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Physical Properties of Enantiomers

Physical Properties of Enantiomers. Enantiomers share identical physical properties m.p., b.p., n D , density, heats of formation etc. Example: Lactic Acid m.p. = 53 °C b.p. = 122 °C. Lactic Acid. Enantiomers rotate a plane of polarized light in equal, but opposite directions

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Physical Properties of Enantiomers

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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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