# Isomerization

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

In [chemistry](/source/chemistry), '''isomerization''' or '''isomerisation''' is the process in which a [molecule](/source/molecule), [polyatomic ion](/source/polyatomic_ion)   or molecular fragment is transformed into an [isomer](/source/isomer) with a different [chemical structure](/source/chemical_structure).<ref>{{GoldBookRef |title= isomerization |file= I03295}}</ref> [Enolization](/source/Enolization) is an example of isomerization, as is [tautomer](/source/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](/source/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 [equilibrium](/source/Chemical_equilibrium) with each other. Many values of the standard [free energy](/source/Thermodynamic_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](/source/Stefan_Grimme), Marc Steinmetz, and Martin Korth [J. Org. Chem.](/source/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](/source/Cracking_(chemistry)) process, used in the [petrochemical](/source/petrochemical) industry to convert straight chain [alkanes](/source/alkanes) to [isoparaffin](/source/isoparaffin)s as exemplified in the conversion of [normal octane](/source/Octane) to [2,5-dimethylhexane](/source/2%2C5-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>
:center

Fuels containing branched [hydrocarbon](/source/hydrocarbon)s are favored for internal combustion engines for their higher [octane rating](/source/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](/source/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](/source/resveratrol) converts to the ''cis'' isomer in a [photochemical reaction](/source/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 |bibcode=2007JChEd..84.1159B }}</ref>
: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](/source/Shell_higher_olefin_process) to convert alpha-olefins to internal olefins, which are subjected to [olefin metathesis](/source/olefin_metathesis).

===Other organic examples===
Isomerism is a major topic in sugar chemistry. [Glucose](/source/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" | 120px|class=skin-invert-image{{pb}}α-{{sm|d}}-glucofuranose
| align="center" | 120px|class=skin-invert-image{{pb}}β-{{sm|d}}-glucofuranose
|- class="background color2"
| align="center" | 100px|class=skin-invert-image{{pb}}α-{{sm|d}}-glucopyranose
| align="center" | 100px|class=skin-invert-image{{pb}}β-{{sm|d}}-glucopyranose
|- class="background color5"
|- class="background color2"
|}

[Aldose-ketose isomerism](/source/Aldose-ketose_isomerization), also known as Lobry de Bruyn–van Ekenstein transformation, provides an example in [saccharide chemistry](/source/saccharide_chemistry).<ref>{{Cite web |title=Lobry de Bruyn-van Ekenstein transformation |url=https://www.oxfordreference.com/display/10.1093/oi/authority.20110803100111253 |access-date=2025-07-08 |website=Oxford Reference |language=en }}</ref>
:500px|center

===Inorganic and organometallic chemistry===
:frameless|440x440px|center
The compound with the formula [{{chem2|(C5H5)2Fe2(CO)4}}](/source/Cyclopentadienyliron_dicarbonyl_dimer) exists as three isomers in solution.  In one isomer the CO ligands are terminal.  When a pair of CO are [bridging](/source/bridging_ligand),
cis and trans isomers are possible depending on the location of the [C<sub>5</sub>H<sub>5</sub> groups](/source/cyclopentadienyl_ligand).<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](/source/organometallic_chemistry) is the [linkage isomer](/source/linkage_isomer)ization of decaphenylferrocene, {{chem2|[(\h{5}C5[Ph](/source/phenyl)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=[J. Chem. Soc., Chem. Commun.](/source/Chemical_Communications)|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>

380px|center|Formation of decaphenylferrocene from its linkage isomer

===Biochemistry===
[isomerase](/source/isomerase)s are a general class of [enzyme](/source/enzyme)s that convert a molecule from one [isomer](/source/isomer) to another.

== Kinetic classification ==
From the [kinetic viewpoint](/source/Chemical_kinetics), 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](/source/fluxional_molecule)s, such as the [cyclohexane ring flip](/source/Cyclohexane_conformation) (chair inversion), the [pyramidal inversion](/source/pyramidal_inversion) of ammonia, the [Berry pseudorotation](/source/Berry_mechanism) in pentacoordinate compounds (e.g. PF<sub>5</sub>, Fe(CO)<sub>5</sub>), the [Cope rearrangements of bullvalene](/source/Bullvalene) or the [Ray-Dutt](/source/Ray%E2%80%93Dutt_twist)/[Bailar twist](/source/Bailar_twist)s for the racemization of octahedral complexes with three bidentate chelate rings ([helical chirality](/source/Axial_chirality)).

In the second broad category of isomerizations, the isomers are nonequivalent. Examples include [tautomer](/source/tautomer)izations ([keto-enol](/source/Enol), [lactam-lactim](/source/Lactam), [amide-imidic](/source/Imidic_acid), [enamine-imine](/source/Enamine), [nitroso-oxime](/source/Nitroso), [ketene-ynol](/source/Ketene), etc) in which one isomer is more stable than the other.
thumb|Concentration profile for the reaction mechanism A = B when k1 = k2
This scheme leads to the following system of differential [rate equation](/source/rate_equation)s:

==See also==
*[Base-promoted epoxide isomerization](/source/Base-promoted_epoxide_isomerization)
*[Epimerization](/source/Epimerization)
*[Racemization](/source/Racemization)
*[Tautomerization](/source/Tautomerization)
*[Linkage isomerism](/source/Linkage_isomerism)

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

Category:Chemical reactions

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Adapted from the Wikipedia article [Isomerization](https://en.wikipedia.org/wiki/Isomerization) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Isomerization?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.