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Chapter 5 Stereoisomerism and Chirality

Chapter 5 Stereoisomerism and Chirality. Donald Cram. Source: Michigan State University, Department of Chemistry http://www.chemistry.msu.edu/Portraits/PortraitsHH_collection.shtml. Chapter 5. Skip p. 388-389 Read p. 475-477 for interest

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Chapter 5 Stereoisomerism and Chirality

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  1. Chapter 5Stereoisomerismand Chirality

  2. Donald Cram Source: Michigan State University, Department of Chemistry http://www.chemistry.msu.edu/Portraits/PortraitsHH_collection.shtml

  3. Chapter 5 • Skip p. 388-389 • Read p. 475-477 for interest • Don’t memorize names of sugars (p. 460-465) or amino acids (p. 472-474)

  4. Chapter 5 Problems • 1 a, b, c, e, g, m 2 • 4 a, b, c, d, f, g, i 5 a, b, e 11, 12 • 13 b, c, d 14 - 18 20 a, b, d • 23 - 28 30 32 - 36 • 38 40, 41 43 • 45 48 52 • 59

  5. Sect 5.1: Symmetry and Chirality • If a molecule is superimposable on its mirror image, it is achiral (non-chiral). • If a molecule is not superimposable on its mirror image, it is chiral. Enantiomers are isomers that are non-superimposable mirror-images

  6. Sect 5.2: Enantiomers Four different atoms are attached to a chiral carbon atom. Mirror images are non-superimposable. rotate this molecule ischiral note that the fluorine and bromine have been interchanged in the enantiomer

  7. Stereocenters The previous molecule has a stereocenter, and is chiral. A stereocenter is an atom, or a group of atoms, that can potentially cause a molecule to be chiral. stereocenters - can give rise to chirality

  8. An achiral molecule A carbon atom with three identical groups is achiral! There is a symmetry plane in the plane of the paper.

  9. C l C l C l C l B r C l B r C l plane of symmetry side view edge view

  10. Another achiral molecule This molecule has a plane of symmetry in the plane of the paper. These two structures are superimposable!

  11. Sect 5.3: identification of stereocenters Look for carbon atoms with four different groups! * *

  12. * * *

  13. * * Finding stereocenters plane ( indicates no stereoisomers )

  14. Finding stereocenters * * * * * * The cis isomer (two methyl groups) - achiral! Thetrans isomers has two stereocenters!

  15. n = 8 28 = 256 Finding stereocenters * * * * * * * *

  16. Sect. 5.4 Properties of Enantiomers • Enantiomers interact differently with polarized light. • Enantiomers have equal magnitude, but opposite signs of rotation • Most other properties are identical. • Odor may be different!!

  17. Sect. 5.5 Polarimetry • dissolve sample in a solvent and put sample into polarimeter • obtain sign of rotation • value depends on concentration and path length.

  18. Optical Activity

  19. TYPES OF OPTICAL ACTIVITY new older (+)- d- Dextrorotatory Rotates the plane of plane-polarized light to the right. new older (-)- l- Levorotatory Rotates the plane of plane-polarized light to the left.

  20. Specific Rotation [a]D This equation corrects for differences in cell length and concentration. a [a]D = t cl Specific rotation calculated in this way is a physical property of an optically active substance. a = observed rotation You always get the same c = concentration ( g/mL ) [a]D t value of l = length of cell ( dm ) D = yellow light from sodium lamp t = temperature ( Celsius )

  21. ENANTIOMERS HAVE EQUAL VALUES BUT OPPOSITE SIGNS OF ROTATIONS W W Enantiomers C C Y X X Y Z Z (+)-numbero (-)-numbero dextrorotatory levorotatory The numbers are the same, but have opposite signs. All other physical properties are IDENTICAL.

  22. Racemic mixture an equimolar (50/50) mixture of enantiomers [a]D = 0o the effect of each molecule is cancelled out by its enantiomer

  23. Sect. 5.6 Configuration • Arrangement in space of atoms or groups around the stereocenter of the molecule • Enantiomers have different configurations.

  24. Sect. 5.7 Specification of Configuration: Cahn-Ingold-Prelog rules

  25. Sir Christopher Ingold Source: Michigan State University, Department of Chemistry http://www.chemistry.msu.edu/Portraits/PortraitsHH_collection.shtml

  26. Specification of Configuration • SEQUENCE RULE 1: priority depends on the atomic numbers of the 4 atoms attached to the stereocenter; atom with higher atomic number receives the higher priority. • If two atoms are isotopes of the same element, the heavier isotope is assigned the higher priority.

  27. R S

  28. Specification of Configuration • SEQUENCE RULE 2: If the relative priority of two groups cannot be decided by Rule 1, it shall be determined by a similar comparison of the next atoms in the groups (and so on, if necessary), working outward in ranks from the stereocenter.

  29. Specification of Configuration SEQUENCE RULE 3: A doubly-bonded atom A is treated as if there were two C-A single bonds.Priorities in the expanded representations are assigned on the basis of Rule 2.

  30. Remember! The atoms shown in parentheses (the duplicate representations) do not exist! They are written only for purposes of assigning priorities.

  31. More... A triply-bonded atom A is treated as if there were three C-A bonds, as in:

  32. Corollary of Rule 3 • If no other distinction can be made, a real atom outranks a “fictional” atom. • NOTE CAREFULLY: This exception is used only as a last resort! You will rarely see this happen.

  33. Sect. 5.8: Compounds with more than one stereocenter: 3-Chloro-2-butanol R R S S R S S R

  34. diastereomers 3- Chloro-2-butanol: Fischer formulas (Sect. 5.9) pair of enantiomers-1 pair of enantiomers-2

  35. How Many Stereoisomers Are Possible? maximumnumber of stereoisomers = 2n, where n = number of stereocenters sometimes fewer than this number will exist For the previous example; two stereocenters = 4

  36. Determining the number of possible stereoisomers * * 22 = 4 stereoisomers * * * 23 = 8 stereoisomers

  37. mirror image is identical 2,3-Dichlorobutane S R meso diastereomers S S R R enantiomers

  38. 2,3-Dichlorobutane meso

  39. diastereomers 2,3-Dichlorobutane: Fischer formulas (Sect. 5.9) Meso! “Pair” becomes one! pair of enantiomers

  40. Tartaric Acid (from fermentation of wine): There are three stereo-isomers (+)-tartaric acid (-)-tartaric acid ALSO FOUND (as a minor component) [a]D = 0 meso -tartaric acid

  41. Sect 5.9: Fischer Formulas

  42. C H O H O C H 2 C H O C H O H 2 EVOLUTION OF THE FISCHER PROJECTION “Sawhorse” Projection O H H Fischer Projection Orient the main chain vertically with the most oxidized group at the top. C H O H O H H O H C H O H 2 Substituents will stick out toward you like prongs Main chain bends away from you

  43. OPERATIONS WITH FISCHER PROJECTIONS Mirror images (enantiomers) are created by switching substituents to the other side. enantiomers diastereomers All stereocenters must be switched to get an enantiomer (the mirror inverts them all). If you switch only one of the stereocenters, but not both, you get a diastereomer.

  44. Legal operations with Fischer formulas . 180o Rotation by 180o in the plane of the paper does not change the molecule. No other rotation is allowed.

  45. This molecule is meso! There isn’t an enantiomer The molecule does not have an enantiomer because it is not chiral. It had a plane of symmetry, rendering it a meso molecule. plane of symmetry

  46. To determine the configuration (R/S). Make two “switches”. Place the priority 4 group in one of the vertical positions. #4 at top position #4 at bottom position 2 4 H CHO 1 H O H OHC C H O H 4 2 2 3 C H O H O H R 2 3 1 Both H’s are in back = same result alternatively: 1 2 O H R R CHO 3 2 1 CHO HOCH2 H O H 4 H C H O H 2 4 3

  47. Draw all of the stereoisomers!

  48. Any more?? NO!

  49. There are only four stereoisomers!

  50. Sect 5.10: cyclic compounds

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