isomer
American Heritage Dictionary:
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i·so·mer
(ī'sə-mər) 
n.
isomerically i'so·mer'i·cal·ly adv.
n.
- Chemistry. Any of two or more substances that are composed of the same elements in the same proportions but differ in properties because of differences in the arrangement of atoms.
- Physics. Any of two or more nuclei with the same mass number and atomic number that have different radioactive properties and can exist in any of several energy states for a measurable period of time.
[Greek īsomerēs, having equal share : īso-, iso- + meros, part, share.]
isomeric i'so·mer'ic (-mĕr'ĭk) adj.isomerically i'so·mer'i·cal·ly adv.
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One of two or more substances with identical molecular formulas but different configurations, differing only in the arrangement of their component atoms. It usually refers to stereoisomers (rather than constitutional isomers or tautomers; isomerism, tautomerism), of which there are two types. Optical isomers, or enantiomers ( optical activity), occur in mirror-image pairs. Geometric isomers are often the result of rigidity in the molecular structure; in organic compounds, this is usually due to a double bond ( bonding) or a ring structure. In the case of a double bond between two carbon atoms, if each has two other groups bonded to it and all are rigidly in the same plane, the corresponding groups can be on the same side (cis) of the CC bond or across the CC bond (trans) from each other. An analogous distinction can be made for ring structures that are all in a plane, between isomers whose substituent groups are on the same side and isomers whose substituent groups are on both sides of the plane. Diastereomers that are not enantiomers also fall into this category. Most cis-trans isomers are organic compounds.
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Molecules containing the same atoms but differently arranged, so that the chemical and biochemical properties differ. (1)In positional isomers the functional groups are on different carbon atoms; e.g. leucine and isoleucine.(2)d- and l-isomerism refers to the spatial arrangement of four different chemical groups on the same carbon atom (stereo-isomerism or optical isomerism). r- and s-isomerism is the same, but determined by a set of systematic chemical rules. See D-.(3)Cis- and trans-isomerism refers to the arrangement of groups adjacent to a carbon-carbon double bond; in the cis-isomer the groups are on the same side of the double bond, while in the trans-isomer they are on opposite sides.
Columbia Encyclopedia:
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isomers
isomer (ī'səmər), in chemistry, one of two or more compounds having the same molecular formula but different structures (arrangements of atoms in the molecule). Isomerism is the occurrence of such compounds. Isomerism was first recognized by J. J. Berzelius in 1827. Early work with stereoisomers was carried out by Louis Pasteur, who separated racemic acid into its two optically active tartaric acid components by crystallization (1848). Pasteur's results were given theoretical basis by J. H. Van't Hoff and independently by J. A. le Bel (1864).
The prefix cis- means "same side" and trans- means "opposite side"; they are used when the groups on either side of the double bond are identical or closely related, e.g., methyl and ethyl. Syn- and anti- have similar meanings but are used when the groups are not identical or closely related.
The atom and radical to either side of the carbon atom are visualized as being above the plane of the paper, the central carbon atom in the plane of the paper, and the radicals above and below the central carbon atom below the plane of the paper. Thus it is seen that the two molecules are mirror images of each other and, each being asymmetrical, cannot be superposed on each other. The d- and l- prefixes stand for dextro (right) and levo (left). Two optical isomers, such as these, whose molecules are asymmetrical and are mirror images of each other, are called enantiomorphs. When equal amounts of d- and l-enantiomorphs are mixed, the mixture has no effect on polarized light; such a mixture is called racemic.
The d- and l-tartaric acids are enantiomorphs; each molecule is asymmetrical and is the mirror image of the other. There are two asymmetrical carbon atoms in meso-tartaric acid, but the molecule is symmetrical and does not exhibit optical activity; the optical activity is internally compensated, the effect of one asymmetrical carbon atom balancing the effect of the other. A pair of optical isomers such as d-tartaric acid and meso-tartaric acid, which are not enantiomorphs, are called diastereoisomers. Molecular disymmetry in optical isomers may come from some source other than an asymmetrical carbon atom, e.g., structural rigidity resulting from double bonds or ring structures within a molecule.
General Characteristics
Isomers have the same number of atoms of each element in them and the same atomic weight but differ in other properties. For example, there are two compounds with the molecular formula C2H6O. One is ethanol (also called ethyl alcohol), CH3CH2OH, a colorless liquid alcohol; the other is dimethyl ether, CH3OCH3, a colorless gaseous ether. Among their different properties, ethanol has a boiling point of 78.5°C and a freezing point of −117°C; dimethyl ether has a boiling point of −25°C and a freezing point of −138°C. Ethanol and dimethyl ether are isomers because they differ in the way the atoms are joined together in their molecules:
Isomers are classified as structural isomers, which have the same number of atoms of each element and molecular weight but different bonding patterns (see chemical bond), or as stereoisomers, which have the same number of atoms of each element, molecular weight, and bonding pattern but in which the atoms have different spatial relationships. Tautomers are structural isomers that readily convert from one isomeric form to another and therefore exist in equilibrium.
Structural Isomers
Structural isomers are subdivided as chain, position, and functional group. Chain isomers occur among the alkanes. For example, there are two chain isomers of butane, C4H10. In n-butane, CH3CH2CH2CH3, the carbon atoms are joined in a so-called straight, or unbranched, chain. In isobutane, CH3CH(CH3)2, the carbon atoms are joined in a branched chain; the isobutane molecule can be visualized as a carbon atom bonded to one hydrogen atom and to three methyl (CH3) groups.
Position isomers occur among substituted alkanes and other compounds. For example, 1-propanol, CH3CH2CH2OH, and 2-propanol, CH3CH(OH)CH3, are position isomers, as are 1-butene, CH2-CHCH2CH3, and 2-butene, CH3CH-CHCH3. Position isomers have similar chemical properties since they differ only in the location of the functional group (e.g., the OH in an alcohol or the double bond in an alkene).
Functional group isomers, on the other hand, have very different chemical properties because differences in their structure give rise to different functional groups. Ethanol and dimethyl ether (see the example, above) are functional group isomers.
Stereoisomers
Stereoisomerism occurs when two or more molecules have the same basic arrangement of atoms in their molecules but differ in the way the atoms are arranged in space. There are two types of stereoisomerism. The first type, geometric isomerism, may occur when a compound contains a double bond or some other feature that gives the molecule a certain amount of structural rigidity. Geometric isomers differ in physical properties such as melting point and boiling point. For example, there are two geometric isomers of 2-butene, CH3CH-CHCH3:
The second type of stereoisomerism is optical isomerism. When plane-polarized light is passed through an optical isomer it is rotated into a different plane of polarization. Optical isomers, also know as chiral molecules or enantiomers, exhibit this optical activity in varying degrees. Optical isomers of a given compound are often identical in all physical properties except the direction in which they rotate light. The molecules of optical isomers are asymmetrical. The simplest optical isomers have a single "asymmetrical carbon atom" in their molecules. An asymmetrical carbon atom has four different atoms or radicals bonded to it, arranged approximately at the corners of a tetrahedron centered on the carbon atom. For example, there are two optical isomers of lactic acid:
When there is more than one asymmetrical carbon atom, there may be more than two optical isomers. For example, tartaric acid has two asymmetrical carbon atoms and three optical isomers:
Stereoisomers are important in metabolism; in many cases only one of several isomeric forms of a compound can take part in biochemical reactions. For example, there are 16 stereoisomers of a simple sugar whose molecular formula is C6H12O4. Of these, only d-glucose is readily utilized in human metabolism.
Science Q&A:
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What are isomers?
Isomers are compounds with the same molecular formula but different structures due to the different arrangement of the atoms within the molecules. Structural isomers have atoms connected in different ways. Geometric isomers differ in their symmetry about a double bond. Optical isomers are mirror images of each other.
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Wiley Dictionary of Flavors:
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Isomer
A related structure different only by a simple change in configuration. A branched isomer is related to the straightchained chemical by the substitution of the branch, but its molecular weight and empirical formula are the same. A transisomer of an alcohol differs from its cis counterpart because it contains a less sterically hindered configuration where the R1 and R2 groups are across from the double bond. See Cis (Isomerism), Double Bond, Trans (Isomerism), Ortho, Meta, Para, Optical Isomers, D (Isomer), L (Isomer).
Oxford Dictionary of Biochemistry:
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isomeric
of or pertaining to an isomer; pertaining to the phenomenon of, or displaying, isomerism.
| isomerase, isomer, isomaltulose | |
| isomeric state, isomeric transition, isomerism |
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Any compound exhibiting, or capable of exhibiting, isomerism.
Mosby's Dental Dictionary:
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isomers
(ī'sōmurz)
n.pl
n.pl
1. organic compounds having the same empirical formula—that is, the same number of the same atoms but different structural formulas and therefore different physical and chemical properties. n 2. one of several nuclides having the same number of neutrons and protons but capable of existing, for a measurable time, in different quantum states with different energies and radioactive properties. The isomer of higher energy commonly decays to one with lower energy by a process known as isomeric transition.
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categories related to 'isomer'
- Nuclear and Particle Physics - isomer: any of two or more nuclei with same number of neutrons and protons but different energy states
- Substances, Particles, and Atomic Architecture - isomer: any of two or more compounds with the same molecular formulas but different structures
See crossword solutions for the clue Wikipedia on Answers.com:
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Isomer
This article is about the chemical concept. For "isomerism" of atomic nuclei, see nuclear isomer.
In chemistry, isomers (pronounced /ˈaɪsəməɹz/; from Greek ἰσομερής, isomerès; isos = "equal", méros = "part") are compounds with the same molecular formula but different structural formulas.[1] Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical isomers, etc. (see chart below). There are two main forms of isomerism: structural isomerism and stereoisomerism (spatial isomerism).
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Structural isomers
Main article: Structural isomer
In structural isomers, sometimes referred to as constitutional isomers, the atoms and functional groups are joined together in different ways. Structural isomers have different IUPAC names and may or may not belong to the same functional group.[2] This group includes chain isomerism whereby hydrocarbon chains have variable amounts of branching; position isomerism which deals with the position of a functional group on a chain; and functional group isomerism in which one functional group is split up into different ones.
For example, two position isomers would be 2-fluoropropane and 1-fluoropropane, illustrated on the right.
In skeletal isomers the main carbon chain is different between the two isomers. This type of isomerism is most identifiable in secondary and tertiary alcohol isomers.
Tautomers are structural isomers of the same chemical substance that spontaneously interconvert with each other, even when pure. They have different chemical properties, and consequently, distinct reactions characteristic to each form are observed. If the interconversion reaction is fast enough, tautomers cannot be isolated from each other. An example is when they differ by the position of a proton, such as in keto/enol tautomerism, where the proton is alternately on the carbon or oxygen.
Stereoisomers
Main article: Stereoisomerism
In stereoisomers the bond structure is the same, but the geometrical positioning of atoms and functional groups in space differs. This class includes enantiomers where different isomers are non-superimposable mirror-images of each other, and diastereomers when they are not.
Diastereomerism is again subdivided into "cis-trans isomers", which have restricted rotation within the molecule (typically isomers containing a double bond) and "conformational isomers" (conformers), which can rotate about one or more single bonds within the molecule.
An obsolete term for "cis-trans isomerism" is "geometric isomers". [3]
For compounds with more than two substituents E-Z notation is used instead of cis and trans. If possible, E and Z (written in italic type) is also preferred in compounds with two substituents. [4]
In octahedral coordination compounds, facial-meridional isomerism occurs. The isomers can be fac- (with facial ligands) or mer- (with meridional ligands).
Note that although conformers can be referred to as stereoisomers, they are not stable isomers, since bonds in conformers can easily rotate thus converting one conformer to another which can be either diastereomeric or enantiomeric to the original one.
While structural isomers typically have different chemical properties, stereoisomers behave identically in most chemical reactions, except in their reaction with other stereoisomers. Enzymes however can distinguish between different enantiomers of a compound, and organisms often prefer one isomer over the other. Some stereoisomers also differ in the way they rotate polarized light.
Isomerisation
Isomerisation is the process by which one molecule is transformed into another molecule which has exactly the same atoms, but the atoms are rearranged[5]. In some molecules and under some conditions, isomerisation 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 isomerisation occurs intramolecularly it is considered a rearrangement reaction.
An example of an organometallic isomerisation is the production of decaphenylferrocene, [(η5-C5Ph5)2Fe] from its linkage isomer.[6][7]
Instances of Isomerization
- Isomerizations in hydrocarbon cracking. This is usually employed in organic chemistry, where fuels, such as pentane, a straight-chain isomer, are heated in the presence of a platinum catalyst. The resulting mixture of straight- and branched-chain isomers then have to be separated. An industrial process is also the isomerisation of n-butane into isobutane.
- Trans-cis isomerism. In certain compounds an interconversion of cis and trans isomers can be observed, for instance, with maleic acid and with azobenzene often by photoisomerization. Another example is the photochemical conversion of the trans isomer to the cis isomer of resveratrol [8]:
- Aldose-ketose isomerism in biochemistry.
- Isomerisations between conformational isomers. These take place without an actual rearrangement for instance inconversion of two cyclohexane conformations
- Fluxional molecules display rapid interconversion of isomers e.g. Bullvalene.
- valence isomerisation: the isomerisation of molecules which involve structural changes resulting only from a relocation of single and double bonds. If a dynamic equilibrium is established between the two isomers it is also referred to as valence tautomerism [9]
The energy difference between two isomers is called isomerisation energy. Isomerisations with low energy difference both experimental and computational (in parentheses) are endothermic trans-cis isomerisation of 2-butene with 2.6 (1.2) kcal/mol, cracking of isopentane to n-pentane with 3.6 (4.0) kcal/mol or conversion of trans-2-butene to 1-butene with 2.6 (2.4) kcal/mol.[10]
Examples
Propanol
A simple example of isomerism is given by propanol: it has the formula C3H8O (or C3H7OH) and occurs as two isomers: propan-1-ol (n-propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II)
Note that the position of the oxygen atom differs between the two: it is attached to an end carbon in the first isomer, and to the center carbon in the second.
There is, however, another isomer of C3H8O which has significantly different properties: methoxyethane (methyl-ethyl-ether; III). Unlike the isomers of propanol, methoxyethane has an oxygen connected to two carbons rather than to one carbon and one hydrogen. This makes it an ether, not an alcohol, as it lacks a hydroxyl group, and has chemical properties more similar to other ethers than to either of the above alcohol isomers.
Examples of isomers having different medical properties can be easily found. For example, in the placement of methyl groups. In substituted xanthines, 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 bronchodilation 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 amphetamine. It is used as an appetite reducing medication and has mild or no stimulant properties. However, a different atomic arrangement gives dextromethamphetamine which is a stronger stimulant than amphetamine.
Allene and propyne are examples of isomers containing different bond types. Allene contains two double bonds, whereas propyne contains one triple bond.
Synthesis of fumaric acid
Industrial synthesis of fumaric acid proceeds via the cis-trans isomerization of maleic acid:
In medicinal chemistry and biochemistry, enantiomers are a special concern because they may possess quite different biological activity. The infamous case of thalidomide arose from the effects of the unwanted enantiomer. Many preparative procedure afford a mixture of equal amounts of both enantiomeric forms. In some cases, the enantiomers are separated by chromatography using chiral stationary phases. In other cases, enantioselective syntheses have been developed.
History
Isomerism was first noticed in 1827, when Friedrich Woehler prepared cyanic acid and noted that although its elemental composition was identical to fulminic acid (prepared by Justus von Liebig the previous year), its properties were quite different. This finding challenged the prevailing chemical understanding of the time, which held that chemical compounds could be different only when they had different elemental compositions. After additional discoveries of the same sort were made, such as Woehler's 1828 discovery that urea had the same atomic composition as the chemically distinct ammonium cyanate, Jöns Jakob Berzelius introduced the term isomerism in 1830 to describe the phenomenon.[11]
In 1848, Louis Pasteur separated tiny crystals of tartaric acid into their two mirror-image forms.[12][13] The individual molecules of each were the left and right optical stereoisomers, solutions of which rotate the plane of polarized light to the same degree but in opposite directions.
Other types of isomerism
Other types of isomerism exist outside this scope. Topological isomers called topoisomers are generally large molecules that wind about and form different shaped knots or loops. Molecules with topoisomers include catenanes and DNA. Topoisomerase enzymes can knot DNA and thus change its topology. There are also isotopomers or isotopic isomers that have the same numbers of each type of isotopic substitution but in chemically different positions. In nuclear physics, nuclear isomers are excited states of atomic nuclei. Spin isomers have differing distributions of spin among their constituent atoms.
See also
- Nuclear isomer
- Chirality (chemistry)
- Structural isomerism
- Cis-trans isomerism
- Cyclohexane conformation
- Electromerism
- Ligand isomerism
References
- ^ The word “isomer” was coined by Swedish chemist Jöns Jacob Berzelius (1779-1848) in 1830. See: 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 different properties) Kongliga Svenska Vetenskaps Academiens Handling (Transactions of the Royal Swedish Science Academy), vol. 49, pages 49–80; see especially page 70. Reprinted in German in: 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, vol. 19, pages 305–335; see especially page 326. Reprinted in French in: 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, vol. 46, pages 113–147; see especially page 136.
- ^ Smith, Janice Gorzynski. General, Organic and Biological Chemistry. The McGraw-Hill Companies. 1st ed 2010. pg. 450
- ^ IUPAC definition of geometric isomerism
- ^ IUPAC definition of cis and trans
- ^ Nic, M.; Jirat, J.; Kosata, B., eds. (2006–). "isomerization". IUPAC Compendium of Chemical Terminology (Online ed.). doi:10.1351/goldbook.I03295. ISBN 0-9678550-9-8. http://goldbook.iupac.org/I03295.html.
- ^ Brown, K. N.; Field, L. D.; Lay, P. A.; Lindall, C. M.; Masters, A. F. (1990). "(η5-Pentaphenylcyclopentadienyl){1-(η6-phenyl)-2,3,4,5-tetraphenylcyclopentadienyl}iron(II), [Fe(η5-C5Ph5){(η6-C6H5)C5Ph4}], a linkage isomer of decaphenylferrocene". J. Chem. Soc., Chem. Commun. (5): 408–410. doi:10.1039/C39900000408.
- ^ Field, L. D.; Hambley, T. W.; Humphrey, P. A.; Lindall, C. M.; Gainsford, G. J.; Masters, A. F.; Stpierre, T. G.; Webb, J. (1995). "Decaphenylferrocene". Aust. J. Chem. 48 (4): 851–860. doi:10.1071/CH9950851.
- ^ Resveratrol Photoisomerization: An Integrative Guided-Inquiry Experiment Elyse Bernard, Philip Britz-McKibbin, Nicholas Gernigon Vol. 84 No. 7 July 2007 Journal of Chemical Education 1159.
- ^ cartage.org.lb
- ^ How to Compute Isomerization Energies of Organic Molecules with Quantum Chemical Methods Stefan Grimme, Marc Steinmetz, and Martin Korth J. Org. Chem.; 2007; 72(6) pp 2118 - 2126; (Article) doi:10.1021/jo062446p
- ^ Esteban, Soledad. (2008). "Liebig–Wöhler Controversy and the Concept of Isomerism". J. Chem. Educ. 85 (9): 1201. doi:10.1021/ed085p1201. http://jchemed.chem.wisc.edu/Journal/Issues/2008/Sep/abs1201.html.
- ^ 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, and chemical composition, and the sense of rotary polarization), Annales de Chimie et de Physique, 3rd series, vol. 24, no. 6, pages 442–459.
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Translations:
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Isomer
Nederlands (Dutch)
isomeer (scheikunde)
Français (French)
n. - isomère
Deutsch (German)
n. - (Chem.) Isomer
Ελληνική (Greek)
n. - ισομερές
Português (Portuguese)
n. - isômero (m) (Fís.) (Quím.)
Español (Spanish)
n. - isómero
Svenska (Swedish)
n. - isomer (kem. el. fys.)
中文（简体）(Chinese (Simplified))
异构体
中文（繁體）(Chinese (Traditional))
n. - 異構體
العربيه (Arabic)
(الاسم) الأيسومر : مركب كيميائي
עברית (Hebrew)
n. - כל אחת מהתרכובות בעלות אותו הרכב מולקולרי אולם סידור שונה של אטומים ותכונות שונות, כל אחד מגרעיני האטומים עם אותו משקל ומספר אטומיים אולם מצבי אנרגיה שונים
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