Editing Structural isomer

{{Short description|Chemical compounds with the same atoms but arranged and connected differently}}
{{Textbook|date=August 2020}}

In [[chemistry]], a '''structural isomer''' (or '''constitutional isomer'''  in the [[IUPAC]] nomenclature<ref name="GoldBook1"/>) of a [[chemical compound|compound]] is a compound that contains the same number and type of atoms, but with a different connectivity (i.e. arrangement of [[chemical bond|bonds]]) between them.<ref name=bett2009/><ref name=mumb2018/> The term '''metamer''' was formerly used for the same concept.<ref name=bynum2014/>

For example, [[butanol]] {{chem2|H3C\s(CH2)3\sOH}}, [[methyl propyl ether]] {{chem2|H3C\s(CH2)2\sO\sCH3}}, and [[diethyl ether]] {{chem2|(H3CCH2\s)2O}} have the same [[molecular formula]] {{chem2|C4H10O}} but are three distinct structural isomers.

The concept applies also to polyatomic ions with the same total charge.  A classical example is the [[cyanate]] ion {{chem2|O\dC\dN-}} and the [[fulminate]] ion {{chem2|C-\tN+\sO-}}. It is also extended to ionic compounds, so that (for example) [[ammonium cyanate]]  {{chem2|[NH4]+[O\dC\dN]-}} and [[urea]] {{chem2|(H2N\s)2C\dO}} are considered structural isomers,<ref name=bynum2014>William F. Bynum, E. Janet Browne, Roy Porter (2014): [https://archive.org/details/dictionaryofhist0000unse_u7i5/page/218/mode/1up?q=metamerism ''Dictionary of the History of Science'']. page 218. {{isbn|9781400853410}}</ref> and so are [[methylammonium formate]] {{chem2|[H3C\sNH3]+[HCO2]-}} and [[ammonium acetate]] {{chem2|[NH4]+[H3C\sCO2]-}}.

Structural isomerism is the most radical type of [[isomer]]ism.  It is opposed to [[stereoisomer]]ism, in which the atoms and bonding scheme are the same, but only the relative spatial arrangement of the atoms is different.<ref name=clark2000/><ref name=poppe2016/> Examples of the latter are the [[enantiomer]]s, whose molecules are mirror images of each other, and the ''cis'' and ''trans'' versions of [[2-butene]].

Among the structural isomers, one can distinguish several classes including '''skeletal isomers''', '''positional  isomers''' (or '''regioisomers'''), '''functional isomers''', '''[[tautomer]]s''',{{r|hibb1987}} and '''structural isotopomers'''.{{r|GoldBook2}}

==Skeletal isomerism==
A '''skeletal isomer''' of a compound is a structural isomer that differs from it in the atoms and bonds that are considered to comprise the "skeleton" of the molecule.  For [[organic compound]]s, such as [[alkanes]], that usually means the carbon atoms and the bonds between them.

For example, there are three skeletal isomers of [[pentane]]: ''n''-pentane (often called simply "pentane"), [[isopentane]] (2-methylbutane) and [[neopentane]] (dimethylpropane).<ref name=slan1986/>

{| align="center" class="wikitable skin-invert-image"
|-
|+Skeletal isomers of pentane
|-
|[[File:Pentane-2D-Skeletal.svg|125px]]
|[[File:Isopentane-2D-skeletal.svg|125px]]
|[[File:Neopentane-2D-skeletal.png|100px]]
|-
| ''n''-[[Pentane]]
| [[Isopentane]]
| [[Neopentane]]
|}

If the skeleton is [[cyclic compound|acyclic]], as in the above example, one may use the term  '''chain isomerism'''.

==Position isomerism (regioisomerism)<span class="anchor" id="Regioisomer"></span>==
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{{see also|Arene substitution pattern#Ortho, meta, and para substitution}}
'''Position isomers''' (also '''positional isomers''' or '''regioisomers''') are structural isomers that can be viewed as differing only on the position of a [[functional group]], [[substituent]], or some other feature on the same "parent" structure.<ref name=stok2015/>

For example, replacing one of the 12 hydrogen atoms –H by a [[Alcohol (chemistry)|hydroxyl]] group –OH on the [[pentane|''n''-pentane]] parent molecule can give any of three different position isomers:

{| align="center" class="skin-invert-image"
|-
| [[File:Pentan-1-ol-pos.png|180px]]
| [[File:Pentan-2-ol-pos.png|180px]]
| [[File:Pentan-3-ol-pos.png|180px]]
|-
| style="padding-left:20px;padding-right:20px;text-align:center;" | [[Pentan-1-ol]]
| style="padding-left:20px;padding-right:20px;text-align:center;" | [[Pentan-2-ol]]
| style="padding-left:20px;padding-right:20px;text-align:center;" | [[Pentan-3-ol]]
|}
Another example of regioisomers are [[α-Linolenic acid|α-linolenic]] and [[γ-linolenic acid]]s, both [[octadecatrienoic acid]]s, each of which has three double bonds, but on different positions along the chain.

==Functional isomerism==
'''Functional isomers''' are structural isomers which have different [[functional group]]s, resulting in significantly different chemical and physical properties.<ref name=hinw1997/>

An example is the pair [[propanal]] H<sub>3</sub>C–CH<sub>2</sub>–C(=O)-H and [[acetone]] H<sub>3</sub>C–C(=O)–CH<sub>3</sub>: the first has a –C(=O)H functional group, which makes it an [[aldehyde]], whereas the second has a C–C(=O)–C group, that makes it a [[ketone]].

Another example is the pair [[ethanol]] H<sub>3</sub>C–CH<sub>2</sub>–OH (an [[alcohol (chemistry)|alcohol]]) and [[dimethyl ether]] H<sub>3</sub>C–O–CH<sub>2</sub>H (an [[ether]]). In contrast, [[1-propanol]] and [[2-propanol]] are structural isomers, but  not functional isomers, since they have the same significant functional group (the [[hydroxyl]] –OH) and are both alcohols.

Besides the different chemistry, functional isomers typically have very different [[infrared spectroscopy|infrared spectra]].  The infrared spectrum is largely determined by the vibration modes of the molecule, and functional groups like hydroxyl and esters have very different vibration modes. Thus 1-propanol and 2-propanol have relatively similar infrared spectra because of the hydroxyl group, which are fairly different from that of methyl ethyl ether.{{citation needed|date=August 2020}}

==Structural isotopomers==
{{Main|Isotopomer}}
In chemistry, one usually ignores distinctions between [[isotope]]s of the same element. However, in some situations (for instance in [[Raman spectroscopy|Raman]], [[nuclear magnetic resonance spectroscopy|NMR]], or [[microwave spectroscopy]]) one may treat different isotopes of the same element as different elements. In the second case, two molecules with the same number of atoms of each isotope but distinct bonding schemes are said to be '''structural isotopomers'''.

Thus, for example, [[ethene]] would have no structural isomers under the first interpretation; but replacing two of the hydrogen atoms (<sup>1</sup>H) by [[deuterium]] atoms (<sup>2</sup>H) may yield any of two structural isotopomers (1,1-dideuteroethene and 1,2-dideuteroethene), if both carbon atoms are the same isotope. If, in addition, the two carbons are different isotopes (say, <sup>12</sup>C and <sup>13</sup>C), there would be three distinct structural isotopomers, since 1-<sup>13</sup>C-1,1-dideuteroethene would be different from 1-<sup>13</sup>C-2,2-dideuteroethene. And, in both cases, the 1,2-dideutero structural isotopomer would occur as two stereoisotopomers, ''cis'' and ''trans''.

==Structural equivalence and symmetry==
===Structural equivalence===
Two molecules (including polyatomic ions) A and B '''have the same structure''' if each atom of A can be paired with an atom of B of the same element, in a one-to-one way, so that for every bond in A there is a bond in B, of the same type, between corresponding atoms; and vice versa.<ref name=mumb2018/> This requirement applies also to complex bonds that involve three or more atoms, such as the [[delocalized bond]]ing in the benzene molecule and other aromatic compounds.

Depending on the context, one may require that each atom be paired with an atom of the same isotope, not just of the same element.

Two molecules then can be said to be structural isomers (or, if isotopes matter, structural isotopomers) if they have the same molecular formula but do not have the same structure.

===Structural symmetry and equivalent atoms===
Structural symmetry of a molecule can be defined mathematically as a [[permutation]] of the atoms that exchanges at least two atoms but does not change the molecule's structure. Two atoms then can be said to be structurally [[equivalence relation|equivalent]] if there is a structural symmetry that takes one to the other.<ref name=faul2010/>

Thus, for example, all four hydrogen atoms of [[methane]] are structurally equivalent, because any permutation of them will preserve all the bonds of the molecule.

Likewise, all six hydrogens of [[ethane]] ({{chem|C|2|H|6}}) are structurally equivalent to each other, as are the two carbons; because any hydrogen can be switched with any other, either by a permutation that swaps just those two atoms, or by a permutation that swaps the two carbons and each hydrogen in one methyl group with a different hydrogen on the other methyl. Either operation preserves the structure of the molecule. That is the case also for the hydrogen atoms in [[cyclopentane]], [[allene]], [[2-butyne]], [[hexamethylenetetramine]], [[prismane]], [[cubane]], [[dodecahedrane]], etc.

On the other hand, the hydrogen atoms of [[propane]] are not all structurally equivalent. The six hydrogens attached to the first and third carbons are equivalent, as in ethane, and the two attached to the middle carbon are equivalent to each other; but there is no equivalence between these two [[equivalence class]]es.

===Symmetry and positional isomerism===
Structural equivalences between atoms of a parent molecule reduce the number of positional isomers that can be obtained by replacing those atoms for a different element or group. Thus, for example, the structural equivalence between the six hydrogens of [[ethane]] {{chem|C|2|H|6}} means that there is just one structural isomer of [[ethanol]] {{chem|C|2|H|5|OH}}, not 6. The eight hydrogens of [[propane]] {{chem|C|3|H|8}} are partitioned into two structural equivalence classes (the six on the methyl groups, and the two on the central carbon); therefore there are only two positional isomers of propanol ([[1-propanol]] and [[2-propanol]]). Likewise there are only two positional isomers of [[butanol]], and three of [[pentanol]] or [[hexanol]].

===Symmetry breaking by substitutions===
Once a substitution is made on a parent molecule, its structural symmetry is usually reduced, meaning that atoms that were formerly equivalent may no longer be so. Thus substitution of two or more equivalent atoms by the same element may generate more than one positional isomer.

The classical example is the derivatives of [[benzene]]. Its six hydrogens are all structurally equivalent, and so are the six carbons; because the structure is not changed if the atoms are permuted in ways that correspond to flipping the molecule over or rotating it by multiples of 60 degrees. Therefore, replacing any hydrogen by chlorine yields only one [[chlorobenzene]]. However, with that replacement, the atom permutations that moved that hydrogen are no longer valid. Only one permutation remains, that corresponds to flipping the molecule over while keeping the chlorine fixed. The five remaining hydrogens then fall into three different equivalence classes: the one opposite to the chlorine is a class by itself (called the ''para'' position), the two closest to the chlorine form another class (''ortho''), and the remaining two are the third class (''meta''). Thus a second substitution of hydrogen by chlorine can yield three positional isomers: [[1,2-Dichlorobenzene|1,2- or ''ortho''-]], [[1,3-Dichlorobenzene|1,3- or ''meta''-]], and [[1,4-Dichlorobenzene|1,4- or ''para''-dichlorobenzene]].

{| align="center"
|-
| style="text-align:center;" | [[File:O-Dichlorobenzene-3D-balls.png|100px]]
| style="text-align:center;" | [[File:M-Dichlorobenzene-3D-balls.png|100px]]
| style="text-align:center;" | [[File:P-Dichlorobenzene-3D-balls.png|90px]]
|-
| style="padding-left:20px;padding-right:20px;text-align:center;" | ''ortho''-Dichlorobenzene
| style="padding-left:20px;padding-right:20px;text-align:center;" | ''meta''-Dichlorobenzene
| style="padding-left:20px;padding-right:20px;text-align:center;" | ''para''-Dichlorobenzene
|-
| style="padding-left:20px;padding-right:20px;text-align:center;" | 1,2-Dichlorobenzene
| style="padding-left:20px;padding-right:20px;text-align:center;" | 1,3-Dichlorobenzene
| style="padding-left:20px;padding-right:20px;text-align:center;" | 1,4-Dichlorobenzene
|}

For the same reason, there is only one [[phenol]] (hydroxybenzene), but three [[benzenediol]]s; and one [[toluene]] (methylbenzene), but three [[toluol]]s, and three [[xylene]]s.

On the other hand, the second replacement (by the same substituent) may preserve or even increase the symmetry of the molecule, and thus may preserve or reduce the number of equivalence classes for the next replacement. Thus, the four remaining hydrogens in ''meta''-dichlorobenzene still fall into three classes, while those of ''ortho''- fall into two, and those of ''para''- are all equivalent again. Still, some of these 3 + 2 + 1 = 6 substitutions end up yielding the same structure, so there are only three structurally distinct [[trichlorobenzene]]s: [[1,2,3-Trichlorobenzene|1,2,3-]], [[1,2,4-Trichlorobenzene|1,2,4-]], and [[1,3,5-Trichlorobenzene|1,3,5-]].

{| align="center" class="skin-invert-image"
|-
| style="text-align:center;" | [[File:1,2,3-trichlorobenzene.svg|90px]]
| style="text-align:center;" | [[File:1,2,4-trichlorobenzene.svg|90px]]
| style="text-align:center;" | [[File:1,3,5-Trichlorobenzene.svg|120px]]
|-
| style="padding-left:20px;padding-right:20px;text-align:center;" | 1,2,3-Trichlorobenzene
| style="padding-left:20px;padding-right:20px;text-align:center;" | 1,2,4-Trichlorobenzene
| style="padding-left:20px;padding-right:20px;text-align:center;" | 1,3,5-Trichlorobenzene
|}

If the substituents at each step are different, there will usually be more structural isomers. [[Xylenol]], which is benzene with one hydroxyl substituent and two methyl substituents, has a total of 6 isomers:

{| align="center" class="skin-invert-image"
|-
| style="text-align:center;" | [[File:2,3-Xylenol.svg|100px]]
| style="text-align:center;" | [[File:2,4-Xylenol.svg|100px]]
| style="text-align:center;" | [[File:2,5-dimethylphenol.png|150px]]
|-
| style="padding-left:20px;padding-right:20px;text-align:center;" | 2,3-Xylenol
| style="padding-left:20px;padding-right:20px;text-align:center;" | 2,4-Xylenol
| style="padding-left:20px;padding-right:20px;text-align:center;" | 2,5-Xylenol
|-
| style="text-align:center;" | [[File:2,6-dimethylphenol.png|160px]]
| style="text-align:center;" | [[File:3,4-Xylenol.svg|100px]]
| style="text-align:center;" | [[File:3,5-Xylenol.svg|160px]]
|-
| style="padding-left:20px;padding-right:20px;text-align:center;" | 2,6-Xylenol
| style="padding-left:20px;padding-right:20px;text-align:center;" | 3,4-Xylenol
| style="padding-left:20px;padding-right:20px;text-align:center;" | 3,5-Xylenol
|}

==Isomer enumeration and counting==
Enumerating or counting structural isomers in general is a difficult problem, since one must take into account several bond types (including delocalized ones), cyclic structures, and structures that cannot possibly be realized  due to valence or geometric constraints, and non-separable tautomers.

For example, there are nine structural isomers with molecular formula [[C3H6O|C<sub>3</sub>H<sub>6</sub>O]] having different bond connectivities. Seven of them are air-stable at room temperature, and these are given in the table below.

{| class="wikitable sortable skin-invert-image"
|+Compounds with molecular formula C<sub>3</sub>H<sub>6</sub>O
! Name !! class="unsortable" | [[Molecular structure]] !!  [[Melting point|Melting<br>point]] (°C) !! [[Boiling point|Boiling<br>point]] (°C) !!  class="unsortable" | Comment 
|-
|valign=top| [[Allyl alcohol]]
|valign=top| [[File:Allyl alcohol v2.svg|80px]]
|valign=top| –129
|valign=top| 97
|valign=top| 
|-
|valign=top| [[Cyclopropanol]]
|valign=top| [[File:Cyclopropanol.svg|40px]]
|valign=top| 
|valign=top| 101–102
|valign=top| 
|-
|valign=top| [[Propionaldehyde]]
|valign=top| [[File:Propanal-skeletal.png|60px]]
|valign=top|  –81
|valign=top| 48
|valign=top| Tautomeric with prop-1-en-1-ol, which has both [[Cis–trans isomerism|''cis'' and ''trans'' stereoisomeric forms]]
|-
|valign=top| [[Acetone]]
|valign=top| [[File:Acetone-2D-skeletal.svg|60px]]
|valign=top|  –94.9
|valign=top| 56.53
|valign=top| Tautomeric with propen-2-ol 
|-
|valign=top| [[Oxetane]]
|valign=top| [[File:Oxetane.png|40px]]
|valign=top|  –97
|valign=top| 48
|valign=top| 
|-
|valign=top| [[Propylene oxide]]
|valign=top| [[File:PropyleneOxide.png|60px]]
|valign=top| –112
|valign=top| 34
|valign=top| Has two [[enantiomer|enantiomeric forms]]
|-
|valign=top| [[Methyl vinyl ether]]
|valign=top| [[File:Methyl vinyl ether.svg|80px]]
|valign=top|  –122
|valign=top| 6
|valign=top| 
|}

Two structural isomers are the [[enol]] [[tautomer]]s of the [[carbonyl]] isomers (propionaldehyde and acetone), but these are not stable.<ref>[[CRC Handbook of Chemistry and Physics]] 65th ed.</ref>

==See also==
*[[Coordination isomerism]]
*[[Descriptor (chemistry)]]
*[[Stereoisomer]]
*[[Metamerism (disambiguation)]]

== References ==
<references>

<ref name="GoldBook1">{{cite journal |title=Constitutional isomerism |url=https://goldbook.iupac.org/html/C/C01285.html |website=IUPAC Gold Book |date=2014 |publisher=IUPAC |doi=10.1351/goldbook.C01285 |access-date=19 July 2018|doi-access=free }}</ref>

<ref name= GoldBook2 >{{cite book |chapter=Isotopomer |title=IUPAC Compendium of Chemical Terminology |date=2006 |edition=3rd |doi=10.1351/goldbook.I03352 |doi-access=free}} online ver. 3.0.1 2019.</ref>

<ref name=bett2009>Frederick A. Bettelheim, William H. Brown, Mary K. Campbell, Shawn O. Farrell (2009): ''Introduction to Organic and Biochemistry''. 752 pages. {{isbn|9780495391166}}</ref>

<ref name=mumb2018>Peter P. Mumba (2018): ''Useful Principles in Chemistry for Agriculture and Nursing Students'', 2nd Edition. 281 pages. {{isbn|9781618965288}}</ref>

<ref name=clark2000>Jim Clark (2000). [http://www.chemguide.co.uk/basicorg/isomerism/structural.html "Structural isomerism"] in ''Chemguide'', n.l.</ref>

<ref name=poppe2016>{{cite book| last1=Poppe| first1=Laszlo| last2=Nagy|first2=Jozsef |last3=Hornyanszky|first3=Gabor|last4=Boros|first4=Zoltan|last5=Mihaly|first5=Nogradi|title=Stereochemistry and Stereoselective Synthesis: An Introduction|date=2016|publisher=Wiley-VCH|location=Weinheim, Germany|isbn=978-3-527-33901-3|pages=26–27}}</ref>

<ref name=hibb1987>D. Brynn Hibbert, A.M. James (1987): ''Macmillan Dictionary of Chemistry''. page 263. {{isbn|9781349188178}}</ref>

<ref name=slan1986>Zdenek Slanina (1986): ''[https://books.google.com/books?id=6QfwAAAAMAAJ Contemporary Theory of Chemical Isomerism]''. 254 pages. {{isbn|9789027717078}}</ref>

<ref name=stok2015>H. Stephen Stoker (2015): ''General, Organic, and Biological Chemistry'', 7th edition. 1232 pages. {{isbn|9781305686182}}</ref>

<ref name=faul2010>Jean-Loup Faulon, Andreas Bender (2010): ''Handbook of Chemoinformatics Algorithms''. 454 pages. {{isbn|9781420082999}}</ref>

<ref name=hinw1997>Barry G. Hinwood (1997): ''A Textbook of Science for the Health Professions''. 489 pages. {{isbn|9780748733774}}</ref>

</references>

[[Category:Isomerism]]

[[es:Isomería estructural]]
[[he:איזומרים מרחביים]]