Dr. Wolf's CHM 201 & 202
7-1

Dr. Wolf's CHM 201 & 202
7-2

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!
Dr. Wolf's CHM 201 & 202 7-3

Bromochlorofluoromethane is chiral
Br
Cl
H
It cannot be superposed point for point on its mirror image.
F
Dr. Wolf's CHM 201 & 202 7-4

Bromochlorofluoromethane is chiral
Br
Cl
F
Dr. Wolf's CHM 201 & 202
H
Cl
Br
H
F
To show nonsuperposability, rotate this model 180° around a vertical axis.
7-5

Bromochlorofluoromethane is chiral
Br
Cl
F
H
Cl
F
Br
H
Dr. Wolf's CHM 201 & 202 7-6

Another look
Dr. Wolf's CHM 201 & 202 7-7

Enantiomers nonsuperposable mirror images are called enantiomers and are enantiomers with respect to each other
Dr. Wolf's CHM 201 & 202 7-8

constitutional isomers
stereoisomers
Dr. Wolf's CHM 201 & 202 7-9

constitutional isomers
stereoisomers enantiomers
Dr. Wolf's CHM 201 & 202 diastereomers
7-10

Dr. Wolf's CHM 201 & 202
Chlorodifluoromethane is achiral
7-11

Dr. Wolf's CHM 201 & 202
Chlorodifluoromethane is achiral
The two structures are mirror images, but are not enantiomers, because they can be superposed on each other.
7-12

Dr. Wolf's CHM 201 & 202
7.2
The Chirality Center
7-13

x w
C z y a carbon atom with four different groups attached to it also called: chiral center asymmetric center stereocenter stereogenic center
Dr. Wolf's CHM 201 & 202 7-14

Chirality and chirality centers
A molecule with a single chirality center is chiral.
Bromo chloro fluoro methane is an example.
Cl
H
C
Br
F
Dr. Wolf's CHM 201 & 202 7-15

Chirality and chirality centers
A molecule with a single chirality center is chiral.
2-Butanol is another example.
H
CH
3
C CH
2
CH
3
OH
Dr. Wolf's CHM 201 & 202 7-16

Examples of molecules with 1 chirality center
CH
3
CH
3
CH
2
CH
2
C CH
2
CH
2
CH
2
CH
3
CH
2
CH
3 a chiral alkane
Dr. Wolf's CHM 201 & 202 7-17

Examples of molecules with 1 chirality center
OH
Linalool, a naturally occurring chiral alcohol
Dr. Wolf's CHM 201 & 202 7-18

Examples of molecules with 1 chirality center
H
2
C CHCH
3
O
1,2-Epoxypropane: a chirality center can be part of a ring attached to the chirality center are:
—H
—CH
3
—
OCH
2
—CH
2
O
Dr. Wolf's CHM 201 & 202 7-19

Examples of molecules with 1 chirality center
CH
3
H C
CH
3
Dr. Wolf's CHM 201 & 202
CH
2
Limonene: a chirality center can be part of a ring attached to the chirality center are:
—H
—CH
2
CH
2
—
CH
2
CH=C
—C=C
7-20

Examples of molecules with 1 chirality center
H
D C
T
CH
3
Chiral as a result of isotopic substitution
Dr. Wolf's CHM 201 & 202 7-21

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 7.10-7.13).
Dr. Wolf's CHM 201 & 202 7-22

Dr. Wolf's CHM 201 & 202
7-23

Symmetry tests for achiral structures
Any molecule with a plane of symmetry or a center of symmetry must be achiral .
Dr. Wolf's CHM 201 & 202 7-24

A plane of symmetry bisects a molecule into two mirror image halves. Chlorodifluoromethane has a plane of symmetry.
Dr. Wolf's CHM 201 & 202 7-25

A plane of symmetry bisects a molecule into two mirror image halves.
1-Bromo-1-chloro-2-fluoroethene has a plane of symmetry.
Dr. Wolf's CHM 201 & 202 7-26

Dr. Wolf's CHM 201 & 202
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.
7-27

7.4
Properties of Chiral Molecules:
Optical Activity
Dr. Wolf's CHM 201 & 202 7-28

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.
Dr. Wolf's CHM 201 & 202 7-29

Light has wave properties periodic increase and decrease in amplitude of wave
Dr. Wolf's CHM 201 & 202 7-30

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
Dr. Wolf's CHM 201 & 202 7-31

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
Dr. Wolf's CHM 201 & 202 7-32

Polarization of light
Dr. Wolf's CHM 201 & 202
7-33

Rotation of plane-polarized light

Dr. Wolf's CHM 201 & 202 7-34

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:
[

] =
100
 cl concentration = g/100 mL length in decimeters
Dr. Wolf's CHM 201 & 202 7-35

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 be either an achiral substance or a racemic mixture
Dr. Wolf's CHM 201 & 202 7-36

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 (-)
Dr. Wolf's CHM 201 & 202 7-37

Dr. Wolf's CHM 201 & 202
7.5
Absolute and
Relative Configuration
7-38

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
Dr. Wolf's CHM 201 & 202 7-39

Relative configuration
CH
3
CHCH CH
2
OH
[

] + 33.2
°
H
2
, Pd
CH
3
[
CHCH
2
OH

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.
Dr. Wolf's CHM 201 & 202
CH
3
] + 13.5
°
7-40

HO
H
Two possibilities
H
2
, Pd
HO
H
H OH
H
2
, Pd
H
OH
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.
Dr. Wolf's CHM 201 & 202 7-41

HO
H
Two possibilities
H
2
, Pd
HO
H
H OH
H
2
, Pd
H
OH
Nor can we tell which structure corresponds to
(+)-2-butanol, and which one to (–)-2-butanol.
Dr. Wolf's CHM 201 & 202 7-42

HO
H
Absolute configurations
H
2
, Pd
HO
H
[

H
] +33.2
OH
°
H
2
, Pd
[

] +13.5
°
H
OH
[

] –33.2°
Dr. Wolf's CHM 201 & 202
[

] –13.5°
7-43

Relative configuration
HBr
CH
3
CH
2
CHCH
2
OH
CH
3
[

] -5.8
°
CH
3
[
CH

2
CHCH
CH
] + 4.0
3
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.
Dr. Wolf's CHM 201 & 202
°
2
Br
7-44

Dr. Wolf's CHM 201 & 202
7.6
The Cahn-Ingold-Prelog
R-S
Notational System
7-45

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.
Dr. Wolf's CHM 201 & 202 7-46

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.
Dr. Wolf's CHM 201 & 202 7-47

4
Example
1 1
3 3
2
Order of decreasing rank:
4 > 3 > 2 > 1
2
4
Dr. Wolf's CHM 201 & 202 7-48

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 .
Dr. Wolf's CHM 201 & 202 7-49

Example
1 1
4 3 3
2
2 clockwise
R
Order of decreasing rank:
4

3

2 anticlockwise
S
Dr. Wolf's CHM 201 & 202
4
7-50

Enantiomers of 2-butanol
CH
3
CH
2
H
C OH
H
3
C
( S )-2-Butanol
H
CH
2
CH
3
HO C
CH
3
( R )-2-Butanol
Dr. Wolf's CHM 201 & 202 7-51

Verify this statement by doing Problem 7.7 on page 291.
All four compounds have positive rotations. What are their configurations according to the Cahn-Ingold-Prelog rules?
Dr. Wolf's CHM 201 & 202 7-52

Chirality center in a ring
H
3
C H
R
H
H
—CH
2
C=C > —CH
2
CH
2
> —CH
3
> —H
Dr. Wolf's CHM 201 & 202 7-53