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.{{GoldBookRef |title= isomerization |file= I03295}} Enolization is an example of isomerization, as is tautomerization.{{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}}
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 equilibrium with each other. Many values of the standard free energy difference, , have been calculated, with good agreement between observed and calculated data.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}}
Examples and applications
=Alkanes=
Skeletal isomerization occurs in the cracking process, used in the petrochemical industry to convert straight chain alkanes to isoparaffins as exemplified in the conversion of normal octane to 2,5-dimethylhexane (an "isoparaffin"):{{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 }}
Fuels containing branched hydrocarbons are favored for internal combustion engines for their higher octane rating.{{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 }} 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.{{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 }}
==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.
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! colspan="3"| Isomers of {{sm|d}}-glucose |
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| align="center" | File:Alpha-D-Glucofuranose.svg{{pb}}α-{{sm|d}}-glucofuranose | align="center" | File:Beta-D-Glucofuranose.svg{{pb}}β-{{sm|d}}-glucofuranose |
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| align="center" | File:Alpha-D-Glucopyranose.svg{{pb}}α-{{sm|d}}-glucopyranose | align="center" | File:Beta-D-Glucopyranose.svg{{pb}}β-{{sm|d}}-glucopyranose |
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Aldose-ketose isomerism, also known as Lobry de Bruyn–van Ekenstein transformation, provides an example in saccharide chemistry.{{Citation needed|date=September 2021}}
=Inorganic and organometallic chemistry=
The compound with the formula Cyclopentadienyliron dicarbonyl dimer exists as three isomers in solution. In one isomer the CO ligands are terminal. When a pair of CO are bridging,
cis and trans isomers are possible depending on the location of the C5H5 groups.{{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-C5H5)Fe(CO)2}2]; cd-di-µ-carbonyl-f-carbonyl-ae-di(η-cyclopentadienyl)-b-(triethyl-phosphite)di-iron(Fe–Fe) [(η-C5H5)2Fe2(CO)3P(OEt)3], 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}}
Another example in organometallic chemistry is the linkage isomerization of decaphenylferrocene, {{chem2|[(\h{5}C5Ph5)2Fe]}}.{{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=(η5-Pentaphenylcyclopentadienyl){1-(η6-phenyl)-2,3,4,5-tetraphenylcyclopentadienyl}iron(II), [Fe(η5-C5Ph5){(η6-C6H5)C5Ph4}], a linkage isomer of decaphenylferrocene|journal=J. Chem. Soc., Chem. Commun.|issue=5|pages=408–410|year=1990|doi=10.1039/C39900000408}}{{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}}
File:Formation of decaphenylferrocene from its linkage isomer.svg
Kinetic classification
From the kinetic viewpoint, isomerizations can be classified into two categories.{{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}} 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 molecules, such as the cyclohexane ring flip (chair inversion), the pyramidal inversion of ammonia, the Berry pseudorotation in pentacoordinate compounds (e.g. PF5, Fe(CO)5), the Cope rearrangements of bullvalene or the Ray-Dutt/Bailar twists for the racemization of octahedral complexes with three bidentate chelate rings (helical chirality).
In the second broad category of isomerizations, the isomers are nonequivalent. Examples include tautomerizations (keto-enol, lactam-lactim, amide-imidic, enamine-imine, nitroso-oxime, 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
This scheme leads to the following system of differential rate equations: