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Exploring Physical Properties of Enantiomers: A Comprehensive Guide

Learn about how enantiomers share similar physical properties, optical activity, racemate mixtures, making racemic mixtures, Celexa vs. Lexapro, and the resolution of enantiomers through selective crystallization. Discover the significance of diastereomers and how to differentiate between enantiomers and diastereomers.

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Exploring Physical Properties of Enantiomers: A Comprehensive Guide

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  1. Physical Properties of Enantiomers • Enantiomers share identical physical properties • m.p., b.p., nD, density, heats of formation etc. • Example: Lactic Acid • m.p. = 53°C • b.p. = 122°C Lactic Acid

  2. Enantiomers rotate a plane of polarized light in equal, but oppositedirections • Called “Optical Activity” • Use Polarimeter

  3. Optical Activity • If the sample rotates the plane of polarized light CW → dextrorotatory (+) • Latin: Dexter = Right • If the sample rotates the plane of polarized light CCW → levorotatory (-) • Latin = Laevus = Left • If one enantiomer is +, the other will be – (+)-lactic acid (–)-lactic acid

  4. The sign of optical rotation is unrelated to R and S configuration of a compound (S)-(+)-lactic acid (R)-(–)-lactic acid

  5. Optical rotation (a) is a quantitative measure of optical activity • [a] = a/cl • [a] = specific rotation • a = degrees • c = concentration (g/mL) • l = path length (dm) • Often, temp and wavelength indicated

  6. (S)-(-)-glyceraldehyde [α]20 = -13.5° mL g-1 dm-1 (R)-(+)-glyceraldehyde [α]20 = +13.5° mL g-1 dm-1

  7. Problems • A 1.50 g sample of (S)-(+)-coniine, the toxic extract of poison hemlock, was dissolved in 10.0 mL of ethanol and placed in a sample cell with a 5.00 cm pathlength. The observed rotation at the sodium D line was +1.21°. Calculate the [α]D for coiine. • What will the specific rotation be for the R enantiomer?

  8. Racemic Mixtures • Racemic mixture/Racemate: a mixture containing equal amounts of two enantiomers • typically have different physical properties from that of the pure enantiomers • Example: Lactic Acid • m.p. (R or S) = 53°C • m.p. (R and S)= 17°C • Indicating racemic mixture: • Racemic Lactic Acid • (±)-Lactic Acid • Optical rotation = 0 6.4 Racemates

  9. Making Racemic Mixtures • Racemization: the process of forming a racemic mixture • Easy: mix = amounts of each enantiomer together • More difficult: chemical rxns • Heat, enzymes, acids, bases

  10. Celexa vs. Lexapro • Celexa = racemic mixture • Lexapro = enantiopure S • escitalopram R-(−)-citalopram S-(+)-citalopram

  11. Resolution of Enantiomers • Enantiomeric resolution: the separation of a pair of enantiomers from a racemic mixutre • Also called chiral resolution, optical resolution, and mechanical resolution • Often difficult • Enantiomers have same chemical properties • Separation often through • Selective crystallization • Diastereomeric salt formation

  12. Selective Crystallization • Pasteur’s experiment (1848) • 1882 – “Seeding” of a supersaturated solution • Now often used in industry • Methadone • L-glutamic acid

  13. Diastereomers • Diastereomers/Diastereoisomers: stereoisomers that are not mirror images (enantiomers) of each other • possible when a molecule has two or more asymmetric carbons • Differ in all their physical properties 6.6 Diastereomers

  14. Isomer Identification Flowchart 6.6 Diastereomers

  15. Molecules with more than one chirality center have mirror image stereoisomers that are enantiomers • In addition they can have stereoisomeric forms that are not mirror images, called diastereomers

  16. Diastereomers and Enantiomers 6.6 Diastereomers

  17. Problem: Can you identify the diastereomers?

  18. 2S,3S 2R,3R 2R,3S 2S,3R

  19. Physical Properties of Stereoisomers 6.6 Diastereomers

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