In the Laboratory
`
`Enantiomeric Resolution of (–)-Mandelic Acid
`by (1R,2S)-(–)-Ephedrine
`An Organic Chemistry Laboratory Experiment Illustrating Stereoisomerism
`Marsha R. Baar* and Andrea L. Cerrone-Szakal1
`Department of Chemistry, Muhlenberg College, Allentown, PA 18104; *baar@muhlenberg.edu
`
`W
`
`Racemic thalidomide was administered to pregnant
`women in the 1960s with a devastating effect (1). The (R )
`enantiomer exhibited the desired analgesic properties, but the
`(S ) enantiomer induced fetal malformations or deaths. As a
`result of this tragedy, marketing regulations for synthetic
`drugs became significantly more stringent. Racemates can no
`longer be sold as commercial drugs without characterization
`of each enantiomer and proof that the nonpotent stereoiso-
`mer is devoid of any harmful side effects. Thus pharmaceu-
`tical companies have focused on developing technologies that
`produce single-isomer drugs. These chiral technologies in-
`clude (i) utilization of chiral starting material whose configu-
`ration is maintained throughout the synthesis, (ii) resolution
`of a racemic mixture to give the desired stereoisomer, and
`(iii) asymmetric synthesis where asymmetry is introduced di-
`rectly into a nonchiral material (2).
`Because of the critical, and rapidly expanding, contri-
`bution that chiral technology is playing, we wished to incor-
`porate an illustrative experiment into the undergraduate
`organic lab curriculum. Enantiomeric resolution is an excel-
`lent example of chiral technology appropriate for the intro-
`ductory organic chemistry lab. The chemistry involved is
`covered early in lecture and requires several techniques to ef-
`fect the resolution. Thus this separation problem provides
`the opportunity to apply vacuum filtration, recrystallization,
`extraction, rotary evaporation, melting point, and polarim-
`etry analyses as well as to review acid–base chemistry.
`Enantiomeric resolution is not new to the organic chem-
`istry lab. Several organic chemistry lab texts include an ex-
`periment using this method and a few have been published
`in this Journal (3, 4). The most frequently cited one is Ault’s
`resolution of
`racemic α-phenylethylamine
`(α-
`methylbenzylamine) by (R,R )-(+)-tartaric acid, which was
`published in this Journal and later in Organic Synthesis (4a,
`5). We have repeatedly performed this resolution, but typi-
`cally obtained an optical purity for the high boiling (−)-α-
`phenylethylamine of 70%, not 95% cited by Ault.2 With the
`desire to improve optical purity, we reviewed Ault’s original
`work as well as other enantiomer resolutions in seminal ar-
`ticles (6, 7).
`In all classical resolutions, the initial diastereomeric salt
`collected was subjected to recrystallization, if not multiple
`recrystallizations, to improve purity. Ault described a recrys-
`tallization of the diastereomeric ammonium tartrate salt in
`Organic Synthesis, but he indicated that it required a large
`volume of methanol (450–500 mL for ∼22 g of tartrate salt)
`and required sitting at room temperature overnight for crys-
`tals to reform (5). We repeated the resolution, on 0.25 scale,
`isolating ∼5 g of diastereomeric salt that required 110 mL of
`
`methanol for a recrystallization. Although the recrystalliza-
`tion improved the quality of the salt, the combination of the
`solvent volume and time commitment for this purification
`was prohibitive.3 Perhaps this is why Ault does not include a
`recrystallization step in either his J. Chem. Educ. article or
`lab text (3a, 4a).
`A literature review indicated that there was not a great
`difference in the solubility of the ammonium tartrate salts in
`the α-phenylethylamine–(+)-tartaric acid system, thus a sig-
`nificant quantity of both diastereomers co-precipitated from
`methanol (7). We looked for a resolution system in which
`the selective precipitation of one diastereomer was greater so
`that recrystallization would be either unnecessary or easy to
`accomplish if necessary. The combination of the chiral amine,
`(−)-ephedrine, and (–)-mandelic acid yielded diasteromeric
`salts that had a very attractive precipitation ratio (Scheme I)
`(7). Additional advantages to this combination were that the
`diasteromeric salts underwent a facile recrystallization and the
`resolved product is a solid, not a high-boiling liquid amine,
`so melting point, as well as polarimetry, could be used to ana-
`lyze the product’s optical purity.4
`
`Results and Discussion
`
`We evaluated the success of the enantiomer resolution
`performed with and without a recrystallization. A solution
`of (1R,2S)-(−)-ephedrine and (–)-mandelic acid was prepared
`in 95% ethanol and allowed to sit at room temperature for 4
`hours in a sealed flask. A white precipitate was collected and
`identified as an 80% yield of crude (1R,2S )-(−)-ephedrine
`(R )-(−)-mandelate salt by its melting point of 160–165 ⬚C
`(cor); lit value 170 ⬚C (8). This crude salt was neutralized
`with 6 M HCl and the reaction mixture was extracted with
`tert-butyl methyl ether (TBME). Rotary evaporation of the
`ether produced a white solid whose melting point of 128–
`132 ⬚C identified it as the (R)-(−)-mandelic acid with a spe-
`cific rotation of ᎑144⬚ (c 0.4167, abs EtOH), which
`corresponds to a 90% optical purity.5 When a recrystalliza-
`tion step was inserted, it required only 25 mL of ethanol and
`10 minutes of cooling in an ice bath. The melting point of
`the diastereomeric salt improved to 168–170 ⬚C, and the (R)-
`(−)-mandelic acid’s purity also improved as indicated by its
`melting point of 133.0–134.5 ⬚C and specific rotation of
`᎑155⬚ (c 0.4973, abs EtOH), which corresponds to a 96%
`optical purity.
`Beginning organic chemistry students, performing these
`techniques for the first time, would probably not match these
`results, so a recrystallization was incorporated into the or-
`ganic chemistry lab sequence to ensure the highest optical
`
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`In the Laboratory
`
`Acid–Base Neutralization Reaction Forming Diastereomeric Salts:
`
`CH3
`CH3
`H
`
`NH
`H
`OH
`
`Ph
`
`ⴙ
`
`Ph
`
`OH
`
`CH
`*
`
`CO2H
`
`⫹
`NH2
`H
`OH
`
`CH3
`CH3
`H
`
`Ph
`
`⫺
`CO2
`OH
`
`Ph
`
`H
`
`ⴙ
`
`CH3
`CH3
`H
`
`⫹
`NH2
`H
`OH
`
`Ph
`
`HO
`
`⫺
`CO2
`H
`
`Ph
`
`(1R, 2S)-(ⴚ)-ephedrine
`
`(ⴞ)-mandelic acid
`
`(1R, 2S)-(ⴚ)-ephedrine R-(ⴚ)-mandelate
`
`(1R, 2S)-(ⴚ)-ephedrine S-(ⴙ)-mandelate
`
`Neutralization of (1R,2S)-(ⴚ)-Ephedrine (R)-(ⴚ)-Mandelate Salt:
`
`⫹
`NH2
`H
`OH
`
`CH3
`CH3
`H
`
`Ph
`
`⫺
`CO2
`OH
`
`Ph
`
`H
`
`ⴙ
`
`HCl
`
`⫹
`NH2
`H
`OH
`
`CH3
`CH3
`H
`
`Ph
`
`⫺
`Cl
`
`ⴙ
`
`H
`
`CO2
`H
`OH
`
`Ph
`
`(1R, 2S)-(ⴚ)-ephedrine R-(ⴚ)-mandelate
`
`(1R, 2S)-(ⴚ)-ephedrine hydrochloride
`
`(R)-(ⴚ)-mandelic acid
`
`Scheme I. Enantiomeric resolution of (–)-mandelic acid using (1R,2S)-(−)-ephedrine.
`
`purity. The following results are the averages from the two
`current lab sections (33 students) and are reflective of typi-
`cal results from previous years’ classes (85 students) taught
`by several different faculty. The crude mass of the diastereo-
`meric salt was 86%; after recrystallization the mass was 52%.
`The salt’s purity was excellent as indicated by the expected
`lustrous quality of the crystals and a mp of 168–170 ⬚C. Neu-
`tralization followed by purification gave a 32% yield of (R)-
`(−)-mandelic acid, which melted at 132–134 ⬚C (cor). Specific
`rotations generally ranged from ᎑135⬚ to ᎑160⬚, which corre-
`sponded to optical purities of 85–100%.6
`
`Summary
`
`The incorporation of an experiment involving enantio-
`meric resolution, as an illustration of chiral technology, is an
`excellent early organic chemistry lab experiment because it
`involves familiar acid–base chemistry and reinforces stere-
`ochemistry concepts covered at the beginning of an organic
`chemistry course. It is an excellent framework in which to
`teach fundamental laboratory techniques such as recrystalli-
`zation, extraction, rotary evaporation, polarimetry, and melt-
`ing point analyses. The experiment we describe here has been
`performed by 118 students with consistently excellent opti-
`cal purities. In addition to polarimetry, the success of the
`enantiomer resolution can be judged by melting point. Al-
`though (1R,2S )-(−)-ephedrine is a more expensive chiral re-
`solving agent, in our opinion, the improved separation and
`the ability to evaluate the resolved (R)-(−)-mandelic acid’s
`optical purity by both polarimetry and melting point, justi-
`fies the use of the (–)-mandelic acid–(1R,2S )-(−)-ephedrine
`system to demonstrate enantiomeric resolution. The entire
`sequence requires two full lab periods and 0.5 hour of an
`earlier period in which the initial diastereomeric salt solu-
`tion is prepared.
`
`Hazards
`
`(1R,2S )-(−)-ephedrine, (–)-mandelic acid, ethanol, and
`tert-butyl methyl ether are irritants, with the solvents being
`flammable as well. The 6 M HCl is corrosive, therefore,
`throughout all operations, gloves were worn, students worked
`in hoods, and hotplates served as the heat source. Ephedrine
`is a regulated substance that requires the submission of a
`vendor’s short form describing its intended use before ship-
`ping.
`
`WSupplemental Material
`
`Instructions for the students and notes for the instruc-
`tor are available in this issue of JCE Online.
`
`Notes
`
`1. Andrea L. Cerrone-Szakal (biochemistry major,
`Muhlenberg College class of 2002) is currently a graduate student
`at The Pennsylvania State University.
`2. Optical purity, 70%, was typical for the hundred students
`who performed Ault’s resolution at our institution. Landgrebe (3d )
`also indicated similar results. Needlelike crystals corresponding to
`the other diastereomeric salt always contaminated the desired pris-
`matic salt crystals. Rewarming to dissolve the needles, as Ault sug-
`gests, did not prevent their reformation with cooling.
`3. Our recrystallized salt melted at 190.0–192.0 ⬚C dec (cor)
`whereas Ault cites 179–182 ⬚C.
`4. The literature melting points for (–)-mandelic acid and (R)-
`(−)-mandelic acid are 120–122 ⬚C and 131–133 ⬚C, respectively.
`(Aldrich).
`5. We based our optical purity on polarimetry analysis of com-
`mercial (R)-(−)-mandelic acid, which gave a specific rotation of
`᎑160⬚ (c 0.5110, abs EtOH). This differs from the value of ᎑173⬚
`
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`In the Laboratory
`
`reported by Jarowski and Hartung (8), but it is not clear whether
`they used 95% or absolute ethanol as their solvent.
`6. A range of optical purities was observed, despite excellent
`melting point values. We believe this was primarily due to an eas-
`ily corrected technical error. Some students failed to reweigh their
`samples after removing a quantity for melting point. Thus the sub-
`sequent experimental optical rotations reflected the drop in mass,
`but the students still utilized the original, higher mass in their spe-
`cific rotation calculations, which leads to an artificially decreased
`value. Also ∼10% of our students’ results showed lower optical pu-
`rities with a corresponding depressed melting point.
`
`Acknowledgments
`
`Barbara and Wilson Gum (Muhlenberg alum 1961) sup-
`ported Andrea L. Cerrone-Szakal’s undergraduate summer
`research.
`
`Literature Cited
`
`1. Seyden-Penn, J. Chiral Auxiliaries and Ligands in Asymmetric Syn-
`thesis; John Wiley and Sons, Inc.: New York, 1995; pp xiii–xv.
`2. Richards, A.; McCague, R. Chem. Ind. 1997, June 2, 422–
`425.
`
`3. (a) Ault, A. Techniques and Experiments for Organic Chemistry,
`5th ed.; Waveland Press, Inc.: Prospect Heights, IL, 1994; pp
`339–343. (b) Roberts, R; Gilbert, J.; Rodewald, L.; Wingrove,
`A. Modern Experimental Organic Chemistry, 3rd ed.; Saunders
`College: Philadelphia, PA, 1979; pp 411–414. (c) Miller, J.;
`Neuzil, E. Modern Experimental Organic Chemistry; D. C.
`Heath and Co.: Lexington, MA, 1982; pp 309–315; (d)
`Landgrebe, J. Theory and Practice in the Organic Laboratory,
`3rd ed.; D. C. Heath and Co.: Lexington, MA, 1982; pp 483–
`485. (e) Bell, C.; Taber, D.; Clark, A. Organic Chemistry Labo-
`ratory; Harcourt College Publishers: Philadelphia, PA, 2001;
`pp 309–313. (f ) Durst, H. D.; Gokel, G. Experimental Or-
`ganic Chemistry; McGraw Hill Book Co.: New York, 1980;
`pp 469–471.
`4. (a) Ault, A. J. Chem. Educ. 1965, 42, 269. (b) Cesare, V.;
`Stephani, R. J. Chem. Educ. 1997, 74, 1226. (c) Hanson, J. J.
`Chem. Educ. 2001, 78, 1266.
`5. Ault, A. In Organic Syntheses; Wiberg, K. B., Ed.; Wiley: New
`York, 1969; Vol 49, p 93.
`6. Manske, R.; Johnson, T. J. Am. Chem. Soc. 1929, 51, 1906.
`7. Wilen, S. H. Tables of Resolving Agents and Optical Resolutions;
`Notre Dame University Press: Notre Dame, IN, 1972; and
`references within.
`8. Jarowski, C.; Hartung, W. H. J. Org. Chem. 1943, 8, 564.
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`1042 Journal of Chemical Education • Vol. 82 No. 7 July 2005 • www.JCE.DivCHED.org
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