Editing Isomerization

{{short description|Transformation of the chemical structure of a molecule or ion}}

In [[chemistry]], '''isomerization''' or '''isomerisation''' is the process in which a [[molecule]], [[polyatomic ion]]   or molecular fragment is transformed into an [[isomer]] with a different [[chemical structure]].<ref>{{GoldBookRef |title= isomerization |file= I03295}}</ref> [[Enolization]] is an example of isomerization, as is [[tautomer]]ization.<ref>{{cite book |author=Antonov L |title=Tautomerism: Concepts and Applications in Science and Technology |edition=1st |publisher=Wiley-VCH |location=Weinheim, Germany |year=2016 |isbn=978-3-527-33995-2}}</ref> 

When the [[activation energy]] for the isomerization reaction is sufficiently small, both isomers can often be observed and the equilibrium ratio will shift in a temperature-dependent [[Chemical equilibrium|equilibrium]] with each other. Many values of the standard [[Thermodynamic free energy|free energy]] difference, <math>\Delta G^\circ</math>, have been calculated, with good agreement between observed and calculated data.<ref>''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}}</ref>

==Examples and applications==
===Alkanes===
Skeletal isomerization occurs in the [[Cracking (chemistry)|cracking]] process, used in the [[petrochemical]] industry to convert straight chain alkanes to [[isoparaffin]]s as exemplified in the conversion of [[Octane|normal octane]] to [[2,5-Dimethylhexane|2,5-dimethylhexane]] (an "isoparaffin"):<ref name=Ullmann>{{cite book |doi=10.1002/14356007.a18_051 |chapter=Oil Refining |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2000 |last1=Irion |first1=Walther W. |last2=Neuwirth |first2=Otto S. |isbn=3-527-30673-0 }}</ref>
:[[File:Paraffintoisoparaffin.svg|center]]

Fuels containing branched [[hydrocarbon]]s are favored for internal combustion engines for their higher [[octane rating]].<ref>{{cite encyclopedia |author=Karl Griesbaum |author2=Arno Behr |author3=Dieter Biedenkapp |author4=Heinz-Werner Voges |author5=Dorothea Garbe |author6=Christian Paetz |author7=Gerd Collin |author8=Dieter Mayer |author9=Hartmut Höke |title=Hydrocarbons|encyclopedia=Ullmann's Encyclopedia of Industrial Chemistry|year=2002|publisher=Wiley-VCH|place=Weinheim|doi=10.1002/14356007.a13_227|isbn=3-527-30673-0 }}</ref>  Diesel engines however operate better with straight-chain hydrocarbons.

===Alkenes===
====Cis vs trans====
Trans-alkenes are about 1 kcal/mol more stable than cis-alkenes. An example of this effect is cis- vs trans-2-butene. The difference is attributed to unfavorable non-bonded interactions in the cis isomer.  This effects helps to explain the formation of trans-fats in food processing.  In some cases, the isomerization can be reversed using UV-light.  The ''trans'' isomer of [[resveratrol]] converts to the ''cis'' isomer in a [[photochemical reaction]].<ref>{{cite journal|title=Resveratrol Photoisomerization: An Integrative Guided-Inquiry Experiment'|author=Elyse Bernard, Philip Britz-McKibbin, Nicholas Gernigon|volume=84|year=2007|journal=Journal of Chemical Education|issue=7 |page=1159|doi=10.1021/ed084p1159 }}</ref>
:[[File:Rasveratrol isomerization-en.svg|400px|Resveratrol photoisomerization]]

====Terminal vs internal====
Terminal alkenes prefer to isomerize to internal alkenes:
:{{chem2|H2C\dCHCH2CH3  ->  CH3CH\dCHCH3}}
The conversion essentially does not occur in the absence of metal catalysts. This process is employed in the [[Shell higher olefin process]] to convert alpha-olefins to internal olefins, which are subjected to [[olefin metathesis]].

===Other organic examples===
Isomerism is a major topic in sugar chemistry. [[Glucose]], the most common sugar, exists in four forms.
{| class="wikitable centered"
|- style="background:#FFDEAD;"
! colspan="3"| Isomers of {{sm|d}}-glucose
|- class="background color5"
|- class="background color2"
| align="center" | [[File:Alpha-D-Glucofuranose.svg|120px|class=skin-invert-image]]{{pb}}α-{{sm|d}}-glucofuranose
| align="center" | [[File:Beta-D-Glucofuranose.svg|120px|class=skin-invert-image]]{{pb}}β-{{sm|d}}-glucofuranose
|- class="background color2"
| align="center" | [[File:Alpha-D-Glucopyranose.svg|100px|class=skin-invert-image]]{{pb}}α-{{sm|d}}-glucopyranose
| align="center" | [[File:Beta-D-Glucopyranose.svg|100px|class=skin-invert-image]]{{pb}}β-{{sm|d}}-glucopyranose
|- class="background color5"
|- class="background color2"
|}

[[Aldose-ketose isomerization|Aldose-ketose isomerism]], also known as Lobry de Bruyn–van Ekenstein transformation, provides an example in [[saccharide chemistry]].{{Citation needed|date=September 2021}}
:[[Image:Lobry-de-Bruyn-Alberda-van-Ekenstein-Umlagerung.svg|500px|center]]

===Inorganic and organometallic chemistry===
:[[File:Fp2-isomerization.png|frameless|440x440px|center]]
The compound with the formula [[Cyclopentadienyliron dicarbonyl dimer|{{chem2|(C5H5)2Fe2(CO)4}}]] exists as three isomers in solution.  In one isomer the CO ligands are terminal.  When a pair of CO are [[bridging ligand|bridging]],
cis and trans isomers are possible depending on the location of the [[cyclopentadienyl ligand|C<sub>5</sub>H<sub>5</sub> groups]].<ref name=":2">{{Cite journal|last1=Harris|first1=Daniel C.|last2=Rosenberg|first2=Edward|last3=Roberts|first3=John D.|date=1974|title=Carbon-13 nuclear magnetic resonance spectra and mechanism of bridge–terminal carbonyl exchange in di-''µ''-carbonyl-bis[carbonyl(''η''-cyclopentadienyl)iron](Fe–Fe) [{(''η''-C<sub>5</sub>H<sub>5</sub>)Fe(CO)<sub>2</sub>}<sub>2</sub>]; ''cd''-di-''µ''-carbonyl-''f''-carbonyl-''ae''-di(''η''-cyclopentadienyl)-''b''-(triethyl-phosphite)di-iron(Fe–Fe) [(''η''-C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>Fe<sub>2</sub>(CO)<sub>3</sub>P(OEt)<sub>3</sub>], and some related complexes|journal=Journal of the Chemical Society: Dalton Transactions|language=en|issue=22|pages=2398–2403|doi=10.1039/DT9740002398|issn=0300-9246|url=https://authors.library.caltech.edu/12272/1/HARjcsdt74.pdf}}</ref>

Another example in [[organometallic chemistry]] is the [[linkage isomer]]ization of decaphenylferrocene, {{chem2|[(\h{5}C5[[phenyl|Ph]]5)2Fe]}}.<ref>{{cite journal|last1=Brown|first1=K. N.|last2=Field|first2=L. D.|last3=Lay|first3=P. A.|last4=Lindall|first4=C. M.|last5=Masters|first5=A. F.|title=(&eta;<sup>5</sup>-Pentaphenylcyclopentadienyl){1-(&eta;<sup>6</sup>-phenyl)-2,3,4,5-tetraphenylcyclopentadienyl}iron(II), [Fe(&eta;<sup>5</sup>-C<sub>5</sub>Ph<sub>5</sub>){(&eta;<sup>6</sup>-C<sub>6</sub>H<sub>5</sub>)C<sub>5</sub>Ph<sub>4</sub>}], a linkage isomer of decaphenylferrocene|journal=[[Chemical Communications|J. Chem. Soc., Chem. Commun.]]|issue=5|pages=408–410|year=1990|doi=10.1039/C39900000408}}</ref><ref>{{cite journal|last1=Field|first1=L. D.|last2=Hambley|first2=T. W.|last3=Humphrey|first3=P. A.|last4=Lindall|first4=C. M.|last5=Gainsford|first5=G. J.|last6=Masters|first6=A. F.|last7=Stpierre|first7=T. G.|last8=Webb|first8=J.|title=Decaphenylferrocene|journal=Aust. J. Chem.|volume=48|issue=4|pages=851–860|year=1995|doi=10.1071/CH9950851}}</ref>

[[File:Formation of decaphenylferrocene from its linkage isomer.svg|380px|center|Formation of decaphenylferrocene from its linkage isomer]]
== Kinetic classification ==
From the [[Chemical kinetics|kinetic viewpoint]], isomerizations can be classified into two categories.<ref>{{Cite book |last=Arnaut |first=Luís G. |url=https://www.worldcat.org/title/on1063653763 |title=Chemical kinetics: from molecular structure to chemical reactivity |date=2021 |publisher=Elsevier |isbn=978-0-444-64039-0 |edition=Second |location=Amsterdam, Netherlands ; Cambridge, MA |oclc=on1063653763}}</ref> Cases in the first category involve transformations between equivalent structures. Most chemical species are in principle susceptible to such processes. Many such cases involve [[Fluxional molecule|fluxional molecules]], such as the [[Cyclohexane conformation|cyclohexane ring flip]] (chair inversion), the [[pyramidal inversion]] of ammonia, the [[Berry mechanism|Berry pseudorotation]] in pentacoordinate compounds (e.g. PF<sub>5</sub>, Fe(CO)<sub>5</sub>), the [[Bullvalene|Cope rearrangements of bullvalene]] or the [[Ray–Dutt twist|Ray-Dutt]]/[[Bailar twist|Bailar twists]] for the racemization of octahedral complexes with three bidentate chelate rings ([[Axial chirality|helical chirality]]). 

In the second broad category of isomerizations, the isomers are nonequivalent. Examples include [[Tautomer|tautomerizations]] ([[Enol|keto-enol]], [[Lactam|lactam-lactim]], [[Imidic acid|amide-imidic]], [[Enamine|enamine-imine]], [[Nitroso|nitroso-oxime]], [[Ketene|ketene-ynol]], etc) in which one isomer is more stable than the other.
[[File:Concentration_profile_for_the_scheme_A_=_B_when_k1_=_k2.png|thumb|Concentration profile for the reaction mechanism A = B when k1 = k2]]
This scheme leads to the following system of differential [[Rate equation|rate equations]]:

==See also==
*[[Base-promoted epoxide isomerization]]
*[[Epimerization]]
*[[Racemization]]
*[[Tautomerization]]
*[[Linkage isomerism]]

==References==
{{Reflist}}
{{Reaction mechanisms}}

[[Category:Chemical reactions]]