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Racemic mixture

A racemic mixture, also known as a racemate, is an equimolar 1:1 mixture of two enantiomers—non-superimposable mirror-image isomers of a chiral molecule—that results in optical inactivity due to the equal but opposite rotations of plane-polarized light by each enantiomer.^[1]^[2] These mixtures commonly form during the synthesis of chiral compounds from achiral starting materials and reagents, as the reaction produces both enantiomers in equal proportions without stereoselective control.^[3] In organic chemistry, racemic mixtures are significant because they highlight the principles of chirality and stereoisomerism, often requiring resolution techniques—such as crystallization with chiral resolving agents or chromatographic separation—to isolate individual enantiomers for applications demanding specific stereochemistry.^[2] In pharmacology, as of 2024, approximately 50% of marketed drugs are chiral, with nearly 90% of these marketed as racemates, where the enantiomers may exhibit distinct pharmacodynamic, pharmacokinetic, and toxicological profiles, sometimes leading to one enantiomer being therapeutic while the other is inactive or harmful.^[4]^[5] This has driven regulatory emphasis on evaluating enantiopure drugs to optimize efficacy and safety, as exemplified by cases like thalidomide, where the racemic form contributed to severe adverse effects.^[6]

Fundamentals and Terminology

Definition

A racemic mixture, also known as a racemate, is defined as an equimolar 1:1 mixture of two enantiomers, which are pairs of chiral molecules that are non-superimposable mirror images of each other.^[7]^[8] Chirality arises when a molecule lacks an internal plane of symmetry, allowing it to exist in these enantiomeric forms, each capable of rotating plane-polarized light in opposite directions.^[8] Understanding enantiomers is essential, as they serve as the fundamental building blocks for racemates in stereochemistry. The hallmark property of a racemic mixture is its optical inactivity, resulting from the equal but opposite specific rotations of the constituent enantiomers, which cancel each other out.^[9] This can be expressed mathematically as the specific rotation of the racemate: [ \alpha ]_{\text{rac}} = \frac{[ \alpha ]_R + [ \alpha ]_S}{2} = 0, where [ \alpha ]_R and [ \alpha ]_S are the specific rotations of the respective enantiomers.^[10] Consequently, a racemic mixture does not rotate the plane of polarized light, distinguishing it from optically active pure enantiomers. Racemic mixtures must be differentiated from other stereoisomer combinations, such as diastereomers, which are stereoisomers that are not mirror images and thus do not necessarily cancel optical activity, or scalemic mixtures, which contain unequal ratios of enantiomers and exhibit net optical rotation proportional to the enantiomeric excess.^[11] This equimolar composition is key to the unique stereochemical behavior of racemates in chemical and biological contexts.

Etymology

The term "racemic" derives from the French "racémique," which in turn originates from the Latin racemus, meaning a cluster of grapes, alluding to the substance's discovery as a byproduct of wine production from grape juice.^[12] The name was first coined by French chemist Joseph Louis Gay-Lussac in 1828 to describe an optically inactive form of tartaric acid isolated from the lees of fermented wine, initially termed "acide racémique."^[13] This nomenclature reflected the compound's natural occurrence in grape fermentation rather than a direct structural analogy, though later interpretations linked the "cluster" imagery to the equal proportions of left- and right-handed enantiomers in such mixtures, akin to a bunch containing both types of forms.^[12] In 1848, Louis Pasteur's resolution of racemic acid into its two enantiomeric components established that it was an equimolar mixture, extending the term "racemic" beyond the specific acid to describe similar optically inactive combinations of mirror-image isomers in general.^[12] The introduction of "racemate" specifically for such a mixture occurred later in the 19th century, distinguishing the general concept from "racemic acid," which remained tied to the tartaric compound; the term "racemate" first appeared in English chemical literature in 1835.^[14] Throughout the 19th century, the terminology evolved in chemical publications, with early English adoptions appearing by 1835 in translations and reviews of French works, while in French it was used consistently from Gay-Lussac's original publication onward.^[15]

Nomenclature

In chemical nomenclature, racemic mixtures, also known as racemates, are denoted using specific prefixes to indicate a 1:1 equimolar combination of enantiomers without specifying absolute configuration.^[7] The most common prefixes include "rac-" (e.g., rac-2-butanol), "(±)-" to signify relative configuration, and "dl-" (dextro-levo), particularly for amino acids and carbohydrates (e.g., DL-tartaric acid).^[16] These notations distinguish the mixture from the pure enantiomers, which are labeled with absolute descriptors such as (R)- or (S)-, as in (R)-2-butanol or (S)-2-butanol.^[7] According to IUPAC recommendations, the term "racemate" is preferred over the obsolete "racemic mixture," and the prefix "rac-" is the standard descriptor for naming, superseding alternatives like "RS" or "SR" for relative stereochemistry in most contexts.^[17] For instance, rac-epinephrine refers to the 1:1 mixture of (R)- and (S)-enantiomers, whereas the individual enantiomers are specified as (R)-epinephrine or (S)-epinephrine.^[7] This system ensures clarity in representing the lack of optical activity due to enantiomeric cancellation in the mixture.^[17] In patents and regulatory naming for pharmaceuticals, racemates are typically identified using "rac-" or "(±)-" prefixes to claim the mixture as a distinct entity, often requiring separation strategies for enantiopure forms to avoid prior art limitations.^[18] Regulatory bodies like the FDA employ these notations in drug approvals, such as for racepinephrine hydrochloride, to differentiate racemic formulations from chiral active ingredients.^[18]

Historical Development

Early Discoveries

Racemic acid, also known as paratartaric acid, was first isolated around 1820 by German chemist Karl Kestner as a byproduct during the purification of tartaric acid from wine production.^[19] In 1844, French physicist Jean-Baptiste Biot communicated to the Académie des Sciences the findings of chemist Eilhard Mitscherlich, who had observed that paratartaric acid—later recognized as the racemic form of tartaric acid—crystallized in forms isomorphous to those of natural tartaric acid yet exhibited no optical activity when dissolved.^[20]^[21] This optical inactivity puzzled researchers, as natural tartaric acid, derived from wine production, consistently rotated plane-polarized light to the right, a property Biot himself had established earlier through polarimetry studies on organic substances. Building on this anomaly, Louis Pasteur, then a young chemist, isolated the sodium ammonium salt of paratartaric acid from the mother liquors remaining after tartaric acid crystallization in wine residues during industrial production in 1848.^[22] Under microscopic examination, he identified that the crystals were hemihedral—lacking certain faces—and occurred in two mirror-image varieties that were nonsuperimposable. Using fine tweezers and a magnifying glass, Pasteur painstakingly separated the crystals into two piles, then dissolved each separately; one solution rotated polarized light to the right (dextrorotatory), matching natural tartaric acid, while the other rotated it equally to the left (levorotatory).^[23] This manual separation marked the first resolution of a racemic mixture into its enantiomeric components. In 1848, Pasteur presented these results to the Académie des Sciences in Paris, proposing that the observed optical activity stemmed from molecular asymmetry rather than merely crystalline form, thereby introducing the foundational concept of enantiomers as mirror-image molecular forms.^[24] To further validate his work, Pasteur investigated the origins of paratartaric acid; in 1853, he demonstrated to Biot in the latter's laboratory at the Collège de France that heating a solution of tartaric acid with certain alkaloids could produce racemic acid, confirming the crystallization behavior and linking it to partial racemization processes observed in natural settings.^[25] These experiments solidified the recognition of racemic mixtures as equimolar combinations of enantiomers that neutralize optical effects.

Key Conceptual Advances

In 1874, Joseph Achille Le Bel and Jacobus Henricus van 't Hoff independently proposed the tetrahedral arrangement of four substituents around a carbon atom, providing a structural explanation for the mirror-image relationship of enantiomers in chiral molecules. Le Bel's model emphasized the spatial arrangement as a consequence of equilibrium forces among the substituents, while van 't Hoff explicitly linked the tetrahedral geometry to the optical activity observed in compounds like tartaric acid, predicting that such asymmetry would lead to non-superimposable mirror images. This conceptual breakthrough, building briefly on Louis Pasteur's earlier empirical separation of enantiomers, established the molecular basis for racemic mixtures as equimolar combinations of these enantiomers, which are optically inactive due to mutual cancellation of rotational effects.^[26] By the late 19th century, Otto Wallach contributed a key empirical observation in 1895, formulating what became known as Wallach's rule: racemic crystals typically exhibit higher density than their enantiopure counterparts, suggesting distinct packing in heterochiral lattices. This rule, derived from density comparisons of compounds like carvone derivatives, highlighted the structural differences between racemates and single enantiomers, influencing early understandings of crystallization behavior. Concurrently, the 1880s and 1900s saw the rise of synthetic organic chemistry, where reactions from achiral precursors routinely produced racemic mixtures; a representative example is Albert Ladenburg's 1886 total synthesis of coniine, the first alkaloid achieved synthetically, via reduction of an achiral pyridine derivative to yield the racemic piperidine alkaloid. These developments underscored that laboratory synthesis, lacking chiral bias, naturally generated racemates, prompting advances in resolution techniques.^[27] In the 1920s, advancements in X-ray crystallography provided direct confirmation of racemic crystal structures, building on the tetrahedral model by revealing atomic arrangements in organic solids. Pioneering work by researchers like William Henry Bragg and others determined structures of aliphatic and aromatic compounds, verifying the tetrahedral carbon geometry essential for chirality and demonstrating how racemates form ordered lattices distinct from conglomerates. For instance, early analyses of compounds with potential chiral centers affirmed the spatial configurations predicted decades earlier, solidifying the theoretical framework for racemic mixtures and enabling precise studies of their polymorphic forms.^[28]

Properties

Optical and Spectroscopic Properties

Racemic mixtures exhibit zero net optical rotation due to the equal presence of enantiomers, each contributing rotations of equal magnitude but opposite sign to plane-polarized light.^[29] In polarimetry measurements, this cancellation results in no observable deviation of the light's polarization plane, distinguishing racemates from enantiomerically pure compounds where specific rotation values are non-zero.^[30] The specific rotation of a racemic mixture, denoted as [\alpha]_{\text{rac}}, is thus zero:

[\alpha]_{\text{rac}} = 0

This property arises because the optical activities of the (R)- and (S)-enantiomers sum vectorially to nullify any net effect.^[31] In spectroscopic techniques probing chiral interactions with light, racemic mixtures display characteristic null signals. Circular dichroism (CD) and optical rotatory dispersion (ORD) spectra of racemates show no net absorption or rotation differences between left- and right-circularly polarized light, as the mirror-image spectra of the enantiomers superimpose and cancel.^[32] Similarly, vibrational circular dichroism (VCD) measurements yield no observable signal for racemates, reflecting the absence of differential infrared absorption for circularly polarized light across vibrational modes.^[33] These spectroscopic behaviors confirm the internal compensation within the mixture. Although racemic mixtures are optically inactive, they differ fundamentally from achiral compounds: the former consist of chiral enantiomers in equal proportions, retaining molecular chirality but lacking macroscopic optical activity, whereas achiral molecules possess no inherent handedness and thus exhibit no enantiomeric forms.^[34] This distinction underscores that optical inactivity in racemates stems from enantiomeric balance rather than molecular symmetry.

Thermodynamic and Solubility Properties

Racemic mixtures exhibit distinct thermodynamic properties compared to their enantiopure counterparts, primarily due to differences in crystal lattice structures and intermolecular interactions. In many cases, racemic compounds—where the two enantiomers form a single crystalline phase—display higher melting points than the pure enantiomers, attributed to enhanced lattice energy from heterochiral hydrogen bonding or packing efficiency. For instance, racemic tartaric acid melts at 204–206 °C, significantly higher than the 170 °C melting point of the enantiopure form, reflecting the stability of the racemic crystal lattice.^[35]^[36] Solubility behaviors also vary between racemates and enantiomers, often influenced by whether the racemate forms a compound or a conglomerate. Racemic compounds typically show lower solubility than enantiopure forms due to stronger cohesive forces in the lattice; for example, the solubility of racemic tartaric acid in water at 20 °C is approximately 20.6 g/100 mL, compared to 139 g/100 mL for the enantiopure isomer. In contrast, conglomerates—mechanical mixtures of enantiopure crystals—exhibit additive solubilities equivalent to that of the individual enantiomers, as the phases do not interact in the solid state. Sodium chlorate serves as a classic model for conglomerate formation, where the racemic mixture consists of separate left- and right-handed chiral crystals with identical solubilities to the pure forms.^[35]^[37]^[38] Phase diagrams for racemate-enantiomer systems frequently reveal eutectic behavior, where the solidus line shows invariant points at which the racemic compound or conglomerate coexists with the excess enantiomer in a liquid phase of fixed composition. These eutectics occur at lower temperatures than the melting points of the pure components, facilitating partial separations via fractional crystallization, though the exact positions depend on the system's thermodynamics. Gibbs free energy considerations underscore the relative stability: racemic compounds are often thermodynamically favored over enantiopure solids when the heterochiral interaction energy lowers the overall ΔG, as quantified by the difference in dissolution free energies (ΔΔG_sol), which can predict solubility ratios via the relation α = exp(-ΔΔG_sol / RT), where α is the solubility ratio of enantiopure to racemic forms.^[39]^[39]

Crystallization Behavior

Types of Racemic Crystals

Racemic mixtures can adopt four distinct solid-state forms upon crystallization, each characterized by different arrangements of the enantiomers in the crystal lattice. These types are conglomerate, racemic compound, pseudoracemate, and quasiracemate.^[40] A conglomerate consists of a physical mixture of separate crystals, each composed of a single enantiomer, forming a mechanical mixture rather than a unified lattice. This form occurs in approximately 10% of racemic chiral compounds.^[41] An example is sodium ammonium tartrate tetrahydrate, which crystallizes as a conglomerate below 27.8°C.^[40] The conglomerate nature of this salt was instrumental in Louis Pasteur's 1848 manual separation of enantiomers, later confirmed through structural analysis showing distinct enantiopure crystals.^[42] In conglomerates, spontaneous resolution is possible because individual crystals are enantiomerically pure, enabling separation by mechanical means or selective crystallization processes.^[41] The racemic compound features a homogeneous crystal lattice where the two enantiomers are present in a 1:1 ratio within the same unit cell, often forming a distinct phase with unique properties. This is the most common form for racemic mixtures.^[40] Mandelic acid exemplifies this type, crystallizing from racemic solutions as polymorphs of a racemic compound in water.^[43] A pseudoracemate is a homogeneous solid solution in which the enantiomers mix ideally or nearly ideally in the crystal lattice, without forming a separate compound phase, resulting in diffraction patterns similar to those of the pure enantiomers.^[40] A quasiracemate arises from a 1:1 mixture of two similar but distinct compounds that are quasi-enantiomers—molecules with opposite configurations at corresponding chiral centers but differing slightly in structure—mimicking the behavior of a true racemate in crystallization. These may form compounds, eutectic mixtures, or solid solutions exhibiting properties akin to racemic compounds. The type of racemic crystal can often be predicted using empirical rules such as Wallach's rule. Identification of these forms typically involves X-ray diffraction to determine lattice structures and thermal analysis techniques like differential scanning calorimetry (DSC) to assess melting behavior and phase transitions.^[44]

Wallach's Rule

In 1895, Otto Wallach observed that racemates derived from substances whose enantiomers form unstable solid crystals tend to crystallize as stable racemic compounds, whereas those with stable enantiomeric solids more often form conglomerates.^[45] This empirical insight highlighted the preference for heterochiral interactions in many chiral systems, contributing to the predominance of racemic compounds over conglomerates in crystallization behavior.^[46] Wallach's rule provides a guideline for predicting the crystallization type based on physical properties: if the melting point of the racemate exceeds that of the pure enantiomer, it indicates formation of a racemic compound due to enhanced lattice stability from unlike-enantiomer pairing; conversely, a lower melting point for the racemate suggests a conglomerate.^[44] This correlation stems from the higher density and thermodynamic stability typically observed in racemic compounds, as denser packing raises the melting temperature relative to the enantiopure form.^[45] The rule holds with approximately 90% accuracy across known chiral compounds, as racemic compounds constitute the majority (90-95%) of racemic species, while only about 10% form conglomerates. However, exceptions occur, particularly with pseudoracemates—mixtures of diastereomers mimicking racemates—where the density or melting point trends do not align due to differing intermolecular forces.^[44] Modern validations employ computational modeling, such as lattice energy calculations via density functional theory, to predict and confirm Wallach's predictions by quantifying heterochiral vs. homochiral interactions in crystal lattices.^[47] Historically, the rule originated from density comparisons of a small number of compounds in Wallach's initial studies, reinforcing its utility in organic chemistry.^[46]

Preparation and Separation

Synthesis Methods

Racemic mixtures are commonly synthesized from achiral precursors through non-stereoselective reactions that generate a new chiral center without bias toward either enantiomer, resulting in a 1:1 mixture. For instance, the addition of Grignard reagents to aldehydes produces secondary alcohols as racemates, as the nucleophilic attack occurs equally from both faces of the planar carbonyl group. Another approach involves the intentional racemization of enantiopure compounds, often via base- or acid-catalyzed epimerization at the chiral center. In base-catalyzed racemization, deprotonation at the alpha position forms a planar carbanion intermediate, allowing reprotonation to yield both enantiomers equally; this is particularly effective for compounds with acidic protons adjacent to the chiral site, such as alpha-amino acids or carboxylic acids. Acid-catalyzed mechanisms similarly proceed through protonation to form achiral intermediates like iminium ions in amines.^[48] In industrial synthesis, racemic mixtures are frequently produced by default when chiral catalysts or auxiliaries are omitted, as most organic reactions from achiral starting materials lack inherent stereocontrol. This approach remains prevalent for certain pharmaceuticals where both enantiomers are bioactive or where enantiopure production is not economically justified, such as in the development of anti-infective agents.^[49]^[50] A recent advancement is the solvent-free racemization of the pharmaceutical Levetiracetam using mechanochemistry, achieved via high-energy ball milling with a base catalyst like NaOH. This method attains near-complete racemization in approximately 60 minutes under liquid-assisted grinding conditions, offering a greener alternative to traditional solution-based processes by avoiding organic solvents and enabling in situ monitoring via X-ray diffraction.^[51]

Resolution Techniques

Resolution of racemic mixtures into enantiomerically pure components is essential in stereochemistry, employing a range of classical and modern techniques that exploit differences in physical or chemical properties between enantiomers.^[52] Classical methods include manual sorting of crystals, pioneered by Louis Pasteur in 1848, who separated enantiomers of sodium ammonium tartrate by handpicking individual hemihedral crystals under a magnifying glass, a labor-intensive approach feasible only for conglomerate-forming substances where enantiomers crystallize separately.^[53]^[54] More broadly applicable classical techniques involve diastereomer formation using chiral auxiliaries, such as reacting the racemate with an enantiopure resolving agent to produce diastereomeric salts or derivatives with differing solubilities, followed by fractional crystallization to isolate one diastereomer; the auxiliary is then removed to yield the pure enantiomer.^[55]^[56] This method, exemplified by the use of tartaric acid or brucine as auxiliaries, remains industrially relevant for scalable resolutions despite requiring stoichiometric amounts of the chiral agent.^[57] Chromatographic techniques offer high selectivity for enantioseparation, particularly chiral high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC) using chiral stationary phases (CSPs) that form transient diastereomeric interactions with analytes.^[58] In chiral HPLC, CSPs like polysaccharide-based columns enable baseline separation of enantiomers in analytical and preparative scales, while SFC, employing CO₂ as a mobile phase, provides faster separations with reduced solvent use.^[59] Advances since 2020 have emphasized green chemistry in these methods, including the integration of bio-based or low-toxicity solvents in SFC to minimize environmental impact, alongside novel CSP designs for broader substrate compatibility and higher throughput.^[60]^[61] Enzymatic resolution leverages the stereospecificity of biocatalysts for kinetic resolution, where enzymes selectively react with one enantiomer of the racemate, leaving the other unchanged; lipases and hydrolases are commonly employed for ester hydrolysis or acylation reactions.^[62] For instance, Candida antarctica lipase B catalyzes the acylation of secondary alcohols in organic solvents, achieving high enantioselectivity (E > 100) and yielding products with up to 99% enantiomeric excess after partial conversion.^[63] This approach is advantageous for its mild conditions and compatibility with sensitive substrates, though it typically limits yield to 50% without racemization steps.^[64] Deracemization techniques amplify a slight enantiomeric imbalance in near-racemic mixtures to full enantiopurity, particularly effective for conglomerate crystal types that allow selective growth of one enantiomer's crystals.^[65] Viedma ripening, developed in 2005, involves attrition-enhanced crystallization in solution with racemization, where grinding or stirring breaks larger crystals into fragments that preferentially dissolve and recrystallize, leading to complete deracemization of amino acids or salts over hours to days.^[66] This nonlinear process, driven by Ostwald ripening modified by attrition, has been optimized with temperature cycling for broader applicability to racemic compounds convertible to conglomerates.^[67] The efficiency of these resolution techniques is quantified by enantiomeric excess (ee), defined as:

\text{ee} = \left( \frac{|\text{R} - \text{S}|}{\text{R} + \text{S}} \right) \times 100\%

where R and S represent the mole fractions of the respective enantiomers, providing a measure of optical purity from 0% (racemic) to 100% (enantiopure).^[68]

Applications and Significance

In Pharmaceuticals

Racemic mixtures have played a significant role in pharmaceutical development, often presenting both opportunities and challenges due to the potential for enantiomers to exhibit differing pharmacological activities, toxicities, and metabolic profiles. In the mid-20th century, many drugs were synthesized and marketed as racemates without full consideration of stereochemistry, leading to unintended consequences. A prominent historical example is thalidomide, introduced in the 1950s as a racemic sedative and antiemetic for morning sickness in pregnancy. The (S)-enantiomer was responsible for severe teratogenic effects, causing birth defects in over 10,000 children worldwide before its withdrawal in 1961. This tragedy underscored the risks of racemic administration and spurred advancements toward enantiopure formulations, though thalidomide itself racemizes rapidly in vivo, complicating pure enantiomer use.^[69] Another illustrative case is ibuprofen, a widely used nonsteroidal anti-inflammatory drug marketed as a racemic mixture since the 1960s. The (S)-enantiomer is primarily responsible for its anti-inflammatory and analgesic effects by inhibiting cyclooxygenase enzymes, while the (R)-enantiomer is largely inactive but undergoes partial metabolic inversion to the active (S)-form in vivo. Despite this, the presence of the (R)-enantiomer can delay onset of action compared to the pure (S)-enantiomer.^[70]^[71] The thalidomide disaster influenced regulatory frameworks, culminating in the U.S. Food and Drug Administration's (FDA) 1992 policy statement on the development of new stereoisomeric drugs. This guidance encouraged the evaluation of single enantiomers over racemates when feasible, emphasizing that racemic mixtures should not be assumed equivalent to their enantiopure counterparts in terms of safety and efficacy. The policy allows flexibility for racemates if they demonstrate comparable benefits but promotes chiral synthesis or resolution techniques to isolate active enantiomers, thereby reducing risks from inactive or harmful stereoisomers.^[49] Despite these shifts, racemic mixtures retain advantages in pharmaceuticals, including simpler and more cost-effective synthesis routes, as separating enantiomers can increase production complexity and expense. As of 2020, approximately 50% of marketed drugs remain racemates, reflecting their practicality when enantiomers show similar or complementary activities without significant adverse effects.^[72] Pharmacokinetic differences between enantiomers in racemates can further complicate therapeutic outcomes. For instance, warfarin, an anticoagulant administered as a racemate, exhibits stereoselective metabolism: the (R)-enantiomer has a longer plasma half-life (37–89 hours) compared to the (S)-enantiomer (21–43 hours), leading to prolonged exposure and requiring careful dose adjustments to avoid bleeding risks.^[73] A notable example of a "chiral switch" strategy—reformulating a racemic drug into its active enantiomer—is esomeprazole, the (S)-enantiomer of the proton pump inhibitor omeprazole. Omeprazole, launched in 1989 as a racemate for acid-related disorders, saw only the (S)-enantiomer contribute substantially to its efficacy. Esomeprazole, approved in 2001, demonstrated improved acid suppression and healing rates in clinical trials, allowing extended market exclusivity despite higher costs. This approach highlights how resolving racemates can enhance therapeutic profiles, often leveraging techniques like chromatographic separation detailed elsewhere.^[74]^[75]

Recent Developments

In recent years, FDA approvals for chiral drugs have continued to favor single enantiomers over racemic mixtures. From 2020 to 2022, approximately 56% of small molecule new molecular entities approved by the FDA were single enantiomers, while racemates accounted for only 4.1% (four drugs: viloxazine, nifurtimox, amisulpride, and gadopiclenol), reflecting a stable but low prevalence of racemics compared to earlier periods where single enantiomers already dominated at around 59-62%.^[76] All chiral drugs approved in this timeframe featured carbon-based stereocenters, underscoring the persistence of traditional chiral motifs in pharmaceutical development despite advances in stereocontrol.^[76] Advances in deracemization techniques have enhanced the scalability of producing enantiopure compounds from racemic mixtures. These approaches, including Viedma ripening and dynamic preferential crystallization, have been applied to conglomerate and racemic compound systems. Enantioseparation technologies have seen enhancements in speed and precision through supercritical fluid chromatography (SFC) and computational optimization of chiral stationary phases (CSPs). Recent developments in 2023-2024 include hybrid organic-inorganic CSPs for SFC, offering improved resolution and green solvent compatibility for pharmaceutical analyses, while structure-based predictions using 3D molecular conformations enable AI-assisted design of CSPs tailored for faster enantioseparation of complex chiral molecules.^[77] These innovations reduce separation times and costs, particularly for analytical-scale quality control in drug development.^[77] This trend continued through 2023-2025, with the FDA maintaining an average of approximately one new racemate approval per year and the EMA approving no new racemates since 2016.^[78]^[79] Looking ahead, there is growing recognition of racemic mixtures' role in pharmaceuticals where enantiomer separation proves uneconomical or technically challenging, especially for complex APIs with irrelevant stereochemistry or established safety profiles, signaling a balanced future where racemics complement single-enantiomer strategies in niche applications.^[80]

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Interpreting vibrational circular dichroism spectra: the Cai•factor for ...
Mar 21, 2023 · Perfect VCD spectra of racemic or achiral substances show no signals. These experimental spectra are approximate mirror images. The signals ...
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5.8: Optical Activity, Racemic Mixtures, and Separation of Chiral ...
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[PDF] Stereochemistry
... melting points, solubilities, and heats of fusion, are often different. Thus racemic tartaric acid has a melting point of. 204–206 C and a solubility in ...
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Solid–Liquid Equilibrium of d- and l-Tartaric Acid and Their ...
May 11, 2012 · As shown in. Figure 8, the melting point of L-TA is 170.2 °C, and the melting ... of the same enantiomer, which results in that the melting point.Missing: enantiopure | Show results with:enantiopure
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Racemic tartaric acid - ChemBK
Stable in air, density 1.697, its solubility in water 20.6(20°C), ether solubility of about 1%, ethanol solubility of 5.01(25°C). Use, Widely used in food, ...
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Viedma ripening: a reliable crystallisation method to reach single ...
Jul 13, 2015 · Viedma discovered that the initially racemic mixture of sodium chlorate crystals was, over a period of several days, completely transformed ...
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Thermochemistry of Racemic and Enantiopure Organic Crystals for ...
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Conglomerate Crystallization in the Cambridge Structural Database ...
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Crystal structures of dextrorotatory and racemic sodium ammonium ...
Crystal structures of dextrorotatory and racemic sodium ammonium tartrate. Reiko Kuroda and Stephen F. Mason. Abstract. The crystal structures of the title ...
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Application of Crystallization Inhibitors to Chiral Separations. 1 ...
We have chosen the system R,S-mandelic acid (MA) in water which, from racemic solutions, crystallizes as two polymorphs of a racemic compound. ... With 10% ...
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On the validity of Wallach's rule - ACS Publications
On the validity of Wallach's rule: on the density and stability of racemic crystals compared with their chiral counterparts.
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On the Validity of Calling Wallach's Rule Wallach's Rule - Ernst ...
The paper gives credit to Wallach's publication in Liebigs Annalen der Chemie in 1895, in which, among other work, a comparison of crystal density of racemates ...
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Predictive crystallography at scale: mapping, validating, and ... - NIH
Computational studies of polymorphism have evaluated the typical lattice energy ... racemic structures, consistent with the empirical Wallach's rule. 3.4 ...
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Controlled racemization and asymmetric transformation of α ...
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Anti-infectives Developed as Racemic Drugs in the 21st Century
Jun 12, 2023 · This viewpoint outlines the case for developing new chemical entities (NCEs) as racemates in infectious diseases and where both enantiomers and racemate retain ...
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Strategies for chiral separation: from racemate to enantiomer
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Part 6: Resolution of Enantiomers - Chiralpedia
Sep 15, 2025 · tartaric acid and its derivatives, have been historically used. ... vs racemic and that pure enantiomers seldom drastically outperform ...
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[PDF] Processes to separate enantiomers - CORE
[83]. In principle, Pasteur's technique of hand sorting crystals of the two enantiomers is still a feasible technique to separate racemic conglomerates that ...
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[PDF] Isolation of enantiomers via diastereomer crystallisation
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CRC Handbook of Optical Resolutions via Diastereomeric Salt ...
[10] The techniques employed for the production of pure enantiomers can be divided into two approaches: a 'chiral approach' (chiral pool, [11] use of chiral ...Missing: sorting | Show results with:sorting
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5.8: Racemic Mixtures and the Resolution of Enantiomers
Jan 28, 2023 · A racemic mixture is a 50:50 mixture of two enantiomers. Because they are mirror images, each enantiomer rotates plane-polarized light in an equal but opposite ...
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Recent Advances in Separation and Analysis of Chiral Compounds
Jan 10, 2023 · This biennial review summarizes the development in the separation and analysis of chiral compounds from September 2020 to September 2022.
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Recent Advances in Enantiorecognition and Enantioseparation ...
Jan 2, 2025 · Recent advancements in these techniques have significantly improved enantioseparation efficiency and resolution. Chiral stationary phases (CSPs) ...
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Emerging applications of quantitative supercritical fluid ...
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Preparative Supercritical Fluid Chromatography for Chiral Separations
Sep 19, 2024 · Preparative SFC offers fast, green and efficient chiral separations for pharmaceutical development.
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Resolution of racemic mixtures via lipase catalysis in organic solvents
Applications of enzymatic and non-enzymatic methods to access enantiomerically pure compounds using kinetic resolution and racemisation. Tetrahedron 2012 ...Missing: hydrolases | Show results with:hydrolases
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Lipase Catalysed Kinetic Resolution of Racemic 1,2-Diols ... - NIH
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Application of Lipases in Kinetic Resolution of Racemates
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Deracemization of a Racemic Compound via Its Conglomerate ...
In the present study, the use of temperature cycling on conglomerate forming salts as an alternative technique for Viedma Ripening was studied. The racemic ...
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Speeding up Viedma Deracemization through Water‐catalyzed and ...
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Review Recent advances in crystallization-induced deracemization ...
This review provides a comprehensive overview of crystallization-based deracemization approaches that combine enantioselective crystallization and racemization ...
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Aug 29, 2014 · The composition of these mixtures is described by the enantiomeric excess, which is the difference between the relative abundance of the two enantiomers.Application to organic chemistry? · Enantiomeric Excess and...
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ibuprofen | Ligand page | IUPHAR/BPS Guide to PHARMACOLOGY
The approved drug is a racemic mixture of the (R)- and (S)-stereoisomers. The (S)-enantiomer is more active, and the (R)-enantiomer is converted in the body to ...
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Comparative pharmacology of S(+)-ibuprofen and (RS) - PubMed
Although the S(+)-enantiomer is capable of inhibiting cyclooxygenase (COX) at clinically relevant concentrations, R(-)-ibuprofen is not a COX inhibitor. The two ...
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A Look at the Importance of Chirality in Drug Activity - MDPI
Nearly 56% of the pharmaceuticals marketed and used in therapy are chiral compounds and, amongst them, 88% are administered as racemates [6]. Interestingly, 20 ...
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Warfarin: Uses, Interactions, Mechanism of Action | DrugBank Online
R-warfarin is cleared more slowly than S-warfarin, at about half the rate. T1/2 for R-warfarin is 37-89 hours. T1/2 for S-warfarin is 21-43 hours.
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A case study of AstraZeneca's omeprazole/esomeprazole chiral ...
Jun 24, 2022 · To describe the chiral switch, an evergreening strategy used by AstraZeneca to position enantiopure esomeprazole as the new proton pump inhibitor market leader.
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Inside the isomers: the tale of chiral switches - Australian Prescriber
For example, esomeprazole is an enantiomer of the racemate omeprazole. The term chiral switching has been coined to describe the development of single ...Inside The Isomers: The Tale... · Introduction · Esomeprazole
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Chirality of New Drug Approvals (2013–2022) - ACS Publications
This perspective discusses trends related to chirality that have developed in the past decade (2013–2022) of new drug approvals.
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https://go.drugbank.com/drugs/DB00682
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The Chiral Switch: A Pharmaceutical Tactic to Prolong Exclusivity
Summary of each segment: