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.
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) ]
Examples and applications
Alkanes
Skeletal isomerization occurs in the cracking process, used in the
petrochemical industry to convert straight chain alkanes to
as exemplified in the conversion of
Octane to 2,5-dimethylhexane (an "isoparaffin"):
Fuels containing branched are favored for internal combustion engines for their higher octane rating.
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.
Terminal vs internal
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 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.
|
|
|
| α--glucofuranose | β--glucofuranose |
| α--glucopyranose | β--glucopyranose |
|
|
Aldose-ketose isomerism, also known as Lobry de Bruyn–van Ekenstein transformation, provides an example in saccharide chemistry.
Inorganic and organometallic chemistry
The compound with the formula exists as three isomers in solution. In one isomer the CO ligands are terminal. When a pair of CO are
bridging ligand,
cis and trans isomers are possible depending on the location of the C
5H
5 groups.
Another example in organometallic chemistry is the of decaphenylferrocene, .
Kinetic classification
From the kinetic viewpoint, isomerizations can be classified into two categories.
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 mechanism in pentacoordinate compounds (e.g. PF
5, Fe(CO)
5), the
Bullvalene or the Ray-Dutt/
Bailar twist for the racemization of octahedral complexes with three bidentate chelate rings (
Axial chirality).
In the second broad category of isomerizations, the isomers are nonequivalent. Examples include Tautomer (Enol, Lactam, Imidic acid, Enamine, Nitroso, Ketene, etc) in which one isomer is more stable than the other.
This scheme leads to the following system of differential Rate equation:
See also
