Enantiomer

An enantiomer is one member of a pair of stereoisomers whose three-dimensional structures are mirror images that cannot be superposed. The two members have the same atomic composition and connectivity but opposite molecular handedness, analogous to left and right hands. This relationship is called enantiomerism, and the underlying geometric property is chirality. Enantiomers generally have identical ordinary physical properties in achiral environments but can behave differently when interacting with other chiral substances. (goldbook.iupac.org)

Structural basis

Enantiomerism is a form of isomerism involving spatial arrangement rather than differences in which atoms are bonded together. A pair of mirror-image drawings is not necessarily a pair of enantiomers: if rotation and translation allow all corresponding atoms to coincide, the drawings represent the same structure. A mirror image must be both structurally corresponding and non-superposable to constitute a distinct enantiomer. (iupac.qmul.ac.uk)

A common source of chirality is a tetrahedral carbon atom attached to four different substituents. Such an atom is a stereogenic center. Its substituents can occupy two mirror-related arrangements. However, chirality is a property of the complete molecule, not simply a count of particular atoms: molecules with several stereogenic centers can be achiral because of their overall symmetry. (iupac.org)

Enantiomers do not require a tetrahedral carbon center. Chirality can also arise from:

• Axial chirality, as in suitably substituted allenes and biphenyls.
• Planar chirality, as in certain constrained cyclic structures.
• Helical chirality, in structures with opposite senses of helical winding.
• Coordination geometry, including mirror-related arrangements of ligands around a metal center. (iupac.qmul.ac.uk)

Whether mirror-related arrangements can be isolated separately also depends on how rapidly they interconvert. atropisomerism, for example, arises when restricted rotation makes conformational stereoisomers sufficiently persistent to be distinguished as separate chemical species. (iupac.qmul.ac.uk)

Relationship to other stereoisomers

Enantiomers must be distinguished from diastereomers, which are stereoisomers that are not mirror images. Diastereomers can differ in physical properties and in their reactions with both achiral and chiral reagents. Consequently, techniques that separate diastereomers do not necessarily separate an enantiomeric pair. (iupac.qmul.ac.uk)

A meso compound contains stereogenic centers but is achiral as a whole. Tartaric acid illustrates the distinction: its two chiral forms constitute an enantiomeric pair, while meso-tartaric acid is a separate, achiral stereoisomer. The absence of optical rotation in a meso compound reflects its molecular symmetry, not cancellation between two different enantiomers in a mixture. (iupac.org)

For ordinary tetrahedral stereogenic centers, reflection reverses their configurations. Nevertheless, identifying an enantiomer requires examining the entire structure, including any symmetry that could make apparently different representations identical. (iupac.org)

Naming and configuration

The absolute configuration of a stereogenic center is commonly described using R and S, assigned through the Cahn–Ingold–Prelog priority rules. These rules rank substituents and specify their spatial order. Molecules containing several centers generally require a descriptor and position number for each relevant center. (iupac.org)

Three naming conventions describe different things:

There is no general rule that an R configuration rotates light positively or that an S configuration rotates it negatively. Likewise, D and L do not indicate the sign of optical rotation. Confusing these conventions can lead to incorrect identification of an enantiomer. (iupac.qmul.ac.uk)

Physical properties and optical activity

Under comparable achiral conditions, pure enantiomers generally have the same melting point, boiling point, solubility in achiral solvents, and ordinary chemical reactivity. Their important optical distinction is that they rotate the plane of linearly polarized light by equal amounts in opposite directions under the same measurement conditions. This phenomenon is optical activity. (iupac.org)

Optical rotation depends on experimental conditions, including wavelength, temperature, solvent, concentration, and path length. A rotation measurement therefore requires these conditions to be specified. Rotation alone does not assign an R or S configuration. (iupac.qmul.ac.uk)

Chiral substances can also be investigated through circular dichroism, which measures differences in absorption of left- and right-circularly polarized light, and other chiroptical methods. These measurements provide information unavailable from ordinary achiral measurements. (iupac.qmul.ac.uk)

The equality of pure-enantiomer properties should not be confused with the properties of their mixtures. A racemic solid may have a different crystal arrangement and melting behavior from an enantiopure solid. (publications.iupac.org)

Racemates and enantiomeric composition

A racemate, or racemic mixture, contains equal amounts of two enantiomers. Their contributions to optical rotation cancel under ordinary solution measurement conditions. An enantiopure sample contains only one handedness within the limits of detection; an enantioenriched sample contains unequal amounts of the pair. (iupac.qmul.ac.uk)

Enantiomeric excess, abbreviated ee, quantifies this imbalance. For amounts $n_A$ and $n_B$ of the two enantiomers,

or, expressed as a percentage,

A 50:50 mixture has 0% ee, while a 90:10 mixture has 80% ee. The identity of the predominant enantiomer must be stated separately. Enantiomeric excess describes the relative amounts of the pair, not the sample’s overall chemical purity. (old.goldbook.iupac.org)

Optical purity, determined from a sample’s rotation relative to that of a pure enantiomer, is not unconditionally interchangeable with ee: the relationship requires an appropriate connection between composition and measured rotation. (iupac.qmul.ac.uk)

Separation and preparation

Separating an enantiomeric mixture into its components is called chiral resolution. Because enantiomers are ordinarily equivalent in achiral surroundings, separation commonly introduces a chiral selector or converts them into derivatives that have distinguishable properties. (iupac.qmul.ac.uk)

Important approaches include:

• Chiral chromatography: different interactions with a chiral selector produce different retention or migration behavior.
• Diastereomer formation: reaction with a single-enantiomer reagent produces diastereomeric derivatives that can be separated and subsequently converted back.
• Crystallization-based resolution: applicable when the relevant solid-state behavior permits separation.
• Kinetic resolution: a chiral reagent or catalyst reacts with one enantiomer faster than with the other. (old.iupac.org)

Alternatively, asymmetric catalysis can preferentially produce one enantiomer during synthesis. A chiral catalyst creates unequal reaction pathways leading to the two mirror-image products, allowing an enantioenriched product to be obtained without first preparing and resolving a racemate. Catalytic asymmetric synthesis was recognized by the 2001 Nobel Prize in Chemistry. (iupac.qmul.ac.uk)

Racemization is the conversion of an enantioenriched material toward a racemic composition. It is distinct from simple physical mixing: molecular interconversion changes the stereochemical composition over time and can affect the stability of an isolated enantiomer. (iupac.qmul.ac.uk)

Biological and pharmaceutical significance

Biological systems can distinguish enantiomers because many of their interacting components are chiral. An enantiomer and its mirror image can interact differently with a chiral binding site, making enantiomerism important in molecular recognition and pharmacology. (nobelprize.org)

For a chiral drug, the two enantiomers may differ in potency, biological effects, absorption, distribution, metabolism, or excretion. Some pairs have similar useful activities; others differ substantially. There is no universal rule that one enantiomer is beneficial and the other inactive or harmful, nor that a single-enantiomer product is necessarily preferable to a mixture. The properties of each pair require experimental characterization, including investigation of possible interconversion. (fda.gov)

Historical development

In 1848, Louis Pasteur separated mirror-related crystals of sodium ammonium tartrate. Solutions prepared from the separated crystal types rotated polarized light in opposite directions, establishing a connection between the observed crystal handedness and molecular dissymmetry. This was a foundational demonstration of enantiomer separation. (crystal.flack.ch)

In 1874, Jacobus Henricus van ’t Hoff and Joseph Achille Le Bel independently advanced spatial explanations of the asymmetric carbon atom. Their work connected three-dimensional molecular arrangement with optical isomerism and helped establish the structural basis of stereochemistry. (crystal.flack.ch)

References

1. Principles of Chemical Nomenclatureiupac.org
2. Basic Terminology of Stereochemistry — D and Eiupac.qmul.ac.uk
3. Basic Terminology of Stereochemistry — B and Ciupac.qmul.ac.uk
4. Basic Terminology of Stereochemistry — R and Siupac.qmul.ac.uk
5. Basic Terminology of Stereochemistry — N to Qiupac.qmul.ac.uk
6. Basic Terminology of Stereochemistry — Aiupac.qmul.ac.uk
7. Basic Terminology of Stereochemistry — F to Miupac.qmul.ac.uk
8. Development of New Stereoisomeric Drugsfda.gov