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isomer

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).

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[symbol]CHCH2CH3, and 2-butene, CH3CH[symbol]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[symbol]CHCH3: 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 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: 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.

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: 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.

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.

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isomers

isomers 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.

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isomer

i·so·mer / ˈīsəmər/ • n. 1. Chem. each of two or more compounds with the same formula but a different arrangement of atoms in the molecule and different properties. 2. Physics each of two or more atomic nuclei that have the same atomic number and the same mass number but different energy states. DERIVATIVES: i·so·mer·ic / ˌīsəˈmerik/ adj. i·som·er·ism / īˈsäməˌrizəm/ n. i·som·er·ize / īˈsäməˌrīz/ v. ORIGIN: mid 19th cent.: from Greek isomerēs ‘sharing equally,’ from isos ‘equal’ + meros ‘a share.’

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isomers

isomers Chemical compounds that have the same molecular formulae but different molecular structures or different arrangements of atoms in space. Structural isomers have different molecular structures, i.e. they may be different types of compound or they may simply differ in the position of the functional group in the molecule. Structural isomers generally have different physical and chemical properties. Stereoisomers have the same formula and functional groups, but differ in the arrangement of groups in space. Optical isomers are examples of stereoisomers (see optical activity).

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isomers

isomers Chemical compounds having the same molecular formula but different properties due to the different arrangement of atoms within the molecules. Structural isomers have atoms connected in different ways. Geometric isomers, also called cis-trans isomers, differ in their symmetry about a double bond. Optical isomers are mirror images of each other.

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isomer

isomer Either or any of two or more compounds that have the same molecular composition but different molecular structure. Isomers differ from each other in their physical and chemical properties.

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isomer

isomer Either or any of two or more compounds that have the same molecular composition but different molecular structure. Isomers differ from each other in their physical and chemical properties.

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isomer

isomer Either or any of 2 or more compounds that have the same molecular composition but different molecular structure. Isomers differ from each other in their physical and chemical properties.

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isomer

isomerdormer, former, korma, Norma, performer, pro-forma, stormer, transformer, trauma, warmer •sixth-former • barnstormer •aroma, carcinoma, chroma, coma, comber, diploma, glaucoma, Homer, lymphoma, melanoma, misnomer, Oklahoma, Omagh, roamer, Roma, romer, sarcoma, soma •beachcomber •bloomer, boomer, consumer, Duma, humour (US humor), Nkrumah, perfumer, puma, roomer, rumour (US rumor), satsuma, stumer, Sumer, tumour (US tumor) •zeugma • fulmar •bummer, comer, drummer, hummer, midsummer, mummer, plumber, rummer, strummer, summa, summer •latecomer • newcomer • agama •welcomer •astronomer, monomer •ashrama • isomer • gossamer •customer •affirmer, Burma, derma, Irma, murmur, squirmer, terra firma, wormer

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Isomer

Isomer

Isomer is the term used to describe two or more chemical compounds which can be represented by the same chemical formula. There are two main types of isomers. Structural isomers differ from one another by the attachment of atoms on the molecule. Stereoisomers differ from on another by the location of the atoms in space.

In the early 1800s two chemists, Friedreich Wohler (18001882) and Justus Liebig (18031873), realized that two chemical compounds might have the same elemental composition yet differ in the order in which the atoms were linked together. Therefore, it is possible that a given chemical formula may describe more than one compound. For example, propyl alcohol and iso-propyl alcohol both are represented by the same formula ([CH3]2CHOH), but they are different compounds with different properties depending on whether the alcohol group (also known as the hydroxyl group) is located on a terminal (end) carbon atom or on the middle carbon atom. This form of isomerism is known as positional, or structural, isomerism. Positional isomerism occurs because the various sites where groups are attached are not equivalent. This principle is demonstrated by the molecule known as benzene, which consists six carbon atoms arranged in a ring. These carbon atoms provide benzene with six different positions where other chemical groups can be substituted for hydrogen atoms. A substituted benzene ring such as toluene can accept another substituent on any of the other five carbon atoms; but because two pairs are equivalent, there are only three possible isomers (ortho, meta, and para).

Another type of structural isomerism is chain isomerism. This type occurs among chemical compounds known as alkanes, which consist of chains of carbon atoms. These carbon atom chains can be configured as either a straight or branched chain with exactly the same overall chemical formula. These different structural configurations are isomers of each other. Although the properties of isomers of a given formula are similar, the compounds are nonetheless distinct. Similarly, the location of the double bond in alkenes and the triple bond in alkynes determines another form of positional isomerism.

Isomers may also be stereoisomers which differ from one another by the spacial position of their atoms. There are two subcategories of stereoisomers; geometric isomers and optical isomers. Both occur in molecules in which the atoms are attached in the same order but have different spatial relationships. For example, if there is a double bond present between two carbon atoms to which are linked hydroxyl atoms, the hydroxyl

KEY TERMS

Geometric isomers Stereoisomers whose mirror images are superimposable.

Optical isomers Stereoisomers whose mirror images are nonsuperimposable.

Stereochemistry The study of stereoisomers.

Stereoisomers Isomers which differ from on another by the location of the atoms in space.

Structural isomers Isomers which differ from one another by the attachment of atoms on the molecule.

groups can be attached to the same side of this double bond or they may be oriented on opposite sides. If the groups are aligned on the same side of the double bond the compound is said to be a cis isomer (from the Latin on this side, if they are on opposite sides it is trans (from Latin across). In certain compounds, the atoms are free to rotate about this double bond, which gives rise to multiple isomeric configurations. If the mirror images of these different configurations are superimposable, the isomers are said to be geometric. If the mirror images are nonsuperimposable they are said to be optical. Optical isomers are distinguishable by the way they interact with a beam of polarized light. Such isomers are the subject of the branch of chemistry known as stereochemistry.

Stereochemistry is the study of the spatial arrangement of atoms in molecules and the effect of their orientation on the physical and chemical properties of those compounds. The study of isomers provides information that is useful in improving the efficiency of the reactions or in the search for new types of reactions or chemical species. Through evaluation of isomeric compounds, chemists gain useful information about chemical reactions and learn how certain bonds are broken and formed or what kinds of intermediates are involved.

Perry Romanowski

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Isomer

Isomer

Isomer is the term used to describe two or more chemical compounds which can be represented by the same chemical formula . There are two main types of isomers: structural isomers which differ from one another by the attachment of atoms on the molecule ; and stereoisomers which differ from on another by the location of the atoms in space .

Chemical compounds can be represented by a formula which qualitatively and quantitatively describes its component elements. For example, the formula for water is H2O, which indicates that the compound contains two hydrogen atoms attached to one oxygen atom. In the early 1800s two chemists, Friedreich Wohler and Justus Liebig, realized that two chemical compounds might have the same elemental composition yet differ in the order in which the atoms were linked together. Therefore, it is possible that a given chemical formula may describe more than one compound. For example, propyl alcohol and isopropyl alcohol both are represented by the same formula ([CH3]2CHOH), but they are different compounds with different properties depending on whether the alcohol group (also known as the hydroxyl group) is located on a terminal (end) carbon atom or on the middle carbon atom. This form of isomerism is known as positional, or structural, isomerism. Positional isomerism occurs because the various sites where groups are attached are not equivalent. This principle is demonstrated by the molecule known as benzene which consists six carbon atoms arranged in a ring. These carbon atoms provide benzene with six different positions where other chemical groups can be substituted for hydrogen atoms. A substituted benzene ring, such as toluene, can accept another substituent on any of the other five carbon atoms; but because two pairs are equivalent, there are only three possible isomers. These are designated as ortho, meta, and para.

Chain isomerism, another type of structural isomerism, occurs among chemical compounds known as alkanes, which consist of chains of carbon atoms. These carbon atom chains can be configured as either a straight or branched chain with exactly the same overall chemical formula. These different structural configurations are isomers of each other. Although the properties of isomers of a given formula are similar, the compounds are nonetheless distinct. Similarly, the location of the double bond in alkenes and the triple bond in alkynes determines another form of positional isomerism.

Isomers may also be stereoisomers which differ from one another by spacial position of their atoms. There are two subcategories of stereoisomers, geometric isomers and optical isomers. Both geometric and optical isomers occur in molecules in which the atoms are attached in the same order but have different spatial relationships. For example, picture a chain of carbon atoms which has two hydroxyl groups attached to the first two positions. Furthermore, assume there is a double bond present between the 1 and 2 carbons. The hydroxyl groups can be attached to the same side of this double bond, (e.g., both on the "top" or both on the "bottom"). Or they may be oriented on opposite sides such that one resides on top and the other on the bottom. If the groups are aligned on the same side of the double bond the compound is said to be a cis isomer (from the Latin "on this side"); if they are on opposite sides it is trans (from Latin "across.") In certain compounds, the atoms are free to rotate about this double bond, which gives rise to multiple isomeric configurations. If the mirror images of these different configurations are superimposable, the isomers are said to be geometric. If the mirror images are nonsuperimposable they are said to be optical. Optical isomers are distinguishable by the way they interact with a beam of polarized light . Such isomers are the subject of the branch of chemistry known as stereochemistry .

Stereochemistry is the study of the spatial arrangement of atoms in molecules and the effect of their orientation on the physical and chemical properties of those compounds. However, the three dimensional nature of this spacial orientation was not really understood until 1871 when two independent chemists, Hendricus van't Hoff and Joseph Achille le Bel, proposed their theories of stereoisomerism, or how isomers are structured.

The study of isomers provides information which is useful in improving the efficiency of the reactions or in the search for new types of reactions or chemical species . Through evaluation of isomeric compounds, chemists gain useful information about chemical reactions and learn how certain bonds are broken and formed or what kinds of intermediates are involved.

See also Formula, chemical; Formula, structural.

Perry Romanowski

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Geometric isomers

—Stereoisomers whose mirror images are superimposable.

Optical isomers

—Stereoisomers whose mirror images are nonsuperimposable.

Stereochemistry

—The study of stereoisomers.

Stereoisomers

—Isomers which differ from on another by the location of the atoms in space.

Structural isomers

—Isomers which differ from one another by the attachment of atoms on the molecule.

Cite this article
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"Isomer." The Gale Encyclopedia of Science. . Encyclopedia.com. 11 Dec. 2018 <https://www.encyclopedia.com>.

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