{{Textbook|date=August 2020}}
In [[chemistry]], a '''structural isomer''' (or '''constitutional isomer''' in the [[IUPAC]] nomenclature[
For example, [[butanol]] {{chem|H|3|C}}–{{chem|(CH|2|)|3}}–OH, [[methyl propyl ether]] {{chem|H|3|C}}–{{chem|(CH|2|)|2}}–O–{{chem|CH|3}}, and [[diethyl ether]] ({{chem|H|3|C}}–{{chem|CH|2}}–)2O have the same [[molecular formula]] {{chem|C|4|H|10|O}} 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 O=C=N− and the [[fulminate]] ion C−≡N+O−.
Structural isomerism is the most radical type of [[isomer|isomerism]]. It is opposed to [[stereoisomer|stereoisomerism]], in which the atoms and bonding scheme are the same, but only the relative spatial arrangement of the atoms is different.][ 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 '''positional isomers''' (or '''regioisomers'''), '''functional isomers''', and '''skeletal isomers'''.][
==Chain isomerism==
In '''chain isomerism''', or '''skeletal isomerism''', components of the (usually carbon) skeleton are distinctly re-ordered to create different structures. [[Pentane]] exists as three isomers: ''n''-pentane (often called simply "pentane"), [[isopentane]] (2-methylbutane) and [[neopentane]] (dimethylpropane).][
{| align="center" class="wikitable"
|-
|+Skeletal isomerism of pentane
|-
|[[File:Pentane-2D-Skeletal.svg|125px]]
|[[File:2-methylbutane-2D-skeletal.svg|125px]]
|[[File:Neopentane-2D-skeletal.png|100px]]
|-
| ''n''-[[Pentane]]
| [[Isopentane]]
| [[Neopentane]]
|}
=={{anchor|Regioisomer}}Position isomerism (regioisomerism) ==
{{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 a "parent" structure. ][
For example, replacement one of the 12 hydrogen atoms –H by a [[alcohol|hydroxyl]] group –OH on the [[pentane|''n''-pentane]] parent molecule can give any of three different position isomers:
{| align="center"
|-
| [[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]]
|}
===Structural symmetry===
There are only three pentanols, not five or 12, because replacing each of the 12 hydrogens by hydroxyls yields many molecules with the same structure.
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 types (elements) of the atoms, or the types of bonds between them. Two atoms then can be said to be structurally [[equivalent]] if there is a structural symmetry that takes one to the other. ][
As the pentol example above shows, if a parent molecule has two or more structurally equivalent atoms, then replacing any of those atoms by an atom of some other element (or by a functional group with a fixed structure) will yield the same positional isomer.
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. Thus there is just one structural isomer of [[chloromethane]] {{chem|CH|3|Cl}}.
Likewise, all six hydrogens of [[ethane]] ({{chem|C|2|H|6}}) are structurally equivalent (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 all the bonds. Thus there is only one [[ethanol]] {{chem|CH|5|OH}} -- not 2 or 6. That is the case also for the hydrogen atoms [[cyclopentane]], [[allene]], [[2-butyne]], [[hexamethylenetetramine]], [[prismane]], [[cubane]], [[dodecahedrane]], etc.
On the other hand, the hydrogen atoms of [[propane]] are not all equivalent. The six hydrogens attached to the first an 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. Therefore, substituting a single hydrogen by a hydroxyl on propane can generate two, and only two, positional isomers ([[1-propanol]] and [[2-propanol]]).
===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"
|-
| 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"
|-
| 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
|}
==Functional group isomerism==
'''Functional isomers''' are structural isomers that have the same [[molecular formula]] (that is, the same number of atoms of the same elements), but the atoms are connected in different ways so that the groupings are dissimilar. These groups of atoms are called [[functional groups]], functionalities.][
For example, [[cyclohexane]] and [[1-hexene]] both have the formula C6H12. These two are considered functional group isomers because cyclohexane is a [[cycloalkane]] and 1-hexene is an [[alkene]].{{dubious|date=November 2014}}
{| align="center" class="wikitable"
|-
|+Example of functional group isomerism
|-
| [[File:Cyclohexane-2D-skeletal.svg|75px]]
| [[File:1-hexene.svg|125px]]
|-
| [[Cyclohexane]]
| [[1-Hexene]]
|}
For two molecules to be functional isomers, they must contain key groups of atoms arranged in particular ways. Some of the best examples come from organic chemistry. C2H6O is a molecular formula. Depending on how the atoms are arranged, it can represent two different compounds [[dimethyl ether]] CH3-O-CH3 or [[ethanol]] CH3CH2-O-H. Dimethyl [[ether]] and ethanol are functional [[isomer]]s. The first is an ether. The [[carbon]] chain-[[oxygen]]-carbon chain functionality is called an ether. The second is an alcohol. The carbon chain-oxygen-[[hydrogen]] functionality is called an [[alcohol]].
If the functionalities stay the same, but their locations change, the structural isomers are not functional isomers. 1-Propanol and 2-propanol are structural isomers, but they are not functional isomers. Both of them are alcohols. The functional group (carbon chain-O-H) is present in both of these compounds, but they are not the same.
While some chemists use the terms structural isomer and [[functional isomer]] interchangeably, not all structural isomers are functional isomers.
Functional isomers are most often identified in chemistry using [[infrared spectroscopy]]. [[Infrared radiation]] corresponds to the energies associated primarily with molecular vibration. The alcohol functionality has a very distinct vibration called OH-stretch that is due to hydrogen bonding. All alcohols in liquid and solid form absorb infrared radiation at certain wavelengths.
Compounds with the same functional groups will all absorb certain wavelengths of infrared light because of the vibrations associated with those groups. In fact, the infrared spectrum is divided into two regions. The first part is called the functional group region. Dimethyl ether and ethanol would have dissimilar infrared spectra in the functional group region.
The second part of the infrared spectrum is called the fingerprint region; it is associated with types of motion allowed by the symmetry of the molecule and influenced by the bond energies. The fingerprint region is more specific to an individual compound. Even though 1-propanol and 2-propanol have similar infrared spectra in the functional group region, they differ in the fingerprint region.
In simple terms, functional isomers are structural isomers that have different functional groups like alcohol and ether.
==Isomer counting==
As an example of isomer counting, there are nine structural isomers with molecular formula [[C3H6O|C3H6O]] having different bond connectivities. Seven of them are air-stable at room temperature, and these are given in the table below. An additional two structural isomers are the [[enol]] [[tautomer]]s of the carbonyl isomers (propionaldehyde and acetone), but these are not stable.][[[CRC Handbook of Chemistry and Physics]] 65Th Ed.]
{| class="wikitable sortable"
! [[Chemical compound]] !! class="unsortable" | [[Molecular structure]] !! [[Melting point|Melting
point]] (°C) !! [[Boiling point|Boiling
point]] (°C) !! class="unsortable" | Comment
|-
|valign=top| [[Allyl alcohol]]
|valign=top| [[File:Allyl-alcohol.png|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 propen-1-ol, which has both [[Cis–trans isomerism|''cis'' and ''trans'' stereoisomeric isomeric 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:Methylvinylether.png|80px]]
|valign=top| –122
|valign=top| 6
|valign=top|
|}
==See also==
*[[Coordination isomerism]]
*[[Stereoisomer]]
*[[Metamerism]]
== References ==
[{{cite web |title=Constitutional isomerism |url=https://goldbook.iupac.org/html/C/C01285.html |website=IUPAC Gold Book |publisher=IUPAC |accessdate=19 July 2018}}]
[Frederick A. Bettelheim, William H. Brown, Mary K. Campbell, Shawn O. Farrell (2009): ''Introduction to Organic and Biochemistry''. 752 pages. {{isbn|9780495391166}}]
[Peter P. Mumba (2018): ''Useful Principles in Chemistry for Agriculture and Nursing Students'', 2nd Edition. 281 pages. {{isbn|9781618965288}}]
[Jim Clark (2000). [http://www.chemguide.co.uk/basicorg/isomerism/structural.html "Structural isomerism"] in ''Chemguide'', n.l.]
[{{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}}]
[D. Brynn Hibbert, A.M. James (1987): ''Macmillan Dictionary of Chemistry''. 532 pages. {{isbn|9781349188178}}]
[Zdenek Slanina (1986): ''[https://books.google.com.br/books?id=6QfwAAAAMAAJ Contemporary Theory of Chemical Isomerism]''. 254 pages. {{isbn|9789027717078}}]
[H. Stephen Stoker (2015): ''General, Organic, and Biological Chemistry'', 7th edition. 1232 pages. {{isbn|9781305686182}}]
[Jean-Loup Faulon, Andreas Bender (2010): ''Handbook of Chemoinformatics Algorithms''. 454 pages. {{isbn|9781420082999}}]
[Barry G. Hinwood (1997): ''A Textbook of Science for the Health Professions''. 489 pages. {{isbn|9780748733774}}]
[[Category:Isomerism]]
[[es:Isomería estructural]]
[[he:איזומרים מרחביים]]