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Chapter 7 Stereochemistry

Chapter 7 Stereochemistry. 7.1 Molecular Chirality: Enantiomers. Chirality. A molecule is chiral if its two mirror image forms are not superposable upon one another. ASYMMETRIC! A molecule is achiral if its two mirror image forms are superposable. SYMMETRIC!.

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Chapter 7 Stereochemistry

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  1. Chapter 7Stereochemistry

  2. 7.1Molecular Chirality: Enantiomers

  3. Chirality A molecule is chiral if its two mirror image forms are notsuperposable upon one another. ASYMMETRIC! A molecule is achiral if its two mirror image forms are superposable. SYMMETRIC!

  4. Bromochlorofluoromethane is chiral It cannot be superposed point for point on its mirror image. Cl Br H F

  5. Bromochlorofluoromethane is chiral To show nonsuperposability, rotate this model 180° around a vertical axis. Cl Cl Br Br H H F F

  6. Bromochlorofluoromethane is chiral Cl Br Cl Br H H F F

  7. Another look

  8. Enantiomers nonsuperposable mirror images are called enantiomers are enantiomers with respect to each other and

  9. Isomers constitutional isomers stereoisomers

  10. Isomers constitutional isomers stereoisomers enantiomers diastereomers

  11. Chlorodifluoromethaneis achiral

  12. Chlorodifluoromethaneis achiral The two structures are mirror images, but are not enantiomers, because they can be superposed on each other.

  13. 7.2The Chirality Center

  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.

  17. CH3 CH2CH2CH2CH3 CH3CH2CH2 C CH2CH3 Examples of molecules with 1 chirality center a chiral alkane

  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 attached to the chirality center are: —H —CH3 —OCH2 —CH2O 1,2-Epoxypropane: a chirality center can be part of a ring

  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 —C=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 (Sections 7.10-7.13).

  23. 7.3Symmetry in Achiral Structures

  24. Symmetry tests for achiral structures Any molecule with a plane of symmetryor a center of symmetry must be achiral.

  25. Plane of symmetry A plane of symmetry bisects a molecule into two mirror image halves. Chlorodifluoromethane has a plane of symmetry.

  26. Plane of symmetry A plane of symmetry bisects a molecule into two mirror image halves.1-Bromo-1-chloro-2-fluoroethene has a planeof symmetry.

  27. Center of symmetry A point in the center of themolecule is a center of symmetry if a line drawn from it to any element, when extended an equal distance in the opposite direction, encounters an identical element.

  28. 7.4Properties of Chiral Molecules:Optical Activity

  29. 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.

  30. Light has wave properties periodic increase and decrease in amplitude of wave

  31. 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

  32. 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

  33. Nicol prism Polarization of light

  34. Rotation of plane-polarized light

  35. 100  concentration = g/100 mL length in decimeters [] = cl Specific rotation observed rotation () depends on the number of molecules encountered and is proportional to:path length (l), and concentration (c) therefore, define specific rotation [] as:

  36. Racemic mixture a mixture containing equal quantities of enantiomers is called a racemic mixture a racemic mixture is optically inactive( = 0) a sample that is optically inactive can beeither an achiral substance or a racemicmixture

  37. Optical purity an optically pure substance consists exclusively of a single enantiomer enantiomeric excess = % one enantiomer – % other enantiomer % optical purity = enantiomeric excess e.g. 75% (-) – 25% (+) = 50% opt. pure (-)

  38. 7.5AbsoluteandRelative Configuration

  39. 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

  40. CH3CHCH CH3CHCH2CH3 CH2 OH OH Relative configuration H2,Pd No bonds are made or broken at the stereogenic 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. [a] + 33.2° [a] + 13.5°

  41. H HO H HO OH OH H H Two possibilities 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. H2, Pd

  42. H HO H HO OH OH H H Two possibilities H2, Pd Nor can we tell which structure corresponds to(+)-2-butanol, and which one to (–)-2-butanol. H2, Pd

  43. H HO H HO OH OH H H Absolute configurations H2, Pd [a] +13.5° [a] +33.2° H2, Pd [a] –33.2° [a] –13.5°

  44. CH3CH2CHCH2OH CH3CH2CHCH2Br CH3 CH3 Relative configuration HBr Not all compounds that have the same relativeconfiguration have the same sign of rotation. No bondsare made or broken at the stereogenic center in thereaction shown, so the relative positions of the atoms are the same. Yet the sign of rotation changes. [a] -5.8° [a] + 4.0°

  45. 7.6 The Cahn-Ingold-Prelog R-S Notational System

  46. Two requirements for a systemfor specifying absolute configuration 1. need rules for ranking substituents at stereogenic 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.

  47. The Cahn-Ingold-Prelog Rules(Table 7.1) 1. Rank the substituents at the stereogenic center according to same rules used in E-Z notation. 2. Orient the molecule so that lowest-ranked substituent points away from you.

  48. 1 1 4 3 3 4 2 2 Example Order of decreasing rank:4 > 3 > 2 > 1

  49. The Cahn-Ingold-Prelog Rules(Table 7.1) • 1. Rank the substituents at the stereogenic 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 anticlockwise, the configuration is S.

  50. 1 1 4 3 3 4 2 2 Example Order of decreasing rank:4®3® 2 anticlockwise clockwise R S

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