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Optical Activity

Optical Activity. Enantiomers are different compounds: Same boiling point, melting point, density Same refractive index Rotate plane polarized light in opposite directions (polarimetry) Different interaction with other chiral molecules Enzymes Taste buds, scent. Optical Activity.

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Optical Activity

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  1. Optical Activity • Enantiomers are different compounds: • Same boiling point, melting point, density • Same refractive index • Rotate plane polarized light in opposite directions (polarimetry) • Different interaction with other chiral molecules • Enzymes • Taste buds, scent

  2. Optical Activity • Polarimetry is a laboratory technique that measures the interaction between a compound and plane polarized light. • Since enantiomers interact with plane polarized light differently, polarimetry can be used to distinquish between enantiomers.

  3. Optical Activity • “Regular” (unpolarized) light vibrates in all directions. • Plane-polarized light: • light composed of waves that vibrate in only a single plane • obtained by passing unpolarized light through a polarizing filter

  4. Optical Activity • When plane polarized light passes through a solution containing a single chiral compound, the chiral compound causes the plane of vibration to rotate. • Polarimeter

  5. Optical Activity • Chiral compounds areoptically active: • capable of rotating the plane of polarized light • Enantiomers rotate the plane of polarized light by exactly the same amount but in opposite directions. (R)-2-butanol (S)-2-butanol -13.5o rotation +13.5o rotation

  6. Optical Activity • Compounds that rotate the plane of polarized light to the right (clockwise) are called dextrorotatory. • d • (+)IUPAC convention • Compounds that rotate the plane of polarized light to the left (counterclockwise) are called levorotatory. • l • (-)IUPAC convention

  7. Optical Activity • The direction and magnitude of rotation must be determined experimentally. • There is NO CORRELATION between (R) and (S) configuration and the direction of rotation. +13.5o rotation -13.5o rotation (+)-2-butanol (-)-2-butanol (S)-(+)-2-butanol (R)-(-)-2-butanol

  8. (R)-(+)-thyroxine inactive Optical Activity (S)-(-)-thyroxine biologically active Unlike (R)-(-)-2-butanol, (R)-thyroxine rotates light to the right.

  9. Optical Activity • The angular rotation observed in a polarimeter depends on: • the optical activity of the compound • the concentration of the sample • the path length of the sample cell • A compound’s specific rotation [a] can be used as a characteristic physical property of a compound: • the rotation observed using a 10-cm sample cell and a concentration of 1 g/mL.

  10. Optical Activity where a = specific rotation c = concentration in g/mL l = path length in dm a (observed) = rotation observed for a specific sample

  11. Optical Activity Example: A solution of 2.0 g of (+)-glyceraldehyde in 10.0 mL of water was placed in a 100. mm polarimeter tube. Using the sodium D line, a rotation of 1.74o was observed at 25oC. Calculate the specific rotation of (+)-glyceraldehyde.

  12. Optical Activity Given:a (obs) = 1.74o Find: [a]

  13. Optical Activity • A mixture containing equal amounts of (+)-2-butanol and (-)-2-butanol gives an observed rotation of zero degrees • Just like an achiral molecule +13.5o rotation -13.5o rotation (S)-(+)-2-butanol (R)-(-)-2-butanol

  14. Optical Activity • A solution containing equal amounts of two enantiomers is called a racemic mixture. • Racemate • (+) pair • (dl) pair • Racemic mixtures are optically inactive. • Racemic mixtures are designated using the prefix (+): • (+)-2-butanol

  15. Optical Activity • Racemic mixtures are often formed during chemical reactions when the reactants and catalysts used are achiral.

  16. Optical Activity • Some mixtures are neither optically pure (all one enantiomer) nor racemic (equal mixture of both enantiomers). • Optical purity: • Ratio of the rotation of a mixture to the rotation of a pure enantiomer • o.p. = observed rotation x 100% rotation of pure enantiomer

  17. Optical Activity Example: (-)-2-butanol has a specific rotation of - 13.5o while the specific rotation of (+)-2-butanol is +13.5o. A mixture containing (+) and (-)-2-butanol has an observed rotation of – 8.55o. Does the mixture contain more (+) or more (-)-2-butanol? Calculate the optical purity of the mixture.

  18. Optical Activity • Another method to express (or determine) the relative amounts of enantiomers present in a mixture is enantiomeric excess. • Numerically identical to optical purity e.e. = o.p. = excess of one over the other x 100% entire mixture

  19. Optical Activity Example: Calculate the e.e of a mixture containing 25% (+)-2-butanol and 75% (-)-2-butanol.

  20. Optical Activity Example: Calculate the relative proportions of (+)-2-butanol and (-)-2-butanol required to give an observed rotation of +0.45o if the specific rotation of (+)-2-butanol is 13.5o.

  21. Optical Activity • Any (or all) of a set of diastereomers may be optically active(if it has a non-superimposable mirror image) • Pairs of optically active diastereomers rotate light by different amounts. (+)-glucose + 52.5o (+)-galactose + 83.9o

  22. Separation of Stereoisomers & Structural Isomers • Structural isomers and diastereomers have different physical properties: • BP, MP, density, refractive index, solubility • Can be separated through conventional means (distillation, recrystallization, chromatography) MP = 158oC MP = 256oC

  23. Resolution of Enantiomers • Since enantiomers have identical physical properties, they cannot be separated by conventional methods. • Distillation and recrystallization fail. • The process of separating enantiomers is called resolution. • Two methods: • chemical resolution • chromatographic resolution

  24. Resolution of Enantiomers • Chemical resolution of enantiomers: • temporarily convert both enantiomers into diastereomers • react with an enantiomerically pure (natural) product • separate the diastereomers based on differences in physical properties • convert each diastereomer back into the original enantiomer

  25. Resolution of Enantiomers

  26. Resolution of Enantiomers • Chromatographic resolution of enantiomers: • Prepare column containing stationary phase coated with a chiral compound • Enantiomers form diastereomeric complexes with the chiral stationary phase • Separate the diastereomeric complexes based on differences in affinity for stationary phase • strongly complexed:elutes slowly • weakly complexed:elutes more quickly

  27. Chiral Compounds w/o Asymmetric Atoms • Although most chiral compounds have at least one asymmetric atom, there are some chiral compounds that have zero asymmetric atoms: • conformation enantiomers • allenes

  28. Chiral Compounds w/o Asymmetric Atoms • Conformational enantiomers: • compounds that are so bulky or so highly strained that they cannot easily confert from one chiral conformation to the mirror-image conformation • “locked” into one conformation

  29. Chiral Compounds w/o Asymmetric Atoms • Allenes: • compounds containing a C=C=C unit • central carbon is sp hybridized • linear

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