Inchemistry,isomerization orisomerisation is the process in which amolecule,polyatomic ion or molecular fragment is transformed into anisomer with a differentchemical structure.[1]Enolization is an example of isomerization, as istautomerization.[2]
When theactivation energy for the isomerization reaction is sufficiently small, both isomers can often be observed and the equilibrium ratio will shift in a temperature-dependentequilibrium with each other. Many values of the standardfree energy difference,, have been calculated, with good agreement between observed and calculated data.[3]
Skeletal isomerization occurs in thecracking process, used in thepetrochemical industry to convert straight chainalkanes toisoparaffins as exemplified in the conversion ofnormal octane to2,5-dimethylhexane (an "isoparaffin"):[4]
Fuels containing branchedhydrocarbons are favored for internal combustion engines for their higheroctane rating.[5] Diesel engines however operate better with straight-chain hydrocarbons.
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. Thetrans isomer ofresveratrol converts to thecis isomer in aphotochemical reaction.[6]
Terminal alkenes prefer to isomerize to internal alkenes:
The conversion essentially does not occur in the absence of metal catalysts. This process is employed in theShell higher olefin process to convert alpha-olefins to internal olefins, which are subjected toolefin metathesis.
Isomerism is a major topic in sugar chemistry.Glucose, the most common sugar, exists in four forms.
| Isomers ofd-glucose | ||
|---|---|---|
 α-d-glucofuranose |  β-d-glucofuranose | |
 α-d-glucopyranose |  β-d-glucopyranose | |
Aldose-ketose isomerism, also known as Lobry de Bruyn–van Ekenstein transformation, provides an example insaccharide chemistry.[7]
The compound with the formula(C5H5)2Fe2(CO)4 exists as three isomers in solution. In one isomer the CO ligands are terminal. When a pair of CO arebridging,cis and trans isomers are possible depending on the location of theC5H5 groups.[8]
Another example inorganometallic chemistry is thelinkage isomerization of decaphenylferrocene,[(η5-C5Ph5)2Fe].[9][10]
From thekinetic viewpoint, isomerizations can be classified into two categories.[11] Cases in the first category involve transformations between equivalent structures. Most chemical species are in principle susceptible to such processes. Many such cases involvefluxional molecules, such as thecyclohexane ring flip (chair inversion), thepyramidal inversion of ammonia, theBerry pseudorotation in pentacoordinate compounds (e.g. PF5, Fe(CO)5), theCope rearrangements of bullvalene or theRay-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 includetautomerizations (keto-enol,lactam-lactim,amide-imidic,enamine-imine,nitroso-oxime,ketene-ynol, etc) in which one isomer is more stable than the other.
This scheme leads to the following system of differentialrate equations:
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