Transcript The Chirality Center

Chapter 7
Stereochemistry
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7.1
Molecular Chirality:
Enantiomers
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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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Bromochlorofluoromethane is chiral
Cl
Br
H
It cannot be
superposed point
for point on its
mirror image.
F
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Bromochlorofluoromethane is chiral
Cl
Cl
Br
Br
H
F
H
F
To show
nonsuperposability, rotate
this model 180° around a
vertical axis.
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Bromochlorofluoromethane is chiral
Cl
Br
Cl
Br
H
F
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H
F
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Another look
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Enantiomers
nonsuperposable mirror images are
called enantiomers
and
are enantiomers with respect to each other
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Isomers
constitutional
isomers
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stereoisomers
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Isomers
constitutional
isomers
enantiomers
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stereoisomers
diastereomers
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Chlorodifluoromethane
is achiral
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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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7.2
The Chirality Center
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The Chirality Center
a carbon atom with four
different groups attached to it
w
x
C
y
z
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also called:
chiral center
asymmetric center
stereocenter
stereogenic center
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Chirality and chirality centers
A molecule with a single chirality center
is chiral.
Bromochlorofluoromethane is an example.
H
Cl
C
F
Br
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Chirality and chirality centers
A molecule with a single chirality center
is chiral.
2-Butanol is another example.
H
CH3
C
CH2CH3
OH
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Examples of molecules with 1 chirality center
CH3
CH3CH2CH2
C
CH2CH2CH2CH3
CH2CH3
a chiral alkane
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Examples of molecules with 1 chirality center
OH
Linalool, a naturally occurring chiral alcohol
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Examples of molecules with 1 chirality center
H2C
CHCH3
O
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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Examples of molecules with 1 chirality center
Limonene: a chirality
center can be part of a
ring
CH3
H
C
CH3
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CH2
attached to the
chirality center are:
—H
—CH2CH2
—CH2CH=C
—C=C
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Examples of molecules with 1 chirality center
H
D
C
CH3
T
Chiral as a result of isotopic substitution
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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).
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7.3
Symmetry in Achiral
Structures
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Symmetry tests for achiral structures
Any molecule with a plane of symmetry
or a center of symmetry must be achiral.
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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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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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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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7.4
Properties of Chiral Molecules:
Optical Activity
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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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Light
has wave properties
periodic increase and decrease in amplitude of
wave
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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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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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Polarization of light
Nicol prism
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Rotation of plane-polarized light

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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
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concentration = g/100 mL
length in decimeters
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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
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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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7.5
Absolute
and
Relative Configuration
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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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Relative configuration
CH3CHCH
CH2
OH
[] + 33.2°
H2, Pd
CH3CHCH2CH3
OH
[] + 13.5°
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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Two possibilities
HO
H
H
OH
H2, Pd
HO
H2, Pd
H
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.
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Two possibilities
HO
H
H
OH
H2, Pd
HO
H2, Pd
H
H
OH
Nor can we tell which structure corresponds to
(+)-2-butanol, and which one to (–)-2-butanol.
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Absolute configurations
HO
H
H2, Pd
OH
[] –33.2°
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H
[] +13.5°
[] +33.2°
H
HO
H2, Pd
H
OH
[] –13.5°
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Relative configuration
HBr
CH3CH2CHCH2OH
CH3
[] -5.8°
CH3CH2CHCH2Br
CH3
[] + 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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7.6
The Cahn-Ingold-Prelog
R-S
Notational System
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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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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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Example
1
1
3
4
4
3
2
2
Order of decreasing rank:
4>3>2 >1
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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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Example
1
1
3
4
4
3
2
2
Order of decreasing rank:
432
clockwise
R
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anticlockwise
S
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Enantiomers of 2-butanol
H
H
CH2CH3
CH3CH2
C
OH
H3C
(S)-2-Butanol
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HO
C
CH3
(R)-2-Butanol
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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 291.
All four compounds have positive rotations. What are their
configurations according to the Cahn-Ingold-Prelog rules?
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Chirality center in a ring
H3C
H
R
H
H
—CH2C=C > —CH2CH2 > —CH3 > —H
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