1. Chapter 7 Stereochemistry
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2. 7.1 Molecular Chirality: Enantiomers
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3. 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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4. Bromochlorofluoromethane is chiralCl
It cannot be superposed point for point on its mirror image. Br H F
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5. Bromochlorofluoromethane is chiralCl Cl Br Br H H F F
To show nonsuperposability, rotate this model 180° around a vertical axis.
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6. Bromochlorofluoromethane is chiralCl Br Br Cl H H F F
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7. Another look
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8. nonsuperposable mirror images are called enantiomers
and
are enantiomers with respect to each other
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9. Isomers constitutional isomers stereoisomers 8
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10. Isomers constitutional isomers stereoisomers enantiomers diastereomers
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11. Chlorodifluoromethane is achiral
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12. Chlorodifluoromethane is achiral
The two structures are mirror images, but are not enantiomers, because they can be superposed on each other.
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13. 7.2 The Chirality Center
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14. a carbon atom with four different groups attached to it also called:
The Chirality Center
a carbon atom with four different groups attached to it
also called:
chiral center asymmetric center stereocenter
stereogenic center w x y z C
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15. Chirality and chirality centers
A molecule with a single chirality center is chiral.
Bromochlorofluoromethane is an example. Cl F Br H C
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16. Chirality and chirality centers
A molecule with a single chirality center is chiral.
2-Butanol is another example.
CH3 OH H C
CH2CH3
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17. Examples of molecules with 1 chirality center
CH2CH3
CH2CH2CH2CH3
CH3CH2CH2
a chiral alkane
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18. Examples of molecules with 1 chirality centerOH
Linalool, a naturally occurring chiral alcohol
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19. Examples of molecules with 1 chirality center
CHCH3
1,2-Epoxypropane: a chirality center
can be part of a ring
attached to the chirality center are: —H
—CH3
—OCH2
—CH2O
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20. Examples of molecules with 1 chirality center
Limonene: a chirality center can be part of a ring
CH3 H C
CH2
attached to the chirality center are: —H
—CH2CH2
—CH2CH=C
—C=C
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21. Examples of molecules with 1 chirality centerD T H
Chiral as a result of isotopic substitution
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22. A molecule with a single chirality center must be chiral.
But, a molecule with two or more chirality centers may be chiral or it may not (Sections ).
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23. 7.3 Symmetry in Achiral Structures
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24. Symmetry tests for achiral structures
Any molecule with a plane of symmetry or a center of symmetry must be achiral.
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25. Plane of symmetry
A plane of symmetry bisects a molecule into two mirror image halves. Chlorodifluoromethane has a plane of symmetry.
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26. Plane of symmetry
A plane of symmetry bisects a molecule into two mirror image halves. 1-Bromo-1-chloro-2-fluoroethene has a plane of symmetry.
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27. Center of symmetry
A point in the center of the molecule 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.
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28. 7.4 Properties of Chiral Molecules: Optical Activity
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29. Optical Activity
A substance is optically active if it rotates the plane of polarized light.
In order for a substance to exhibit optical activity, it must be chiral and one enantiomer must be present in excess of the other.
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30. periodic increase and decrease in amplitude of wave
Light
has wave properties
periodic increase and decrease in amplitude of wave
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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
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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
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33. Polarization of light
Nicol prism
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34. Rotation of plane-polarized light
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35. therefore, define specific rotation [] as:
observed rotation () depends on the number of molecules encountered and is proportional to: path length (l), and concentration (c)
therefore, define specific rotation [] as:
100  cl
concentration = g/100 mL
length in decimeters
[] =
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36. a racemic mixture is optically inactive ( = 0)
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 be either an achiral substance or a racemic mixture
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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 (-)
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38. 7.5 Absolute and Relative Configuration
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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
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40. Relative configuration
H2, Pd
CH3CHCH OH
CH2
CH3CHCH2CH3 OH
[a] °
[a] °
No bonds are made or broken at the stereogenic center in 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 two have the same relative configuration.
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41. Two possibilities H HO H HO H2, Pd H OH H OH 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.
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42. Two possibilities H HO H HO H2, Pd H OH H OH H2, Pd
Nor can we tell which structure corresponds to (+)-2-butanol, and which one to (–)-2-butanol.
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43. Absolute configurationsH HO H HO
H2, Pd
[a] +13.5°
[a] +33.2° H OH H OH
H2, Pd
[a] –33.2°
[a] –13.5°
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44. Relative configuration
HBr
CH3CH2CHCH2OH
CH3
CH3CH2CHCH2Br
CH3
[a] -5.8°
[a] + 4.0°
Not all compounds that have the same relative configuration have the same sign of rotation. No bonds are made or broken at the stereogenic center in the reaction shown, so the relative positions of the atoms are the same. Yet the sign of rotation changes.
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45. The Cahn-Ingold-Prelog
7.6
The Cahn-Ingold-Prelog
R-S
Notational System
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46. Two requirements for a system for 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.
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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.
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48. Order of decreasing rank: 4 > 3 > 2 > 1
Example 4 3 2 1 4 3 2 1
Order of decreasing rank: 4 > 3 > 2 > 1
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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.
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50. Order of decreasing rank: 4 ® 3 ® 2
Example 4 3 2 1 4 3 2 1
Order of decreasing rank: 4 ® 3 ® 2
clockwise
anticlockwise R S
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51. Enantiomers of 2-butanolC OH
H3C H
CH3CH2 C HO
CH3 H
CH2CH3
(S)-2-Butanol
(R)-2-Butanol
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52. Very important!
Two different compounds with the same sign of rotation need not have the same configuration.
Verify this statement by doing Problem 7.7 on page All four compounds have positive rotations. What are their configurations according to the Cahn-Ingold-Prelog rules?
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53. Chirality center in a ring
H3C R
—CH2C=C > —CH2CH2 > —CH3 > —H
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