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Isomerization
Isomerization
Isomerization
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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.[1] Enolization is an example of isomerization, as is tautomerization.[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.[3]

Examples and applications

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Alkanes

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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"):[4]

Fuels containing branched hydrocarbons are favored for internal combustion engines for their higher octane rating.[5] Diesel engines however operate better with straight-chain hydrocarbons.

Alkenes

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Cis vs trans

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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.[6]

Resveratrol photoisomerization

Terminal vs internal

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Terminal alkenes prefer to isomerize to internal alkenes:

H2C=CHCH2CH3 → CH3CH=CHCH3

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

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Isomerism is a major topic in sugar chemistry. Glucose, the most common sugar, exists in four forms.

Isomers of d-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 in saccharide chemistry.[7]

Inorganic and organometallic chemistry

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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 are bridging, cis and trans isomers are possible depending on the location of the C5H5 groups.[8]

Another example in organometallic chemistry is the linkage isomerization of decaphenylferrocene, [(η5-C5Ph5)2Fe].[9][10]

Formation of decaphenylferrocene from its linkage isomer
Formation of decaphenylferrocene from its linkage isomer

Kinetic classification

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From the kinetic 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 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.

Concentration profile for the reaction mechanism A = B when k1 = k2

This scheme leads to the following system of differential rate equations:

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Isomerization

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Isomerization is a chemical process in which a compound is converted into one of its isomers through the rearrangement of atoms or functional groups, preserving the molecular formula but altering the structure or spatial arrangement. This transformation can occur spontaneously, via thermal, photochemical, or catalytic means, or through enzymatic action in biological systems. Isomerization reactions are broadly classified into two main types: constitutional isomerization, which involves changes in the connectivity of atoms (also known as structural or chain isomerization), and stereoisomerization, which affects the spatial configuration without altering bonding, including cis-trans (geometric) isomerization and enantiomerization (conversion between mirror-image forms). Examples include the shift from n-butane to in hydrocarbons or the cis-to-trans conversion in alkenes. These reactions are fundamental in , enabling the synthesis of compounds with distinct physical, chemical, and biological properties. In biology, isomerization plays a critical role in metabolic pathways, catalyzed by enzymes called isomerases that facilitate interconversions essential for processes like , vision (e.g., the of 11-cis-retinal to all-trans-retinal), and . Isomers often exhibit different pharmacological effects, making stereoselective isomerization vital for developing effective and safe therapeutics, as seen in the enantiomer-specific activity of drugs like ibuprofen. Industrially, isomerization is a of , where light fractions (C5-C6 paraffins) are isomerized to branched forms using platinum-based catalysts under pressure, boosting numbers for high-quality without increasing aromatics or olefins. This process, often combined with and , enhances and meets environmental standards, with global production exceeding millions of barrels daily. Photochemical and thermal isomerizations also find applications in , such as in photoresponsive polymers and liquid crystals.

Fundamentals

Definition and Scope

Isomerization is a in which a compound is converted into one or more of its isomers, resulting in products that share the same molecular formula but differ in atomic connectivity or spatial arrangement. This process typically involves the rearrangement of bonds within the molecule, often requiring energy input such as heat, light, or to overcome activation barriers. At its foundation, isomerization presupposes the existence of isomers, which are compounds with identical molecular formulas but distinct structures. Constitutional isomers differ in the connectivity of their atoms, as exemplified by n-butane (CH₃CH₂CH₂CH₃) and isobutane ((CH₃)₂CHCH₃), where the carbon skeleton varies. In contrast, stereoisomers have the same connectivity but differ in the three-dimensional arrangement of atoms, such as cis and trans configurations around a . The scope of isomerization encompasses deliberate transformations, frequently facilitated by catalysts, that target specific isomeric forms for synthetic or industrial purposes, distinguishing it from spontaneous or equilibrium-driven processes. Unlike tautomerization, which involves rapid, reversible interconversion of tautomers via low-energy heterolytic rearrangements often in equilibrium, isomerization generally proceeds more slowly and can be directed toward stable products. It also contrasts with , a chain-growth reaction that links monomers to form larger molecules with increased molecular weight, thereby altering the overall formula rather than rearranging a single molecule. Early observations of isomerization emerged in 19th-century , with notable examples including the conversion of (cis-butenedioic acid) to (trans-butenedioic acid) achieved through heating or halogen catalysis in the late 1800s by chemists such as Johann Wislicenus.

Relevant Isomer Types

Constitutional isomers, also referred to as structural isomers, feature the same molecular formula but differ in the connectivity of their atoms, making them prime candidates for isomerization reactions that rearrange atomic linkages. A prominent example is chain branching in alkanes, where straight-chain hydrocarbons like n-hexane are converted to branched variants such as , altering the carbon skeleton while preserving the overall formula. Positional isomers, a subclass of constitutional isomers, involve shifts in the location of functional groups or multiple bonds; for instance, the repositioning of a hydroxyl group from a primary to a secondary carbon in alcohols, as seen in certain catalytic rearrangements, exemplifies this type. Stereoisomers maintain identical atomic connectivity but vary in spatial configuration, and their isomerization interconverts these arrangements. Geometric isomers, including cis-trans forms in s and coordination compounds, undergo isomerization by overcoming restricted rotations around double bonds or metal-ligand bonds, such as the cis-to-trans conversion in non-conjugated dienes facilitated by alkene assistance. Optical isomers, or enantiomers, experience isomerization through , where a chiral center inverts to produce a , as demonstrated in dynamic kinetic resolutions of chiral amines using metal catalysts. The feasibility of isomerization hinges on energy barriers that separate the wells of distinct s, with the process entailing to surpass these hurdles and favor more stable configurations. Tautomeric s, like keto-enol pairs, constitute a boundary case of constitutional isomerism characterized by low interconversion barriers and rapid equilibrium, often distinguishing them from slower, deliberate isomerization processes. These types underscore the versatility of isomerization in organic systems, notably in enhancing like branching in alkanes for combustion efficiency.

Reaction Mechanisms

Thermal and Photochemical Mechanisms

Thermal isomerization of alkenes often proceeds through the around the C=C , typically involving a intermediate that allows for cis-trans interconversion without external catalysts. This mechanism requires overcoming a high barrier, generally in the range of 200-300 kJ/mol, as exemplified by the ~272 kJ/mol barrier for rotation. In processes like thermal cracking of hydrocarbons, isomerization can occur via free radical mechanisms, where initial bond homolysis generates radicals that rearrange through shifts or other radical processes, contributing to skeletal shifts. The equilibrium distribution of isomers in such thermal processes is influenced by , where isomers with higher conformational flexibility or rotational freedom are favored at elevated temperatures due to the -TΔS term in the . Photochemical mechanisms enable isomerization at lower energies by exciting molecules to reactive states, often leading to stereoisomerization through bond rotations. In stilbene, UV irradiation promotes the trans isomer to an excited , where twisting around the central C=C bond occurs, followed by relaxation to the cis . The efficiency of such photoisomerizations is quantified by the Φ, defined as: Φ=number of isomers formednumber of photons absorbed\Phi = \frac{\text{number of isomers formed}}{\text{number of photons absorbed}} This metric highlights the competition between productive isomerization and non-radiative decay pathways. Additionally, Norrish type I reactions in ketones provide a photochemical route to skeletal isomerization, where excitation cleaves the C-C bond adjacent to the carbonyl, generating acyl and alkyl radicals that recombine in rearranged configurations, such as ring expansions or transpositions. These non-catalytic paths contrast with catalytic methods that lower activation energies for more selective outcomes at milder conditions.

Catalytic Mechanisms

Catalytic mechanisms in isomerization reactions typically involve the use of acids, bases, or metal complexes to lower energies and enable selective transformations under milder conditions compared to processes, which often require high temperatures for non-catalyzed rearrangements. Acid-catalyzed isomerization proceeds via of a substrate, such as an or alcohol, to generate a intermediate that rearranges through 1,2-shifts to a more stable form. For instance, in systems, protonation can lead to hydride shifts, where a migrates from an adjacent carbon, as exemplified by the conversion of a primary or secondary carbocation to a tertiary one: a primary carbocation like R-CH₂-CH₂⁺ undergoes a hydride shift to form R-CH⁺-CH₃. The Wagner-Meerwein rearrangement represents a specific case of such acid-catalyzed processes, where protonation of an alcohol followed by loss of water forms a that undergoes a 1,2-alkyl shift to yield a rearranged , commonly observed in chemistry. Base-catalyzed mechanisms rely on to form or intermediates, facilitating positional isomerization, particularly in carbonyl compounds or allylic systems. In keto-enol tautomerism, a base abstracts an α-hydrogen from the carbonyl, generating a resonance-stabilized ; subsequent on the α-carbon or oxygen leads to the isomeric or back to the keto form, enabling double-bond migration. This process is reversible and catalyzed by bases like , contrasting with the irreversible shifts in acid pathways. Metal-catalyzed isomerization often involves transition metals forming π-allyl complexes, which allow for efficient double-bond migration. In such mechanisms, a metal adds across the to form an alkyl intermediate, followed by β- elimination to reposition the ; for example, or catalysts generate η³-π-allyl that isomerize terminal to internal ones with high selectivity. Ziegler-Natta type catalysts, typically titanium-based with aluminum alkyls, enable stereoisomerization during olefin by coordinating the in a chiral environment, directing 1,2- or 1,4-insertion to produce isotactic or syndiotactic polymers. Key performance metrics for these catalysts include turnover numbers (), which quantify the number of substrate molecules converted per site before deactivation, often reaching thousands in efficient systems like complexes for allylic isomerization. effects, such as adsorbing on sites of bifunctional catalysts, can reduce activity by 30-35% by blocking reactive centers. Bifunctional catalysts like Pt/Al₂O₃ combine metal sites for dehydrogenation of alkanes to alkenes with sites for skeletal rearrangement, enabling dual-step isomerization in a single process. Recent post-2020 advances in zeolite-based catalysts, such as hierarchical variants, have enhanced shape-selective isomerization of n-alkanes by improving diffusion and site distribution, achieving high selectivities for monobranched products.

Applications in Organic Chemistry

Alkane Isomerization

Alkane isomerization involves the rearrangement of straight-chain saturated hydrocarbons into branched isomers to enhance properties, particularly through the conversion of n-alkanes to their branched counterparts, such as n-pentane to , proceeding via intermediates formed on acidic catalysts. This process typically occurs under controlled conditions where the alkane adsorbs onto the catalyst surface, undergoes to generate a , and rearranges via 1,2-hydride or methyl shifts before to yield the branched product. The reaction is thermodynamically favored for branching due to increased stability of the branched structures. A representative example is the isomerization of n-butane to , depicted as: \ceCH3−CH2−CH2−CH3−>(CH3)2CH−CH3\ce{CH3-CH2-CH2-CH3 -> (CH3)2CH-CH3} with an change of approximately -8.6 kJ/mol, reflecting the energetic preference for the branched form. Common catalysts include solid acids such as zeolites, which provide Brønsted acid sites for formation, or bifunctional systems like Pt-loaded zeolites that facilitate dehydrogenation-hydrogenation steps alongside skeletal rearrangement. This isomerization significantly improves the of fuels, as branched alkanes resist autoignition better than linear ones, a development that played a key role in producing high-octane aviation gasoline during the . However, challenges include coke formation from oligomerization of intermediates, leading to catalyst deactivation by pore blockage and reduced acidity.

Alkene and Other Unsaturated Systems

Isomerization in systems primarily involves the repositioning of the carbon-carbon or the reconfiguration of orientations around it. Positional isomerization, often termed double bond migration, converts terminal alkenes to more internal isomers through a series of 1,3- or 1,2-shifts. For instance, undergoes isomerization to 2-butene via the formation of an allylic intermediate, where a metal adds across the , followed by β- elimination to relocate the π-bond. This process is driven by thermodynamic stability, as internal alkenes exhibit lower free energies due to increased and reduced steric strain compared to terminal counterparts. Geometric isomerization in alkenes entails the interconversion between cis and trans configurations, which is restricted by the rigidity of the but can be facilitated under catalytic conditions. The reaction cis-RCH=CHR → trans-RCH=CHR proceeds via reversible addition-elimination mechanisms that temporarily saturate the , allowing . The trans isomer is thermodynamically favored, with a typical free energy difference ΔG ≈ -3 kJ/mol for disubstituted systems like 2-butene, arising from minimized steric repulsion between substituents. This preference influences reaction equilibria, often requiring contra-thermodynamic strategies for cis-selective outcomes. In other unsaturated systems, isomerization extends to triple bonds and conjugated structures. Alkynes can tautomerize to through base- or metal-catalyzed 1,2-hydride shifts, forming cumulated double bonds. Similarly, non-conjugated dienes rearrange to thermodynamically stable conjugated dienes, enhancing π-delocalization; this occurs via sequential allylic rearrangements, as seen in the conversion of 1,4-pentadiene to 1,3-pentadiene under . Catalysts for these transformations span homogeneous and heterogeneous regimes. Homogeneous systems, such as ruthenium(II) hydride complexes (e.g., [Ru(H)(Cl)(CO)(PPh3)3]), enable selective double bond migrations at low loadings (ppm levels) through reversible π-alkene coordination and hydride transfer, achieving high turnover numbers in polar solvents. Heterogeneous catalysts, including metal oxides like MgO or supported nickel hydrides, promote surface-mediated isomerization via acid-base sites or metal-alkyl intermediates, offering advantages in scalability and recyclability for industrial feedstocks. A notable application is the selective Z-isomerization of s using catalysts to access specific isomers for synthetic production.

Functional Group Rearrangements

Functional group rearrangements represent a class of isomerization reactions in where a migrates from one atom to an adjacent or nearby atom within the , often facilitated by acid or thermal conditions, leading to skeletal reorganization while preserving the overall carbon framework. These transformations are pivotal for constructing complex molecular architectures, particularly when traditional bond-forming strategies are inefficient. Unlike simple positional isomerizations, functional group migrations typically involve or concerted mechanisms that dictate the and of the product. A prominent example is the , where vicinal diols (1,2-diols) undergo dehydration under acidic conditions to form carbonyl compounds, with one alkyl or aryl group migrating to the adjacent carbon. Discovered in the late , this reaction proceeds via protonation of one hydroxyl group, loss of water to generate a , and subsequent 1,2-migration of the antiperiplanar group to yield a , such as the conversion of pinacol to . The semi-pinacol variant extends this to α-hydroxy systems with a better , where neighboring group participation stabilizes the intermediate through anchimeric assistance, enhancing reaction efficiency and stereocontrol. For instance, in semi-pinacol rearrangements, a vicinal or can donate electrons to form a bridged , promoting selective migration. The exemplifies nitrogen-containing migration, transforming into amides via , where the group anti to the hydroxyl migrates with retention of configuration. This stereospecific 1,2-shift is crucial for synthesizing lactams, as seen in the conversion of to ε-caprolactam, a key industrial precursor for nylon-6. Mechanistically, of the oxygen facilitates departure of water and migration of the anti to the electron-deficient , yielding the trans-amide product. Similarly, the involves a [3,3]-sigmatropic shift in allyl vinyl ethers, such as allyl phenyl ether rearranging thermally to o-allylphenol, a pericyclic process that is suprafacial and stereospecific, proceeding through a chair-like . Key concepts in these rearrangements distinguish 1,2-shifts, common in pinacol and Beckmann processes for adjacent atom migrations, from 1,3-shifts in Claisen rearrangements, which involve allylic systems and maintain at chiral centers through concerted pathways. In chiral molecules, these reactions often exhibit high ; for example, the migrating group in pinacol-type shifts retains its configuration, while Claisen proceeds with inversion at the allylic terminus due to the sigmatropic nature. Such specificity is vital for asymmetric synthesis. An notable application lies in synthesis, where the isomerization of to via acid-catalyzed 1,2-methyl shift has been industrial since the , enabling production of synthetic precursors. Recent advancements include enzyme-mimicking catalysts that enable these rearrangements under mild conditions, reducing energy demands and improving selectivity. For the , calcium-based catalysts facilitate the process at ambient temperatures, mimicking enzymatic active sites by stabilizing transition states without harsh acids. These developments draw from general catalytic mechanisms to enhance in .

Applications in Inorganic and Organometallic Chemistry

Coordination Compound Isomerization

Coordination compound isomerization encompasses structural rearrangements within the of metal complexes, primarily geometric and linkage types, which were first systematically elucidated by in the early 1910s through his pioneering work on in inorganic compounds. Werner's resolution of optical isomers in cobalt(III) ammine complexes and identification of cis-trans forms validated the application of stereochemical principles to inorganic systems, earning him the 1913 . These isomerizations highlight the rigidity of metal-ligand bonds and their influence on complex properties, distinct from dynamic processes in organometallics. Geometric isomerism arises in coordination complexes with specific geometries, such as square planar and octahedral, where ligands occupy different spatial positions relative to each other. In square planar Pt(II) complexes like [Pt(NH₃)₂Cl₂], the cis isomer features adjacent identical ligands, while the trans has them opposite, affecting reactivity and , as seen in the anticancer drug (cis form). For octahedral Co(III) complexes, such as [Co(NH₃)₄Cl₂]⁺, the cis isomer has chlorides adjacent (90° angle), whereas the trans positions them opposite (180°), with the cis and trans forms exhibiting different stabilities; the cis isomer is more prevalent at equilibrium in (~88% cis) due to ligand field and effects. These configurations influence spectroscopic and magnetic properties, underscoring the role of geometry in determining viability. Linkage isomerism occurs when ambidentate ligands, capable of binding through multiple donor atoms, switch coordination modes without altering the overall composition. A classic example involves the nitrite ligand (NO₂⁻) in cobalt(III) complexes, where it coordinates via nitrogen as nitro ([Co(NH₃)₅(NO₂)]²⁺) or via oxygen as nitrito ([Co(NH₃)₅(ONO)]²⁺), with the isomerization represented as [Co(NH₃)₅(NO₂)]²⁺ ⇌ [Co(NH₃)₅(ONO)]²⁺. The nitro form is thermodynamically more stable, while the nitrito is metastable and can be induced by thermal or photochemical means. The mechanisms of these isomerizations typically proceed via dissociative (D) or associative (A) pathways, analogous to substitution but adapted for intramolecular shifts. In paths, a metal- bond temporarily breaks, forming a five-coordinate intermediate in octahedral systems, followed by recoordination in the alternative mode; this is common for with high activation barriers around 100-120 kJ/mol for nitro-nitrito conversion. Associative mechanisms involve direct attack by the alternative donor atom without full dissociation, often favored in square planar complexes due to lower steric hindrance, with exchange barriers influenced by the metal's electronic configuration. Crystal field stabilization energy (CFSE) modulates isomer stability by favoring configurations that minimize d-orbital splitting penalties; for instance, in d⁶ low-spin octahedral Co(III) complexes, the relative stabilities of cis and trans geometric isomers are influenced by symmetrical fields and other factors such as .

Fluxional Processes in Organometallics

Fluxional processes in organometallic compounds involve rapid, intramolecular rearrangements that lead to time-averaged structures, often observable through spectroscopic techniques such as (NMR). These dynamic isomerizations, occurring on timescales faster than structural characterization methods, enable ligand exchanges or geometric interconversions without bond breaking, distinguishing them from dissociative mechanisms. In organometallics, fluxionality facilitates adaptive coordination geometries, which are crucial for reactivity in catalytic cycles. A prominent example of fluxionality is the Berry pseudorotation in five-coordinate complexes, where trigonal bipyramidal geometries undergo permutation of axial and equatorial ligands via a square pyramidal transition state. This mechanism, first proposed by , interconverts the 20 possible isomers of a general AX5 species through a low-energy pathway. In organometallic systems like Fe(CO)5, Berry pseudorotation results in averaging of carbonyl environments, with experimental activation barriers around 2-3 kcal/mol, enabling very rapid dynamics even at low temperatures. Hapticity changes, or haptotropic shifts, represent another key fluxional process, particularly in complexes featuring π-bound ligands like cyclopentadienyl (Cp). In these systems, the ligand can slip between η⁵ (pentahapto, full ring coordination) and η³ (trihapto, localized allyl-like binding) modes, altering the metal's to accommodate substrates or stabilize intermediates. For instance, in Cp complexes, such shifts occur with barriers around 15-18 kcal/mol, enabling fluxional behavior that supports selective bond activations. This dynamic slippage is often induced by electronic changes, such as oxidation or coordination of additional ligands. Detection of fluxional behavior relies heavily on variable-temperature NMR spectroscopy, where coalescence temperatures (T_c) mark the point at which exchanging sites become magnetically equivalent, broadening and then sharpening signals. The free energy of activation (ΔG‡) for these processes can be estimated using the Eyring equation adapted for fast exchange: ΔG‡=RTc[23.76+ln⁡(Tc2πδν)]\Delta G^\ddagger = RT_c \left[ 23.76 + \ln \left( \frac{T_c}{\sqrt{2} \pi \delta \nu} \right) \right]
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