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Properties of Chiral Molecules:
Optical Activity
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.
Light
has wave properties
periodic increase and decrease in amplitude
of wave
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
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
Polarization of light
Polarization of light
Nicol prism
Polarization of light
Nicol prism
Rotation of plane-polarized light
Rotation of plane-polarized light
Rotation of plane-polarized light
Rotation of plane-polarized light
a
Specific rotation
observed rotation (a) depends on the number
of molecules encountered and is proportional to:
path length (l), and
concentration (c)
Specific rotation
observed rotation (a) depends on the number
of molecules encountered and is proportional to:
path length (l), and
concentration (c)
therefore, define specific rotation [a] as:
[a] =
100 a
cl
concentration = g/100 mL
length in decimeters
Racemic mixture
a mixture containing equal quantities
of enantiomers is called a racemic mixture
a racemic mixture is optically inactive
(a = 0)
a sample that is optically inactive can be
either an achiral substance or a racemic
mixture
Optical purity
an optically pure substance consists exclusively
of a single enantiomer
enantiomeric excess =
% one enantiomer – % other enantiomer
% optical purity = enantiomeric excess
Stereochemistry
Physical Properties of Stereoisomers
• Since enantiomers have identical physical properties, they cannot
be separated by common physical techniques like distillation.
• Diastereomers and constitutional isomers have different physical
properties, and therefore can be separated by common physical
techniques.
Figure 5.12
The physical properties of the
three stereoisomers of
tartaric acid
Stereochemistry
Chemical Properties of Enantiomers
• Two enantiomers have exactly the same chemical properties except
for their reaction with chiral non-racemic reagents.
• Many drugs are chiral and often must react with a chiral receptor or
chiral enzyme to be effective. One enantiomer of a drug may
effectively treat a disease whereas its mirror image may be
ineffective or toxic.
Stereochemistry
Enantiomers and the Sense of Smell
• Research suggests that the odor of a particular molecule
is determined more by its shape than by the presence of
a particular functional group.
• Because enantiomers interact with chiral smell
receptors, some enantiomers have different odors.
Absolute
and
Relative Configuration
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.
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
Relative configuration
Pd
CH3CHCH
CH2
OH
[a] + 33.2°
CH3CHCH2CH3
OH
[a] + 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.
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.
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.
Absolute configurations
HO
H
H2, Pd
OH
[a] –13.5°
H
[a] +33.2°
[a] +13.5°
H
HO
H2, Pd
H
OH
[a] –33.2°
Relative configuration
HBr
CH3CH2CHCH2OH
CH3
[a] -5.8°
CH3CH2CHCH2Br
CH3
[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.