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CHE 240 Unit IV Stereochemistry, Substitution and Elimination Reactions CHAPTER FIVE

CHE 240 Unit IV Stereochemistry, Substitution and Elimination Reactions CHAPTER FIVE. Terrence P. Sherlock Burlington County College 2004. Chiral Carbons. Tetrahedral carbons with 4 different attached groups are chiral.

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CHE 240 Unit IV Stereochemistry, Substitution and Elimination Reactions CHAPTER FIVE

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  1. CHE 240Unit IVStereochemistry, Substitution and Elimination ReactionsCHAPTER FIVE Terrence P. Sherlock Burlington County College 2004

  2. Chiral Carbons • Tetrahedral carbons with 4 different attached groups are chiral. • Its mirror image will be a different compound (enantiomer). => Chapter 5

  3. Mirror Planes of Symmetry • If two groups are the same, carbon is achiral. (animation) • A molecule with an internal mirror plane cannot be chiral.* Caution! If there is no plane of symmetry, molecule may be chiral or achiral. See if mirror image can be superimposed. => Chapter 5

  4. => (R), (S) Nomenclature • Different molecules (enantiomers) must have different names. • Usually only one enantiomer will be biologically active. • Configuration around the chiral carbon is specified with (R) and (S). Chapter 5

  5. Cahn-Ingold-Prelog Rules • Assign a priority number to each group attached to the chiral carbon. • Atom with highest atomic number assigned the highest priority #1. • In case of ties, look at the next atoms along the chain. • Double and triple bonds are treated like bonds to duplicate atoms. => Chapter 5

  6. Assign (R) or (S) • Working in 3D, rotate molecule so that lowest priority group is in back. • Draw an arrow from highest to lowest priority group. • Clockwise = (R), Counterclockwise = (S) => Chapter 5

  7. Properties of Enantiomers • Same boiling point, melting point, density • Same refractive index • Different direction of rotation in polarimeter • Different interaction with other chiral molecules • Enzymes • Taste buds, scent => Chapter 5

  8. Optical Activity • Rotation of plane-polarized light • Enantiomers rotate light in opposite directions, but same number of degrees. => Chapter 5

  9. Polarimetry • Use monochromatic light, usually sodium D • Movable polarizing filter to measure angle • Clockwise = dextrorotatory = d or (+) • Counterclockwise = levorotatory = l or (-) • Not related to (R) and (S) => Chapter 5

  10. Biological Discrimination => Chapter 5

  11. Racemic Mixtures • Equal quantities of d- and l- enantiomers. • Notation: (d,l) or () • No optical activity. • The mixture may have different b.p. and m.p. from the enantiomers! => Chapter 5

  12. Racemic Products If optically inactive reagents combine to form a chiral molecule, a racemic mixture of enantiomers is formed. => Chapter 5

  13. Nonmobile Conformers If the conformer is sterically hindered, it may exist as enantiomers. => Chapter 5

  14. Fischer Projections • Flat drawing that represents a 3D molecule • A chiral carbon is at the intersection of horizontal and vertical lines. • Horizontal lines are forward, out-of-plane. • Vertical lines are behind the plane. Chapter 5

  15. Fischer Rules • Carbon chain is on the vertical line. • Highest oxidized carbon at top. • Rotation of 180 in plane doesn’t change molecule. • Do not rotate 90! • Do not turn over out of plane! => Chapter 5

  16. => Fischer Mirror Images • Easy to draw, easy to find enantiomers, easy to find internal mirror planes. • Examples: Chapter 5

  17. (S) => (S) Fischer (R) and (S) • Lowest priority (usually H) comes forward, so assignment rules are backwards! • Clockwise 1-2-3 is (S) and counterclockwise 1-2-3 is (R). • Example: Chapter 5

  18. Diastereomers • Stereoisomers that are not mirror images. • Geometric isomers (cis-trans) • Molecules with 2 or more chiral carbons. => Chapter 5

  19. Alkenes Cis-trans isomers are not mirror images, so these are diastereomers. => Chapter 5

  20. => Ring Compounds • Cis-trans isomers possible. • May also have enantiomers. • Example: trans-1,3-dimethylcylohexane Chapter 5

  21. Two or More Chiral Carbons • Enantiomer? Diastereomer? Meso? Assign (R) or (S) to each chiral carbon. • Enantiomers have opposite configurations at each corresponding chiral carbon. • Diastereomers have some matching, some opposite configurations. • Meso compounds have internal mirror plane. • Maximum number is 2n, where n = the number of chiral carbons. => Chapter 5

  22. => Examples Chapter 5

  23. Fischer-Rosanoff Convention • Before 1951, only relative configurations could be known. • Sugars and amino acids with same relative configuration as (+)-glyceraldehyde were assigned D and same as (-)-glyceraldehyde were assigned L. • With X-ray crystallography, now know absolute configurations: D is (R) and L is (S). • No relationship to dextro- or levorotatory. => Chapter 5

  24. => * * D and L Assignments * Chapter 5

  25. Properties of Diastereomers • Diastereomers have different physical properties: m.p., b.p. • They can be separated easily. • Enantiomers differ only in reaction with other chiral molecules and the direction in which polarized light is rotated. • Enantiomers are difficult to separate.=> Chapter 5

  26. Resolution of Enantiomers React a racemic mixture with a chiral compound to form diastereomers, which can be separated. => Chapter 5

  27. ChromatographicResolution of Enantiomers => Chapter 5

  28. POWER POINT IMAGES FROM “ORGANIC CHEMISTRY, 5TH EDITION”L.G. WADEALL MATERIALS USED WITH PERMISSION OF AUTHORPRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGEORGANIC CHEMISTRY COURSEBY:ANNALICIA POEHLER STEFANIE LAYMAN CALY MARTIN Chapter 5

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