5. Chirality A molecule is chiral if its two mirror image forms are not superposable upon one another. A molecule is achiral if its two mirror image forms are superposable.
14. w x y C z The Chirality Center a carbon atom with fourdifferent groups attached to it also called: chiral centerasymmetric centerstereocenter stereogenic center
15. H F Cl C Br Chirality and chirality centers A molecule with a single chirality center is chiral. Bromochlorofluoromethane is an example.
16. H CH3 CH2CH3 C OH Chirality and chirality centers A molecule with a single chirality center is chiral. 2-Butanol is another example.
18. OH Examples of molecules with 1 chirality center Linalool, a naturally occurring chiral alcohol
19. H2C CHCH3 O Examples of molecules with 1 chirality center 1,2-Epoxypropane: a chirality center can be part of a ring attached to the chirality center are: —H —CH3 —OCH2 —CH2O
20. CH3 H C CH2 CH3 Examples of molecules with 1 chirality center Limonene: a chirality center can be part of a ring attached to thechirality center are: —H —CH2CH2 —CH2CH= —C=
21. H CH3 D C T Examples of molecules with 1 chirality center Chiral as a result of isotopic substitution
22. A molecule with a single chirality centermust be chiral. But, a molecule with two or more chirality centers may be chiral or it may not.
24. Optical Activity A substance is optically active if it rotates the plane of polarized light. In order for a substance to exhibit opticalactivity, it must be chiral and one enantiomer must be present in excess of the other.
25. Light has wave properties periodic increase and decrease in amplitude of wave
26. Light optical activity is usually measured using light having a wavelength of 589 nm this is the wavelength of the yellow light from a sodium lamp and is called the D line of sodium
27. Polarized light ordinary (nonpolarized) light consists of many beams vibrating in different planes plane-polarized light consists of only those beams that vibrate in the same plane
31. Configuration Relative configuration compares the arrangement of atoms in space of one compound with those of another. Absolute configuration is the precise arrangement of atoms in space.
32. Configuration Relative configuration compares the arrangement of atoms in space of one compound with those of another.until the 1950s, all configurations were relative Absolute configuration is the precise arrangement of atoms in space. we can now determine the absolute configuration of almost any compound
33. CH3CHCH2CH3 CH3CHCH CH2 OH OH Relative configuration Pd [] + 33.2° [] + 13.5° No bonds are made or broken at the chirality centerin this experiment. Therefore, when (+)-3-buten-2-ol and (+)-2-butanol have the same sign of rotation, the arrangement of atoms in space is analogous. The twohave the same relative configuration.
34. H HO H HO OH OH H H Two possibilities H2, Pd H2, Pd But in the absence of additional information, we can't tell which structure corresponds to(+)-3-buten-2-ol, and which one to (–)-3-buten-2-ol.
35. H HO H HO OH OH H H Two possibilities H2, Pd H2, Pd Nor can we tell which structure corresponds to(+)-2-butanol, and which one to (–)-2-butanol.
36. H HO H HO OH OH H H Absolute configurations H2, Pd [] +33.2° [] +13.5° H2, Pd [] –13.5° [] –33.2°
37. CH3CH2CHCH2Br CH3CH2CHCH2OH CH3 CH3 Relative configuration HBr [] -5.8° [] + 4.0° Not all compounds that have the same relativeconfiguration have the same sign of rotation. No bondsare made or broken at the chirality center in thereaction shown, so the relative positions of the atoms are the same. Yet the sign of rotation changes.
39. Two requirements for a systemfor specifying absolute configuration 1. need rules for ranking substituents at chirality center in order of decreasing precedence 2. need convention for orienting molecule so that order of appearance of substituents can be compared with rank The system that is used was devised by R. S. Cahn, Sir Christopher Ingold, and V. Prelog.
40. The Cahn-Ingold-Prelog Rules 1. Rank the substituents at the chirality center according to same rules used in E-Z notation. 2. Orient the molecule so that lowest-ranked substituent points away from you.
41. 1 1 4 3 3 4 2 2 Example Order of decreasing rank:4 > 3 > 2 > 1
42. The Cahn-Ingold-Prelog Rules 1. Rank the substituents at the chirality center according to same rules used in E-Z notation. 2. Orient the molecule so that lowest-ranked substituent points away from you. 3. If the order of decreasing precedence traces a clockwise path, the absolute configuration is R. If the path is counterclockwise, the configuration is S.
43. 1 1 4 3 3 4 2 2 counterclockwise clockwise R S Example Order of decreasing rank:43 2
44. H H CH2CH3 CH3CH2 C HO C OH CH3 H3C (S)-2-Butanol (R)-2-Butanol Enantiomers of 2-butanol
45. Very important! Two different compounds with the same sign of rotation need not have the same configuration. Verify this statement by doing Problem. All four compounds have positive rotations. What are their configurations according to the Cahn-Ingold-Prelog rules?
46. H H3C R H H Chirality center in a ring —CH2C=C > —CH2CH2 > —CH3 > —H
47. Fischer Projections Purpose of Fischer projections is to show configuration at chirality center without necessity of drawing wedges and dashes or using models.
48. Rules for Fischer projections H Cl Br F Arrange the molecule so that horizontal bonds at chirality center point toward you and vertical bonds point away from you.
49. Rules for Fischer projections H Br Cl F Projection of molecule on page is a cross. When represented this way it is understood that horizontal bonds project outward, vertical bonds are back.
50. Rules for Fischer projections H Br Cl F Projection of molecule on page is a cross. When represented this way it is understood that horizontal bonds project outward, vertical bonds are back.
52. Physical properties of enantiomers Same: melting point, boiling point, density, etc Different: properties that depend on shape of molecule (biological-physiological properties) can be different
53. Odor CH3 CH3 O O H3C H3C CH2 CH2 (–)-Carvonespearmint oil (+)-Carvonecaraway seed oil
54. H H3C CH2CH(CH3)2 C C HO O Chiral drugs Ibuprofen is chiral, but normally sold asa racemic mixture. The S enantiomer is the one responsible for its analgesic and antiinflammatory properties.
56. O CH3CHCHCOH HO OH 2,3-Dihydroxybutanoic acid 2 3 What are all the possible R and S combinations of the two chirality centers in this molecule? Carbon-2 R R S S Carbon-3 R S R S
57. O CH3CHCHCOH HO OH 2,3-Dihydroxybutanoic acid 2 3 4 Combinations = 4 Stereoisomers Carbon-2 R R S S Carbon-3 R S R S
58. O CH3CHCHCOH HO OH 2,3-Dihydroxybutanoic acid 2 3 4 Combinations = 4 Stereoisomers What is the relationship between these stereoisomers? Carbon-2 R R S S Carbon-3 R S R S
59. O CH3CHCHCOH HO OH 2,3-Dihydroxybutanoic acid 2 3 enantiomers: 2R,3R and 2S,3S 2R,3S and 2S,3R Carbon-2 R R S S Carbon-3 R S R S
60. CO2H CO2H [] = -9.5° [] = +9.5° R S HO OH H H enantiomers OH HO H H R S CH3 CH3 CO2H CO2H S R OH HO H H enantiomers OH H HO H R S [] = -17.8° [] = +17.8° CH3 CH3
61. O CH3CHCHCOH HO OH 2,3-Dihydroxybutanoic acid 2 3 but not all relationships are enantiomeric stereoisomers that are not enantiomers are diastereomers Carbon-2 R R S S Carbon-3 R S R S
63. CO2H CO2H R S HO OH H H OH HO H H R S CH3 CH3 diastereomers CO2H CO2H S R OH HO H H OH H HO H R S CH3 CH3 [] = -9.5° [] = +9.5° enantiomers enantiomers [] = -17.8° [] = +17.8°
64. Fischer Projections recall for Fischer projection: horizontal bonds point toward you; vertical bonds point away staggered conformation does not have correct orientation of bonds for Fischer projection CO2H CH3
65. Fischer projections transform molecule to eclipsed conformation in order to construct Fischer projection
67. Two chirality centers in a ring S R S R trans-1-Bromo-1-chlorocyclopropane nonsuperposable mirror images; enantiomers
68. Two chirality centers in a ring S S R R cis-1-Bromo-1-chlorocyclopropane nonsuperposable mirror images; enantiomers
69. Two chirality centers in a ring S S R R cis-1-Bromo-1-chloro-cyclopropane trans-1-Bromo-1-chloro-cyclopropane stereoisomers that are notenantiomers; diastereomers
