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In chemistry, isomers are molecules or polyatomic ions with an identical molecular formula – that is, the same number of atoms of each element – but distinct arrangements of atoms in space.[1] Isomerism refers to the existence or possibility of isomers.

Isomers do not necessarily share similar chemical or physical properties. Two main forms of isomerism are structural (or constitutional) isomerism, in which bonds between the atoms differ; and stereoisomerism (or spatial isomerism), in which the bonds are the same but the relative positions of the atoms differ.

Isomeric relationships form a hierarchy. Two chemicals might be the same constitutional isomer, but upon deeper analysis be stereoisomers of each other. Two molecules that are the same stereoisomer as each other might be in different conformational forms or be different isotopologues. The depth of analysis depends on the field of study or the chemical and physical properties of interest.

The English word "isomer" (/ˈaɪsəmər/) is a back-formation from "isomeric",[2] which was borrowed through German isomerisch[3] from Swedish isomerisk; which in turn was coined from Greek ἰσόμερoς isómeros, with roots isos = "equal", méros = "part".[4]

Two broad types of isomers

Structural isomers

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Main article: Structural isomer

Structural isomers have the same number of atoms of each element (hence the same molecular formula), but the atoms are connected in distinct ways.[5]

Example: C
3H
8O

[edit]

For example, there are three distinct compounds with the molecular formula :

Structural isomers of C 3H 8O: I 1-propanol, II 2-propanol, III ethyl-methyl-ether.
Structural isomers of C
3H
8O: I 1-propanol, II 2-propanol, III ethyl-methyl-ether.

The first two isomers shown of are propanols, that is, alcohols derived from propane. Both have a chain of three carbon atoms connected by single bonds, with the remaining carbon valences being filled by seven hydrogen atoms and by a hydroxyl group comprising the oxygen atom bound to a hydrogen atom. These two isomers differ on which carbon the hydroxyl is bound to: either to an extremity of the carbon chain propan-1-ol (1-propanol, n-propyl alcohol, n-propanol; I) or to the middle carbon propan-2-ol (2-propanol, isopropyl alcohol, isopropanol; II). These can be described by the condensed structural formulas and .

The third isomer of is the ether methoxyethane (ethyl-methyl-ether; III). Unlike the other two, it has the oxygen atom connected to two carbons, and all eight hydrogens bonded directly to carbons. It can be described by the condensed formula .

The alcohol "3-propanol" is not another isomer, since the difference between it and 1-propanol is not real; it is only the result of an arbitrary choice in the direction of numbering the carbons along the chain. For the same reason, "ethoxymethane" is the same molecule as methoxyethane, not another isomer.

1-Propanol and 2-propanol are examples of positional isomers, which differ by the position at which certain features, such as double bonds or functional groups, occur on a "parent" molecule (propane, in that case).

Example: C
3H
4

[edit]

There are also three structural isomers of the hydrocarbon :

I Propadiene II Propyne III Cyclopropene

In two of the isomers, the three carbon atoms are connected in an open chain, but in one of them (propadiene or allene; I) the carbons are connected by two double bonds, while in the other (propyne or methylacetylene; II) they are connected by a single bond and a triple bond. In the third isomer (cyclopropene; III) the three carbons are connected into a ring by two single bonds and a double bond. In all three, the remaining valences of the carbon atoms are satisfied by the four hydrogens.

Again, note that there is only one structural isomer with a triple bond, because the other possible placement of that bond is just drawing the three carbons in a different order. For the same reason, there is only one cyclopropene, not three.

Tautomers

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Tautomers are structural isomers which readily interconvert, so that two or more species co-exist in equilibrium such as

.[6]

Important examples are keto-enol tautomerism and the equilibrium between neutral and zwitterionic forms of an amino acid.

Stereoisomers

[edit]
Two kinds of stereoisomers
Main article: Stereoisomerism

Stereoisomers have the same atoms or isotopes connected by bonds of the same type, but differ in the relative positions of those atoms in space. Two broad types of stereoisomers exist, enantiomers and diastereomers. Enantiomers have identical physical properties but diastereomers do not.[7]

Enantiomers

[edit]

Two compounds are said to be enantiomers if their molecules are mirror images of each other and cannot be made to coincide only by rotations or translations – like a left hand and a right hand. The two shapes are said to be chiral.

A classic example is bromochlorofluoromethane (). The two enantiomers can be distinguished, for example, by whether the path turns clockwise or counterclockwise as seen from the hydrogen atom. In order to change one conformation to the other, at some point those four atoms would have to lie on the same plane – which would require severely straining or breaking their bonds to the carbon atom. The corresponding energy barrier between the two conformations is so high that there is practically no conversion between them at room temperature, and they can be regarded as different configurations.

The compound chlorofluoromethane , in contrast, is not chiral; the mirror image of its molecule is also obtained by a half-turn about a suitable axis.

(R)-2,3-Pentadiene

Another example of a chiral compound is 2,3-pentadiene , a hydrocarbon that contains two overlapping double bonds. The double bonds are such that the three middle carbons are in a straight line, while the first three and last three lie on perpendicular planes. The molecule and its mirror image are not superimposable, even though the molecule has an axis of symmetry. The two enantiomers can be distinguished, for example, by the right-hand rule. This type of isomerism is called axial isomerism.

Enantiomers behave identically in chemical reactions, except when reacting with chiral compounds or in the presence of chiral catalysts, such as most enzymes. For this latter reason, the two enantiomers of most chiral compounds usually have markedly different effects and roles in living organisms. In biochemistry and food science, the two enantiomers of a chiral molecule – such as glucose – are usually identified and treated as very different substances.

Each enantiomer of a chiral compound typically rotates the plane of polarized light that passes through it. The rotation has the same magnitude but opposite senses for the two isomers, and can be a useful way of distinguishing and measuring their concentration in a solution. For this reason, enantiomers were formerly called "optical isomers".[8][9] However, this term is ambiguous and is discouraged by the IUPAC.[10][11]

Some enantiomer pairs (such as those of trans-cyclooctene) can be interconverted by internal motions that change bond lengths and angles only slightly. Other pairs (such as CHFClBr) cannot be interconverted without breaking bonds, and therefore are different configurations.

Diastereomers

[edit]

Stereoisomers that are not enantiomers are called diastereomers. Some diastereomers may contain chiral centers, and some may not.[12]

Cis-trans isomerism

[edit]

A double bond between two carbon atoms forces the remaining four bonds (if they are single) to lie on the same plane, perpendicular to the plane of the bond as defined by its π orbital. If the two bonds on each carbon connect to different atoms, two distinct conformations are possible that differ from each other by a twist of 180 degrees of one of the carbons about the double bond.

The classical example is dichloroethene , specifically the structural isomer that has one chlorine bonded to each carbon. It has two conformational isomers, with the two chlorines on the same side or on opposite sides of the double bond's plane. They are traditionally called cis (from Latin meaning "on this side of") and trans ("on the other side of"), respectively, or Z and E in the IUPAC recommended nomenclature. Conversion between these two forms usually requires temporarily breaking bonds (or turning the double bond into a single bond), so the two are considered different configurations of the molecule.

More generally, cis–trans isomerism (formerly called "geometric isomerism") occurs in molecules where the relative orientation of two distinguishable functional groups is restricted by a somewhat rigid framework of other atoms.[13]

For example, in the cyclic alcohol inositol (a six-fold alcohol of cyclohexane), the six-carbon cyclic backbone largely prevents the hydroxyl and the hydrogen on each carbon from switching places. Therefore, one has different configurational isomers depending on whether each hydroxyl is on "this side" or "the other side" of the ring's mean plane. Discounting isomers that are equivalent under rotations, there are nine isomers that differ by this criterion, and behave as different stable substances (two of them being enantiomers of each other). The most common one in nature (myo-inositol) has the hydroxyls on carbons 1, 2, 3 and 5 on the same side of that plane, and can therefore be called cis-1,2,3,5-trans-4,6-cyclohexanehexol. And each of these cis-trans isomers can possibly have stable "chair" or "boat" conformations (although the barriers between these are significantly lower than those between different cis-trans isomers).

The two isomeric complexes, cisplatin and transplatin, are examples of square planar MX2Y2 molecules with M = Pt.

Cis and trans isomers also occur in inorganic coordination compounds, such as square planar complexes and octahedral complexes.

For more complex organic molecules, the cis and trans labels can be ambiguous. In such cases, a more precise labeling scheme is employed based on the Cahn-Ingold-Prelog priority rules.[14][12]


Isotopes and spin

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Isotopomers

[edit]

Different isotopes of the same element can be considered as different kinds of atoms when enumerating isomers of a molecule or ion. The replacement of one or more atoms by their isotopes can create multiple structural isomers and/or stereoisomers from a single isomer.

For example, replacing two atoms of common hydrogen () by deuterium (, or ) on an ethane molecule yields two distinct structural isomers, depending on whether the substitutions are both on the same carbon (1,1-dideuteroethane, ) or one on each carbon (1,2-dideuteroethane, ); as if the substituent was chlorine instead of deuterium. The two molecules do not interconvert easily and have different properties, such as their microwave spectrum.[15]

Another example would be substituting one atom of deuterium for one of the hydrogens in chlorofluoromethane (). While the original molecule is not chiral and has a single isomer, the substitution creates a pair of chiral enantiomers of , which could be distinguished (at least in theory) by their optical activity.[16]

When two isomers would be identical if all isotopes of each element were replaced by a single isotope, they are described as isotopomers or isotopic isomers.[17] In the above two examples if all were replaced by , the two dideuteroethanes would both become ethane and the two deuterochlorofluoromethanes would both become .

The concept of isotopomers is different from isotopologs or isotopic homologs, which differ in their isotopic composition.[17] For example, and are isotopologues and not isotopomers, and are therefore not isomers of each other.

Spin isomers

[edit]

Another type of isomerism based on nuclear properties is spin isomerism, where molecules differ only in the relative spin magnetic quantum numbers ms of the constituent atomic nuclei. This phenomenon is significant for molecular hydrogen, which can be partially separated into two long-lived states described as spin isomers[18] or nuclear spin isomers:[19] parahydrogen, with the spins of the two nuclei pointing in opposite directions, and orthohydrogen, where the spins point in the same direction.

Applications

[edit]

Isomers having distinct biological properties are common; for example, the placement of methyl groups. In substituted xanthines, theobromine, found in chocolate, is a vasodilator with some effects in common with caffeine; but, if one of the two methyl groups is moved to a different position on the two-ring core, the isomer is theophylline, which has a variety of effects, including bronchodilation and anti-inflammatory action. Another example of this occurs in the phenethylamine-based stimulant drugs. Phentermine is a non-chiral compound with a weaker effect than that of amphetamine. It is used as an appetite-reducing medication and has mild or no stimulant properties. However, an alternate atomic arrangement gives dextromethamphetamine, which is a stronger stimulant than amphetamine.

In medicinal chemistry and biochemistry, enantiomers are a special concern because they may possess distinct biological activity. Many preparative procedures afford a mixture of equal amounts of both enantiomeric forms. In some cases, the enantiomers are separated by chromatography using chiral stationary phases. They may also be separated through the formation of diastereomeric salts. In other cases, enantioselective synthesis have been developed.

As an inorganic example, cisplatin (see structure above) is an important drug used in cancer chemotherapy, whereas the trans isomer (transplatin) has no useful pharmacological activity.

History

[edit]

Isomerism was first observed in 1827, when Friedrich Wöhler prepared silver cyanate and discovered that, although its elemental composition of was identical to silver fulminate (prepared by Justus von Liebig the previous year),[20] its properties were distinct. This finding challenged the prevailing chemical understanding of the time, which held that chemical compounds could be distinct only when their elemental compositions differ. (We now know that the bonding structures of fulminate and cyanate can be approximately described as ≡ and , respectively.)

Additional examples were found in succeeding years, such as Wöhler's 1828 discovery that urea has the same atomic composition () as the chemically distinct ammonium cyanate. (Their structures are now known to be and , respectively.) In 1830 Jöns Jacob Berzelius introduced the term isomerism to describe the phenomenon.[4][21][22][23]

In 1848, Louis Pasteur observed that tartaric acid crystals came into two kinds of shapes that were mirror images of each other. Separating the crystals by hand, he obtained two version of tartaric acid, each of which would crystallize in only one of the two shapes, and rotated the plane of polarized light to the same degree but in opposite directions.[24][25] In 1860, Pasteur explicitly hypothesized that the molecules of isomers might have the same composition but different arrangements of their atoms.[26]

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Isomer

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In chemistry, an isomer is one of several molecular entities that possess the same molecular formula but differ in their connectivity of atoms (constitutional isomers) or in the spatial arrangement of atoms (stereoisomers). These structural variations lead to distinct physical and chemical properties, despite the identical elemental composition. Isomers are broadly classified into two major categories: constitutional isomers and stereoisomers. Constitutional isomers, also known as structural isomers, have the same molecular formula but different bonding arrangements between atoms; for example, butane (CH₃CH₂CH₂CH₃) and isobutane ((CH₃)₂CHCH₃) are constitutional isomers of C₄H₁₀, exhibiting different boiling points due to variations in molecular shape. Stereoisomers, in contrast, share the same connectivity but differ in the three-dimensional orientation of atoms; they are subdivided into geometric isomers (such as cis-trans isomers in alkenes like (Z)-2-butene and (E)-2-butene) and optical isomers (enantiomers, which are non-superimposable mirror images, like D-glucose and L-glucose). Conformational isomers, a subset of stereoisomers, arise from rotation around single bonds and interconvert more readily, as seen in the staggered and eclipsed forms of ethane. The study of isomerism is fundamental to organic and , influencing molecular reactivity, stability, and function. In and , stereoisomers often exhibit profoundly different effects; for instance, one of a may be therapeutic while its is inactive or toxic, as exemplified by the (S)-ibuprofen versus its less active (R)-. reactions, catalyzed by enzymes called isomerases, play critical roles in metabolic pathways, underscoring the biological relevance of these structural differences. Overall, isomerism highlights the diversity possible within a fixed atomic composition, driving advancements in synthesis, , and .

Fundamentals

Definition

In chemistry, an isomer is defined as one of several molecular entities that possess the same molecular formula but differ in their connectivity or spatial arrangement of atoms. This results in distinct physical and chemical properties despite the identical atomic composition. Isomers arise primarily from variations in the bonding patterns (connectivity) between atoms or from different three-dimensional configurations, which can significantly influence reactivity, stability, and . A fundamental prerequisite for understanding isomers is the concept of a molecular formula, which specifies the exact number and types of atoms in a , such as C₄H₁₀ for the isomers. These differences in atomic arrangement lead to compounds that, while sharing the same formula, exhibit unique behaviors under the same conditions. It is important to distinguish isomers from related concepts like isotopes and allotropes. Isotopes refer to variants of the same that have identical atomic numbers but different mass numbers due to varying numbers of neutrons in the nucleus, resulting in the same but altered nuclear properties. In contrast, allotropes are different structural forms of the same element, such as and for carbon, where the atomic connectivity varies but the elemental composition remains uniform. Isomers, therefore, apply to compounds rather than elements or atomic nuclei. Isomers are broadly classified into constitutional isomers, which differ in atomic connectivity, and stereoisomers, which share connectivity but vary in spatial orientation.

Classification

Isomers are broadly classified into two primary categories: constitutional isomers and stereoisomers, based on differences in atomic connectivity and spatial arrangement, respectively. Constitutional isomers, also termed structural isomers, share the same molecular but exhibit variations in the bonding sequence or connectivity of atoms, leading to distinct molecular structures. This category is hierarchically subdivided into skeletal isomers, which differ in the arrangement of the carbon or chain branching; positional isomers, which involve differences in the location of functional groups, double bonds, or substituents along the chain; and functional isomers, which possess different functional groups despite the same overall . In contrast, stereoisomers maintain identical atomic connectivity and molecular but differ in the three-dimensional orientation of atoms or groups in space. Stereoisomers are further classified into enantiomers and diastereomers. Enantiomers are pairs of stereoisomers that are nonsuperimposable mirror images of each other, arising from centers or other asymmetric features. Diastereomers encompass all other stereoisomers that are not enantiomers, including geometric isomers (such as cis-trans isomers in alkenes or rings), which result from restricted rotation around bonds. This classification hinges on the prerequisite that constitutional isomers involve altered connectivity, whereas stereoisomers presuppose identical connectivity with variations solely in spatial configuration. Beyond these classical molecular isomers, related variants include isotopic and nuclear forms, which extend the concept but deviate from the standard definition of identical atomic composition. Isotopomers differ in the positional arrangement of isotopic atoms while maintaining the same isotopic composition, and isotopologues vary in their overall isotopic substitution, though these are not true isomers due to mass differences affecting the molecular formula when isotopes are distinguished. Nuclear isomers, conversely, represent long-lived excited states of atomic nuclei with the same proton and neutron numbers but differing configurations, classified into types such as spin, , K-, and fission isomers based on the hindrance mechanisms for decay; these are not molecular isomers but share terminological roots in .

Constitutional Isomers

Skeletal Isomers

Skeletal isomers, also known as isomers, are a subtype of constitutional isomers in which compounds share the same molecular formula and functional groups but differ in the arrangement or branching of their carbon skeleton. This variation in carbon connectivity leads to distinct molecular shapes while preserving the overall composition. Such isomerism is prevalent among alkanes, saturated hydrocarbons with the general molecular formula CnH2n+2C_nH_{2n+2}, where nn represents the number of carbon atoms. For instance, the C4H10C_4H_{10} isomers n-butane and exemplify skeletal isomerism: n-butane features a linear carbon (CH₃-CH₂-CH₂-CH₃), whereas isobutane has a branched structure ((CH₃)₃CH). These structural differences significantly affect physical properties, such as boiling points, due to variations in molecular and intermolecular forces. n-Butane boils at -0.5°C, while boils at -11.7°C; the branched adopts a more compact, spherical form, reducing surface area for van der Waals interactions and thus requiring less energy to vaporize.

Positional and Functional Isomers

Positional isomers are constitutional isomers that share the same carbon skeleton and functional groups but differ in the position of these groups or multiple bonds along the chain. For example, (\ceCH3CH2CH2OH\ce{CH3CH2CH2OH}) and 2-propanol (\ceCH3CH(OH)CH3\ce{CH3CH(OH)CH3}) both have the molecular formula \ceC3H8O\ce{C3H8O} and a hydroxyl group, but the -OH is attached to different carbon atoms, leading to variations in points and reactivity. Another instance involves alkenes like 1-butene (\ceCH2=CHCH2CH3\ce{CH2=CHCH2CH3}) and 2-butene (\ceCH3CH=CHCH3\ce{CH3CH=CHCH3}), where the double bond's location shifts, affecting stability and addition reactions. Functional isomers, in contrast, possess the same molecular formula but differ in the types of functional groups present, resulting in distinct chemical behaviors despite identical atomic compositions. A classic pair is (\ceCH3CH2OH\ce{CH3CH2OH}) and (\ceCH3OCH3\ce{CH3OCH3}), both \ceC2H6O\ce{C2H6O}, where the former features an alcohol group and the latter an linkage; this leads to ethanol's ability to form bonds, yielding a higher (78.4°C) compared to dimethyl ether's (-24.8°C). Similarly, propanal (\ceCH3CH2CHO\ce{CH3CH2CHO}) and propanone (\ceCH3COCH3\ce{CH3COCH3}), both \ceC3H6O\ce{C3H6O}, represent and functional groups, influencing their oxidation products—propanal oxidizes to propanoic acid, while propanone resists further oxidation under mild conditions. Metamerism represents a subtype of functional isomerism, characterized by differences in the alkyl chain lengths attached to a polyvalent , such as in ethers or amines, while maintaining the same overall formula. For instance, (\ce(CH3CH2)2O\ce{(CH3CH2)2O}) and methyl propyl ether (\ceCH3OCH2CH2CH3\ce{CH3OCH2CH2CH3}), both \ceC4H10O\ce{C4H10O}, exhibit this variation around the oxygen, resulting in subtle differences in and . Metamerism is particularly relevant in compounds with divalent heteroatoms, highlighting how chain distribution impacts physical properties without altering the core . These isomer types often display marked differences in physicochemical properties and reactivity due to their structural variations. In functional isomers like alcohols and ethers, alcohols engage in hydrogen bonding, enhancing in and elevating points relative to ethers of comparable . Reactivity diverges significantly: alcohols undergo oxidation to aldehydes, ketones, or carboxylic acids depending on the conditions, whereas ethers are largely inert to such transformations and resist nucleophilic attack under neutral conditions. Positional isomers, while sharing reactivity patterns, may show nuanced differences, such as 1-propanol's facilitating esterification more readily than the secondary 2-propanol. Overall, these distinctions underscore the importance of precise in predicting compound behavior.

Tautomers

Tautomers represent a specialized subset of constitutional isomers that interconvert rapidly through tautomerization, a involving the relocation of a (or proton) and a concomitant rearrangement of bonds, typically a shifting to maintain valence. This dynamic equilibrium distinguishes tautomers from static isomers, as the structures exist in reversible balance rather than as isolated compounds. The term "tautomer" derives from Greek roots meaning "same" and "part," reflecting their identical molecular formula but differing atomic arrangements. A classic example of tautomerism is keto-enol tautomerism, observed in compounds like acetone. In its keto form, acetone exists as CH3C(O)CH3CH_3C(O)CH_3, featuring a , while the form is CH2=C(OH)CH3CH_2=C(OH)CH_3, with a hydroxyl group attached to a carbon-carbon . The equilibrium strongly favors the keto tautomer, with an (KeqK_{eq}) of approximately 5×10−95 \times 10^{-9} in at , indicating that less than 0.001% of acetone molecules adopt the form under standard conditions. The mechanism of tautomerization generally proceeds via proton transfer, often facilitated by or base to overcome the barrier in neutral conditions. In acid-catalyzed keto-enol interconversion, the carbonyl oxygen is first protonated to form a resonance-stabilized intermediate, followed by from the alpha carbon to yield the ; the reverse path regenerates the keto form. Base-catalyzed mechanisms involve at the alpha carbon to generate an ion, which is then protonated on the oxygen. These pathways highlight the role of labile protons in enabling the bond shifts. Tautomerism significantly influences molecular reactivity, as the distinct functional groups in each form lead to varied chemical behaviors. For instance, the tautomer of acetone exhibits enhanced nucleophilicity at the alpha carbon due to the electron-rich structure, facilitating reactions like electrophilic additions that are less favorable for the keto form. This duality allows to participate in diverse synthetic pathways, such as aldol condensations, where the or acts as a . In biological contexts, tautomerism plays a critical role in nucleic acids, particularly through rare tautomeric forms of DNA bases that can lead to . For example, the standard keto or amino forms of bases like or ensure faithful Watson-Crick base pairing during replication, but transient shifts to or imino tautomers enable mismatched pairings (e.g., with instead of ), with rare tautomeric forms occurring at low fractions (estimated ~10^{-4} to 10^{-6}). Such events underscore tautomerism's impact on genetic fidelity and evolutionary processes.

Stereoisomers

Enantiomers

Enantiomers are one of a pair of stereoisomers that are non-superimposable mirror images of each other. They arise from molecules that exhibit chirality, where the spatial arrangement of atoms cannot be superimposed on its mirror image. Unlike constitutional isomers, enantiomers share the same molecular formula and connectivity but differ in the configuration at one or more chiral centers. Chirality in enantiomers typically requires the presence of at least one chiral center, most commonly a tetrahedral carbon atom bonded to four different substituents, resulting in a stereogenic center. This asymmetry leads to the two possible configurations, often designated as (R) and (S) according to the Cahn-Ingold-Prelog priority rules. Without such a chiral element, molecules lack the handedness necessary for enantiomerism, and their mirror images are superimposable. Enantiomers possess identical physical properties, such as melting points, boiling points, and solubilities, but they differ in their interaction with plane-polarized light, rotating it in opposite directions—a known as optical activity. The , a measure of this effect, is equal in magnitude but opposite in sign for each . For instance, (S)-(+)- has a specific rotation of +3.8° at 589 nm, while its enantiomer, (R)-(-)-lactic acid, has -3.8° under the same conditions. This optical distinction arises because chiral molecules absorb left- and right-circularly polarized light differently. A , or racemate, consists of equal proportions of both s and exhibits no net due to mutual cancellation. Such mixtures are common in synthesis without chiral control and can be resolved into pure enantiomers using techniques like chiral . Enantiomers of one compound may form diastereomeric relationships with stereoisomers of related compounds, leading to differing properties in those contexts. projections provide a conventional two-dimensional representation of enantiomers, depicting the chiral center as a cross with horizontal bonds projecting forward and vertical bonds receding. For , the (S) enantiomer is shown with the hydroxyl group on the left in the standard orientation, contrasting with the (R) form on the right. This method facilitates visualization of the mirror-image relationship without three-dimensional models.

Diastereomers

Diastereomers are defined as stereoisomers that are not mirror images of one another and thus not enantiomers. They arise in molecules with two or more chiral centers, where the stereoisomers differ in configuration at one or more, but not all, of these centers. This configuration difference leads to distinct spatial arrangements that result in varying physical and chemical properties, unlike the identical properties (except for optical rotation) observed in enantiomers. A classic example of diastereomers is found in tartaric acid, where the (2R,3R)-tartaric acid and the meso form (2R,3S)-tartaric acid differ in configuration at one chiral center. The meso form, being achiral due to an internal plane of symmetry, exhibits different solubility in water compared to the chiral (2R,3R) form; for instance, the meso isomer has lower solubility (125 g/100 mL) compared to the chiral form (135 g/100 mL), allowing separation via fractional crystallization. This difference in properties highlights how diastereomers can be resolved using conventional techniques like chromatography or distillation, in contrast to enantiomers which require specialized methods such as chiral resolution agents. Diastereomers require the presence of multiple stereogenic centers or other elements of to exist, as a single chiral center can only produce enantiomers. The term encompasses a broader range of stereoisomers than just those from chiral centers, including geometric isomers arising from restricted rotation, though the focus here is on chiral variants. A specific subtype of diastereomers is epimers, which are stereoisomers that differ in configuration at only one chiral center while maintaining the same configuration at all others. Epimers are particularly relevant in chemistry, where they influence biological recognition and reactivity.

Geometric Isomers

Geometric isomers, also referred to as cis-trans isomers, are stereoisomers that result from the restricted rotation about a bond, typically a carbon-carbon in alkenes or within cyclic structures like cycloalkanes, leading to distinct spatial arrangements of substituents. This form of isomerism is a subtype of diastereomerism, where the isomers are not mirror images. In alkenes, the rigidity of the double bond prevents rotation, allowing for two configurations when each carbon of the double bond is attached to two different substituents: the cis isomer, in which the higher-priority substituents (or similar groups) are on the same side of the double bond, and the trans isomer, in which they are on opposite sides. A classic example is 2-butene (CH₃-CH=CH-CH₃), where cis-2-butene has both methyl groups on the same side and a boiling point of 3.7 °C, while trans-2-butene has them on opposite sides with a boiling point of 0.9 °C; the difference arises from the greater dipole moment in the cis form, enhancing intermolecular forces./10:_Alkenes/10.04:_Physical_Properties)

Cis-2-butene: Trans-2-butene: CH3 CH3 | \ H-C=C-H H-C=C-H | / CH3 CH3

Cis-2-butene: Trans-2-butene: CH3 CH3 | \ H-C=C-H H-C=C-H | / CH3 CH3

When the two substituents on each carbon of the are different, the cis-trans nomenclature is insufficient, and the E/Z system is employed, based on the Cahn-Ingold-Prelog (CIP) priority rules. These rules, established in a seminal 1966 review, assign priorities to substituents by comparing s at the first point of difference (higher atomic number receives higher priority); if tied, multiple bonds are treated as duplicated atoms for comparison. The (zusammen, "together") designation indicates higher-priority groups on the same side, analogous to cis, while (entgegen, "opposite") indicates they are on opposite sides, analogous to trans. This system ensures unambiguous naming for complex alkenes and is widely applied in . Geometric isomerism is also prevalent in cycloalkanes, where limits conformational flexibility, particularly in disubstituted rings like 1,2-dimethylcyclopentane or 1,3-dimethylcyclohexane. In these cases, cis isomers have substituents on the same face of the ring, while trans isomers have them on opposite faces; for instance, trans-1,2-dimethylcyclopropane is more stable due to reduced steric repulsion compared to its cis counterpart in small rings. Such isomers exhibit different physical properties, including boiling points and solubilities, influencing their roles in materials and biological systems.

Isotopic and Nuclear Variants

Isotopomers and Isotopologues

Isotopologues are molecular entities that differ only in their isotopic composition, specifically the number of isotopic substitutions present in the molecule. For instance, methane isotopologues include CH₄ (all protium), CH₃D (one deuterium substitution), and CH₂D₂ (two deuterium substitutions), where the isotopic variants replace protium with heavier isotopes like deuterium or tritium while maintaining the same connectivity of atoms. These variants arise naturally in low abundances or can be synthesized for specific applications, and their masses differ due to the varying neutron counts in the isotopes, leading to distinct physical properties such as vibrational frequencies and diffusion rates. Isotopomers represent a more specific subclass of isotopologues, defined as isomers that have the same number of each isotopic atom but differ in the positions of those isotopes within the molecule. The term is a contraction of "isotopic isomer," emphasizing their structural similarity except for isotope placement. For example, in ethane (C₂H₆), the isotopomers ¹³CH₃CH₂D and CH₃¹³CH₂D both contain one carbon-12, one carbon-13, one deuterium, and five protiums, but the deuterium and carbon-13 occupy different positions relative to each other. This positional difference becomes relevant in techniques that probe atomic environments, as the isotopes' locations can influence local electronic or magnetic properties without altering the overall molecular formula or bonding. A classic example involves ethanol (C₂H₅OH), where isotopologues such as CH₃CH₂OH (all protium) and CH₃CD₂OH (deuterated methylene group) exhibit nearly identical chemical reactivity but differ in molecular mass and spectroscopic signatures. In nuclear magnetic resonance (NMR) spectroscopy, these isotopic substitutions cause shifts in resonance frequencies due to the isotope effect, where heavier isotopes like deuterium alter the spin-spin coupling and chemical shifts of neighboring protons; for instance, the methylene protons in CH₃CH₂OH appear as a quartet at around 3.7 ppm in ¹H NMR, but in CH₃CHD₂, the signals split differently owing to the reduced coupling from deuterium's lower gyromagnetic ratio. This distinction allows NMR to resolve specific isotopomers, such as those with deuterium at the methyl (CH₂DCH₂OH), methylene (CH₃CHDOH), or hydroxyl (CH₃CH₂OD) positions, enabling precise analysis of isotopic distributions. Despite their classification as isotopic isomers, isotopologues and isotopomers are often distinguished from classical constitutional or stereoisomers because the latter typically assume identical nuclidic composition, whereas isotopic variants involve different atomic masses that subtly affect physical but not chemical properties. Their primary utility lies in and tracing applications, where the mass differences facilitate tracking molecular pathways without significantly perturbing reactivity. In isotope tracing, for example, stable isotopologues like ¹³C- or ²H-labeled metabolites are introduced into biological systems to monitor flux through pathways, as can distinguish enriched isotopologues from natural abundance ones, revealing and substrate utilization. This approach has been instrumental in elucidating dynamic processes in cellular , such as , by quantifying the incorporation of labeled carbons into downstream products.

Spin Isomers

Spin isomers refer to molecular species that differ solely in the orientation of their nuclear spins, leading to distinct quantum states due to the indistinguishability of identical nuclei. In the case of dihydrogen (H₂), protons are fermions, and the total wavefunction must be antisymmetric, coupling nuclear spin symmetry to . Ortho-hydrogen features parallel nuclear spins (total spin quantum number I=1I = 1, ), allowing only odd rotational quantum numbers J=1,3,5,…J = 1, 3, 5, \dots, while para-hydrogen has antiparallel spins (I=0I = 0, ), restricted to even J=0,2,4,…J = 0, 2, 4, \dots./Quantum_Mechanics/11:_Molecules/Ortho_and_Para_hydrogen) The energy difference between the ground states of these isomers arises from the rotational levels, with para-H₂ in the J=0J = 0 state at zero energy and ortho-H₂ in the J=1J = 1 state. The is given by EJ=BJ(J+1),E_J = B J (J + 1), where BB is the rotational constant for H₂ (B≈60.85B \approx 60.85 cm⁻¹). Thus, the ground-state energy separation is ΔE=2B≈122\Delta E = 2B \approx 122 cm⁻¹ (or about 175 K), making ortho-H₂ the higher-energy form. At low temperatures, such as below 20 K, the equilibrium favors para-H₂, but interconversion between ortho and para forms is kinetically hindered in the gas phase, occurring slowly (on the order of days to years) without a catalyst, allowing the isomers to be treated as separate species. This slow equilibration impacts applications like , where unconverted ortho-H₂ leads to boiling anomalies. These spin isomers influence spectroscopic properties, as ortho- and para-H₂ exhibit distinct Raman and spectra due to their differing allowed rotational transitions, enabling their quantification and study in contexts. The distinction highlights Pauli exclusion principles in molecular , affecting thermodynamic properties like specific heat at low temperatures. In modern applications, para-H₂'s high nuclear spin polarization has been utilized in (NMR) , where it serves as a resource for implementing quantum algorithms with nearly pure states. Unlike isotopomers, which involve variations in isotopic mass without altering spin statistics, spin isomers maintain identical atomic composition but differ in spin alignment./Quantum_Mechanics/11:_Molecules/Ortho_and_Para_hydrogen)

Nuclear Isomers

Nuclear isomers are excited states of atomic nuclei that have the same (Z) and (A) as the but possess significantly longer lifetimes due to hindered transitions back to the lower . These metastable states arise when the nucleus is trapped in a configuration where electromagnetic transitions, such as gamma decay, are suppressed by selection rules related to , parity, or other quantum mechanical factors. Unlike typical excited nuclear states that decay almost instantaneously (on the order of 10^{-12} seconds or less), nuclear isomers are conventionally defined as those with half-lives exceeding about 10^{-9} seconds. The primary decay mode for nuclear isomers is isomeric transition, involving the emission of gamma rays to release the excess energy and return to the ground state, though internal conversion or other processes can also occur. In standard notation, the metastable state is denoted by superscripting an "m" after the mass number, such as ^{60m}\ce{Co} for the isomer of cobalt-60, distinguishing it from the ground state ^{60}\ce{Co}. This notation highlights the isomeric nature without altering the elemental symbol or mass indication. Building on isotopic basics, where isotopes share Z but differ in A due to neutron count, nuclear isomers extend this concept to energy configurations within the same nuclide. A prominent example is (^{99m}\ce{Tc}), an isomer of the ground-state with a of approximately 6 hours, decaying primarily by gamma emission at 140 keV. This isomer is produced from the decay of molybdenum-99 and is widely used in (SPECT) for , allowing visualization of organs and tissues with minimal radiation dose due to its short and pure gamma emission. Beyond diagnostics, nuclear isomers hold significance in radiotherapy, where selective separation and utilization can enhance therapeutic efficacy; for instance, isolating the long-lived isomer lutetium-177m from lutetium-177 enables targeted beta-particle therapy for cancers like prostate carcinoma by avoiding unwanted gamma emissions from the isomer. Such applications underscore the potential of nuclear isomers to provide precise energy release for both and treatment, filling gaps in by leveraging metastable states for controlled radiation delivery.

Applications

In Pharmaceuticals and Biology

In pharmaceuticals, the of often depends on the specific , with one form potentially therapeutic while the other is inactive or harmful. For example, , marketed as a in the late for treating in pregnant women, caused severe birth defects in over 10,000 children worldwide due to the teratogenic (S)-, despite the (R)- providing effects. This tragedy highlighted the risks of racemates, as racemize , complicating efforts to administer a pure non-toxic form. Biological systems exhibit , where enzymes distinguish between enantiomers to ensure precise molecular interactions. Naturally occurring proteins consist exclusively of L-amino acids, a maintained through stereospecific enzymatic processes like those catalyzed by synthetases, which reject D-enantiomers during protein synthesis. This selectivity supports proper and function, underscoring why disruptions in can impair biological pathways. Tautomerism in nucleobases contributes to genetic instability by enabling rare or imino forms that disrupt standard Watson-Crick base pairing during . For instance, the tautomer of can pair with instead of , leading to T-A to C-G transition . Such spontaneous tautomerizations, though infrequent, account for a portion of baseline mutation rates in cells. The incident prompted regulatory reforms, including the U.S. Food and Drug Administration's (FDA) 1992 policy on stereoisomeric drugs, which mandates stereospecific assays for evaluation in , , and during development. The guidelines favor single- drugs when one isomer demonstrates superior efficacy or safety over the racemate and require stability testing to detect potential . These standards ensure chiral integrity in and labeling, influencing global approaches to chiral drug approval.

In Materials and Synthesis

Isomers play a crucial role in and synthetic chemistry by enabling the tailoring of physical and chemical properties through structural variations. In polymer synthesis, geometric isomerism significantly influences mechanical behavior; for instance, cis-1,4-polyisoprene forms the elastic , while trans-1,4-polyisoprene yields the rigid , due to differences in chain packing and flexibility. These distinct properties arise from the spatial arrangement around double bonds, allowing selective techniques to produce materials with targeted elasticity or hardness. Constitutional isomers, differing in carbon connectivity, are essential in fuel design to enhance performance. Branched alkanes, such as (isooctane), exhibit higher octane ratings compared to straight-chain n-octane, reducing and improving combustion efficiency in . This isomer-specific branching increases volatility and resistance to autoignition, guiding processes to optimize blends for higher output. Achieving selective formation of desired isomers poses significant synthetic challenges, particularly for enantiomers in chiral materials. Asymmetric catalysis addresses this by enabling stereoselective reactions, as recognized by the 2001 Nobel Prize in Chemistry awarded to William S. Knowles, Ryoji Noyori, and K. Barry Sharpless for developing chiral catalysts that produce enantiomerically pure compounds through and oxidation. These methods, such as Noyori's ruthenium-based catalysts, achieve high enantiomeric excesses (up to 100%) in industrial syntheses, facilitating the production of optically active polymers and ligands for advanced materials. In applications, geometric isomers enable responsive behaviors critical for optoelectronic devices. derivatives, which undergo reversible trans-to-cis , form liquid crystalline phases where the bent cis form disrupts nematic order, inducing phase transitions used in actuators and sensors. This isomerism allows light-controlled switching of , with trans isomers promoting aligned mesophases and cis isomers enabling rapid disordering, as demonstrated in elastomers with minimal cis content (under 4%) triggering complete order-to-disorder changes.

History

Early Concepts

The concept of isomerism emerged in the early amid growing recognition that compounds could share identical empirical formulas yet exhibit distinct properties, challenging prevailing notions of . This period predated modern understandings of atomic structure and electron configurations, relying instead on empirical analyses and observations of reactivity and physical characteristics. In 1830, Swedish chemist coined the term "isomerism" (from Greek roots meaning "equal parts") to describe such compounds, drawing on earlier observations of substances like silver cyanate (AgOCN) and (AgCNO). These silver salts, first synthesized by and in the 1820s, possessed the same elemental composition but differed markedly in stability—fulminate being highly while cyanate was relatively inert—prompting Berzelius to propose isomerism as a fundamental chemical phenomenon. During the 1830s, French chemist Jean-Baptiste André Dumas advanced early ideas about isomerism through his discoveries in substitution reactions, such as the chlorination of alcohol to form ether-like compounds. Dumas' work demonstrated that atoms could be replaced by equivalents like without altering the overall combining capacity, laying groundwork for structural theories that explained why isomers might arise from different atomic arrangements rather than compositional differences alone. In the 1850s, Italian chemist contributed to the understanding of isomerism by incorporating examples like the variants into his advocacy for Avogadro's on molecular weights. Through his 1858 "Sunto di un corso di filosofia chimica," Cannizzaro illustrated how isomers, such as the optically active forms of , supported distinctions between empirical and true molecular formulas, influencing the resolution of atomic weight debates at the 1860 .

Development of Stereochemistry

The development of stereochemistry began with Louis Pasteur's groundbreaking experiments in 1848, when he manually separated the enantiomers of sodium ammonium tartrate by crystallizing the compound and sorting the resulting hemihedral crystals under a , demonstrating that the two forms were mirror images with opposite optical rotations. This resolution marked the first isolation of enantiomers and provided for the existence of molecular handedness, challenging prevailing views that optical activity was a property of the crystalline form rather than the molecule itself. Pasteur's work built on the earlier discovery of optical activity by in 1815, who observed that certain organic solutions rotated plane-polarized light, but it was Pasteur who, in the , explained this as arising from the asymmetric arrangement of atoms within the , introducing the concept of molecular dissymmetry. Pasteur's correlation of crystal morphology with in tartrates solidified the link between spatial structure and physical properties, laying the foundation for as a distinct field. A pivotal theoretical advance came in 1874, when and Joseph Achille Le Bel independently proposed that the carbon atom has a tetrahedral geometry, with its four bonds directed toward the vertices of a , to account for the existence of enantiomers and the observed optical activity in compounds like . This model resolved the puzzle of why certain molecules with identical connectivity exhibited different properties, predicting that four different substituents on a carbon would yield non-superimposable mirror images, thus formalizing the structural basis for . By the mid-20th century, the need for a systematic led to the Cahn-Ingold-Prelog (CIP) rules, introduced in 1956 by Robert Sidney Cahn, Christopher Kelk Ingold, and , which provide a priority-based method to assign absolute configurations (R or S) to chiral centers by ranking substituents according to and other criteria. These rules standardized the designation of enantiomers, enabling precise communication of stereochemical information across the and facilitating advances in synthesis and analysis.

Modern Advances

In the mid-20th century, (NMR) spectroscopy emerged as a cornerstone technique for distinguishing structural and stereoisomers through differences in chemical shifts and resonance multiplicities, enabling precise identification of molecular configurations in solution. Complementing NMR, has advanced significantly since the 1950s, providing atomic-resolution structures that reveal isomer-specific bonding and packing arrangements, particularly in crystalline solids and nanoparticles. For instance, single-crystal analysis has elucidated structural isomerism in gold nanoparticles, highlighting subtle geometric variations otherwise undetectable. For separation, chiral (HPLC) became widely adopted in the late 20th century, utilizing chiral stationary phases to exploit differential interactions between enantiomers and achieve baseline resolutions. This method's high efficiency and scalability have made it indispensable for purifying optical isomers in pharmaceutical production, with recent enhancements incorporating ultra-high-performance variants for faster analyses. Quantum chemistry computations, particularly (DFT) methods developed in the 1990s and refined thereafter, have revolutionized the prediction of equilibria by calculating relative energies and effects with high accuracy. These approaches model proton transfer pathways and predict dominant s in solution, aiding where tautomeric forms influence reactivity and binding affinity. In the 2010s, supramolecular isomerism gained prominence in metal-organic frameworks (MOFs), where subtle changes in orientation or conditions yield frameworks with identical compositions but distinct topologies and porosities. For example, isomers of MOF-74 exhibit tunable pore sizes and guest-binding properties, controlled by synthesis modulators, enabling applications in gas storage and separation. Recent reviews underscore how such isomerism enhances MOF functionality through directed . Advances in ultracold quantum gases have illuminated spin isomers—distinct nuclear spin configurations like ortho and para forms in diatomic molecules—through coherent control and superposition states. In fermionic molecules such as NaK, researchers have achieved stable coherence between spin-isomer states at near-absolute zero, opening pathways to and entanglement studies. Computational and nanoscale applications continue to evolve, with DFT-integrated protocols predicting stable isomers in for 2020s innovations in and sensing. For instance, machine learning-enhanced forecast phase-separated nanostructures, ensuring desired isomer dominance at the atomic scale for . These tools address scalability challenges, providing 2025-relevant insights into isomer-selective synthesis in quantum dots and hybrid .

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