Difference between revisions of "Isomer" - New World Encyclopedia

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In [[chemistry]], '''isomers''' are [[molecule]]s with the same [[chemical formula]] and often with the same kinds of [[chemical bond|bonds]] between atoms, but in which the atoms are arranged differently (analogous to a chemical [[anagram]]). That is to say, they have different [[structural formula|structural formulae]]. Many isomers share similar if not identical properties in most chemical contexts.  This should not be confused with a [[nuclear isomer]], which involves a nucleus at different states of excitement.   
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[[Image:Isomerism.png|thumb|right|350px|Different classes of isomers and their organization.]]
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In [[chemistry]], '''isomers''' are [[molecule]]s with the same [[chemical formula]] and often with the same kinds of [[chemical bond|bonds]] between atoms, but in which the atoms are arranged differently (analogous to a chemical [[anagram]]). That is to say, they have different [[structural formula|structural formulae]]. Many isomers share similar if not identical properties in most chemical contexts.  This should not be confused with a [[nuclear isomer]], which involves a nucleus at different states of excitement.
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== History ==
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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 compound]]s could be different only when they had different elemental compositionsAfter 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'' to describe the phenomenon.
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In 1849, [[Louis Pasteur]] separated tiny crystals of [[tartaric acid]] into their two [[mirror image|mirror-image]] forms. The individual molecules of each were the left and right optical [[stereoisomer]]s, solutions of which rotate the plane of [[polarization|polarized]] [[light]] in opposite directions.
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== Example ==
  
 
A simple example of '''isomerism''' is given by [[propanol]]: it has the formula [[Carbon|C]]<sub>3</sub>[[Hydrogen|H]]<sub>8</sub>[[Oxygen|O]] (or [[Carbon|C]]<sub>3</sub>[[Hydrogen|H]]<sub>7</sub>[[Hydroxyl|OH]]) and two isomers Propan-1-ol (n-propyl alcohol; '''I''') and Propan-2-ol (isopropyl alcohol; '''II''')
 
A simple example of '''isomerism''' is given by [[propanol]]: it has the formula [[Carbon|C]]<sub>3</sub>[[Hydrogen|H]]<sub>8</sub>[[Oxygen|O]] (or [[Carbon|C]]<sub>3</sub>[[Hydrogen|H]]<sub>7</sub>[[Hydroxyl|OH]]) and two isomers Propan-1-ol (n-propyl alcohol; '''I''') and Propan-2-ol (isopropyl alcohol; '''II''')
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== Classification ==
 
== Classification ==
  
[[Image:Isomerism.png|thumb|right|350px|The different types of isomers]]
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There are two main types of isomerism: [[structural isomerism]] and [[stereoisomerism]].  
 
 
There are two main forms of isomerism: [[structural isomerism]] and [[stereoisomerism]].  
 
  
 
In '''structural isomers''', the atoms and [[functional group]]s are joined together in different ways, as in the example of propyl alcohol above. 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.
 
In '''structural isomers''', the atoms and [[functional group]]s are joined together in different ways, as in the example of propyl alcohol above. 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.
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Other types of isomerism exist outside this scope. [[Topological]] isomers called [[topoisomer]]s are generally large molecules that wind about and form different shaped knots or loops. Molecules with topoisomers include [[catenane]]s 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 isomer]]s are excited states of atomic nuclei.
 
Other types of isomerism exist outside this scope. [[Topological]] isomers called [[topoisomer]]s are generally large molecules that wind about and form different shaped knots or loops. Molecules with topoisomers include [[catenane]]s 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 isomer]]s are excited states of atomic nuclei.
  
== History ==
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== Structural isomerism ==
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 compound]]s 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'' to describe the phenomenon.
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'''Structural isomerism''' (or '''constitutional isomerism''') is a form of [[isomer|isomerism]] in which [[molecule]]s with the same [[molecular formula]] have [[atom]]s bonded together in different orders, as opposed to [[stereoisomerism]].
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Three categories of constitutional isomers are skeletal, positional, and functional isomers.
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In '''skeletal isomerism''' (or '''chain isomerism''') components of the (usually carbon) skeleton are distinctly re-ordered to create different structures. For example [[3-methylpentane]] is a chain isomer of [[2-methylpentane]]. [[Pentane]] exists as three isomers: ''n-pentane'', ''[[isopentane]]'' and ''[[neopentane]]''.
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In '''position isomerism''' a [[functional group]] changes position on the chain. In the diagram, [[pentan-2-ol]] has become [[pentan-3-ol]]. Many [[aromatic]] isomers exist because substituents can be positioned on different parts of the benzene ring. Only one isomer of [[phenol]] or hydroxybenzene exists but [[cresol]] or methylphenol has three isomers where the additional methyl group can be placed on three different positions on the ring. [[Xylenol]] has one hydroxyl group and two methyl groups and a total of 6 isomers exist.
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[[image:position_isomer.png|right|300px|example of positional isomerism]]
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In '''functional group isomerism''' a functional group splits up and becomes a different group.  
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Here is an example of functional group isomerism: take [[cyclohexane]], C<sub>6</sub>H<sub>12</sub> and [[1-Hexene]], also C<sub>6</sub>H<sub>12</sub>. These two are considered functional group isomers because cyclohexane is an [[alkane]] and 1-Hexene is an [[alkene]]. Both must have the same [[molecular formula]].
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[[image:C6H12_isomers.JPG|example of functional group isomerism]]
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== See also ==
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* [[Alkane]]
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== References ==
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* Clark, Jim. "Structural isomerism" in ''Chemguide'', n.l., '''2000''', December 7.[http://www.chemguide.co.uk/basicorg/isomerism/structural.html Web article]
  
In 1849, [[Louis Pasteur]] separated tiny crystals of [[tartaric acid]] into their two [[mirror image|mirror-image]] forms. The individual molecules of each were the left and right optical [[stereoisomer]]s, solutions of which rotate the plane of [[polarization|polarized]] [[light]] in opposite directions.
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=== External links ===
  
[[Category:organic chemistry]]
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[[Category:Physical sciences]]
[[Category:stereochemistry]]
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[[Category:Chemistry]]
  
[[ar:متزامر]]
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[[zh:同分异构体]]
 

Revision as of 20:52, 12 April 2007

Different classes of isomers and their organization.

In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently (analogous to a chemical anagram). That is to say, they have different structural formulae. Many isomers share similar if not identical properties in most chemical contexts. This should not be confused with a nuclear isomer, which involves a nucleus at different states of excitement.

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 to describe the phenomenon.

In 1849, Louis Pasteur separated tiny crystals of tartaric acid into their two mirror-image forms. The individual molecules of each were the left and right optical stereoisomers, solutions of which rotate the plane of polarized light in opposite directions.

Example

A simple example of isomerism is given by propanol: it has the formula C3H8O (or C3H7OH) and two isomers Propan-1-ol (n-propyl alcohol; I) and Propan-2-ol (isopropyl alcohol; II)

isomers of propanol

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. The number of possible isomers increases rapidly as the number of atoms increases; for example the next largest alcohol, named butanol (C4H10O), has four different isomers.

In the example above it should also be noted that in both isomers all the bonds are single bonds; there is no type of bond that appears in one isomer and not in the other. Also the number of bonds is the same. From the structures of the two molecules it could be deduced that their chemical stabilities are liable to be identical or nearly so.

There is, however, another isomer of C3H8O which has significantly different properties: methyl ethyl ether (III). Notice that unlike the top two examples, the oxygen is connected to two carbons rather than to one carbon and one hydrogen. As it lacks a hydroxyl group, the above molecule is no longer considered an alcohol but is classified as an ether, and has chemical properties more similar to other ethers than to either of the above alcohol isomers.

Another example of isomers having very different properties can be found in certain xanthines. Theobromine is found in chocolate, but if one of the two methyl groups is moved to a different position on the two-ring core, the isomer is theophylline, used as a bronchodilator.

Allene and propyne are examples of isomers containing different bond types. Allene contains two double bonds, while propyne contains one triple bond.

Classification

There are two main types of isomerism: structural isomerism and stereoisomerism.

In structural isomers, the atoms and functional groups are joined together in different ways, as in the example of propyl alcohol above. 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.

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 mirror-images of each other, and diastereomers when they are not. Diastereomerism is again subdivided into conformational isomerism (conformers) when isomers can interconvert by chemical bond rotations and cis-trans isomerism when this is not possible. Note that although conformers can be referred to as having a diastereomeric relationship, the isomers over all are not diastereomers, since bonds in conformers can be rotated to make them mirror images.

In food chemistry, medicinal chemistry and biochemistry, cis-trans isomerism is always considered. In medicinal chemistry and biochemistry, enantiomers are of particular interest since most changes in these types of isomers are now known to be meaningful in living organisms. Pharmaceutical and university-level researchers have found chromatographical methods to reliably separate these from each other. On an industrial scale, however, these methods are rather costly and are mostly used to filter out potentially harmful isomers.

While structural isomers typically have different chemical properties, stereoisomers behave identically in most chemical reactions. Enzymes however can distinguish between different stereoisomers of a compound, and organisms often prefer one stereoisomer over the other. Some stereoisomers also differ in the way they rotate polarized light.

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.

Structural isomerism

Structural isomerism (or constitutional isomerism) is a form of isomerism in which molecules with the same molecular formula have atoms bonded together in different orders, as opposed to stereoisomerism.

Three categories of constitutional isomers are skeletal, positional, and functional isomers.

In skeletal isomerism (or chain isomerism) components of the (usually carbon) skeleton are distinctly re-ordered to create different structures. For example 3-methylpentane is a chain isomer of 2-methylpentane. Pentane exists as three isomers: n-pentane, isopentane and neopentane.

In position isomerism a functional group changes position on the chain. In the diagram, pentan-2-ol has become pentan-3-ol. Many aromatic isomers exist because substituents can be positioned on different parts of the benzene ring. Only one isomer of phenol or hydroxybenzene exists but cresol or methylphenol has three isomers where the additional methyl group can be placed on three different positions on the ring. Xylenol has one hydroxyl group and two methyl groups and a total of 6 isomers exist.

example of positional isomerism

In functional group isomerism a functional group splits up and becomes a different group. Here is an example of functional group isomerism: take cyclohexane, C6H12 and 1-Hexene, also C6H12. These two are considered functional group isomers because cyclohexane is an alkane and 1-Hexene is an alkene. Both must have the same molecular formula.

example of functional group isomerism

See also

References
ISBN links support NWE through referral fees

  • Clark, Jim. "Structural isomerism" in Chemguide, n.l., 2000, December 7.Web article

External links

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