomers.
Isomers do not necessarily share similar chemical or physical properties. Two main forms of isomerism are structural (or constitutional) isomerism, in which chemical bond between the atoms differ; and stereoisomerism (or spatial isomerism), in which the bonds are the same but the relative positions of the atoms differ.
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 Isotopologue. The depth of analysis depends on the field of study or the chemical and physical properties of interest.
The English word "isomer" () is a back-formation from "isomeric",Merriam-Webster: "isomer" online dictionary entry. Accessed on 2020-08-26 which was borrowed through German language isomerischMerriam-Webster: "isomeric" online dictionary entry. Accessed on 2020-08-26 from Swedish language isomerisk; which in turn was coined from Greek language ἰσόμερoς , with roots = "equal", = "part".
The first two isomers shown of
The third isomer of
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 or , occur on a "parent" molecule (propane, in that case).
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 , 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.
Important examples are keto-enol tautomerism and the equilibrium between neutral and zwitterionic forms of an amino acid.
A classic example is bromochlorofluoromethane (
The compound chlorofluoromethane
Another example of a chiral compound is Pentadiene
Enantiomers behave identically in chemical reactions, except when reacting with chiral compounds or in the presence of chiral catalysts, such as most . For this latter reason, the two enantiomers of most chiral compounds usually have markedly different effects and roles in living organisms. In biochemistry and food science, the two enantiomers of a chiral molecule – such as glucose – are usually identified and treated as very different substances.
Each enantiomer of a chiral compound typically rotates the plane of polarized light that passes through it. The rotation has the same magnitude but opposite senses for the two isomers, and can be a useful way of distinguishing and measuring their concentration in a solution. For this reason, enantiomers were formerly called "optical isomers".Whitten K.W., Gailey K.D. and Davis R.E. "General Chemistry" (4th ed., Saunders College Publishing 1992), p. 976–7 However, this term is ambiguous and is discouraged by the IUPAC.
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.
The classical example is dichloroethene
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.
For example, in the cyclic alcohol inositol
Cis and trans isomers also occur in inorganic coordination compounds, such as square planar
For more complex organic molecules, the cis and trans labels can be ambiguous. In such cases, a more precise labeling scheme is employed based on the Cahn-Ingold-Prelog priority rules.
For example, replacing two atoms of common hydrogen (
Another example would be substituting one atom of deuterium for one of the hydrogens in chlorofluoromethane (
When two isomers would be identical if all isotopes of each element were replaced by a single isotope, they are described as or isotopic isomers. In the above two examples if all
The concept of isotopomers is different from isotopologue or isotopic homologs, which differ in their isotopic composition. For example,
In medicinal chemistry and biochemistry, are a special concern because they may possess distinct biological activity. Many preparative procedures afford a mixture of equal amounts of both enantiomeric forms. In some cases, the enantiomers are separated by chromatography using chiral stationary phases. They may also be separated through the formation of diastereomeric salts. In other cases, enantioselective synthesis have been developed.
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.
Additional examples were found in succeeding years, such as Wöhler's 1828 discovery that urea has the same atomic composition (
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.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. In 1860, Pasteur explicitly hypothesized that the molecules of isomers might have the same composition but different arrangements of their atoms.Pullman (1998). The Atom in the History of Human Thought, p. 230
Structural isomers
Structural isomers have the same number of atoms of each element (hence the same molecular formula), but the atoms are connected in distinct ways.
Example:
For example, there are three distinct compounds with the molecular formula
Example:
There are also three structural isomers of the hydrocarbon I Propadiene II Propyne III Cyclopropene
Tautomers
are structural isomers which readily interconvert, so that two or more species co-exist in equilibrium such as
Stereoisomers
Stereoisomers have the same atoms or isotopes connected by bonds of the same type, but differ in the relative positions of those atoms in space. Two broad types of stereoisomers exist, enantiomers and diastereomers. Enantiomers have identical physical properties but diastereomers do not.
Enantiomers
Two compounds are said to be if their molecules are mirror images of each other and 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 chirality.
Diastereomers
Stereoisomers that are not enantiomers are called . Some diastereomers may contain chiral center, and some may not.
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 π orbital. If the two bonds on each carbon connect to different atoms, two distinct conformations are possible that differ from each other by a twist of 180 degrees of one of the carbons about the double bond.
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.
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 or nuclear spin isomers: parahydrogen, with the spins of the two nuclei pointing in opposite directions, and orthohydrogen, where the spins point in the same direction.
Applications
Isomers having distinct biological properties are common; for example, the placement of . In substituted , theobromine, found in chocolate, is a vasodilator with some effects in common with caffeine; but, if one of the two methyl groups is moved to a different position on the two-ring core, the isomer is theophylline, which has a variety of effects, including bronchodilator and anti-inflammatory action. Another example of this occurs in the phenethylamine-based stimulant drugs. Phentermine is a non-chiral compound with a weaker effect than that of amphetamine. It is used as an appetite-reducing medication and has mild or no stimulant properties. However, an alternate atomic arrangement gives dextromethamphetamine, which is a stronger stimulant than amphetamine.
History
Isomerism was first observed in 1827, when Friedrich Wöhler prepared silver cyanate and discovered that, although its elemental composition of
See also
External links
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