10/4/2010
John E. McMurry
http://www.cengage.com/chemistry/mcmurry
Chapter 5
Stereochemistry at
Tetrahedral Centers
Richard Morrison • University of Georgia, Athens
Handedness
Right and left hands are not identical
• Right and left hands are mirror images of each
other – they are nonsuperimposable mirror
images
• Almost all the molecules in the human body are
handed
• Handedness primarily arises from the tetrahedral
stereochemistry of sp3-hybridized carbon atoms
5.1 Enantiomers and the Tetrahedral
Carbon
Molecular handedness
•
Molecules CH3X and
CH2XY are identical to
their mirror images
•
•
Molecular images can
superimpose on their
mirror images
Molecule CHXYZ is not
identical to its mirror
image
•
Molecular image can
not superimpose on its
mirror image
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Enantiomers and the Tetrahedral Carbon
Enantiomers
• From the Greek enantio, meaning “opposite”
• Stereoisomers in which molecules are not identical to
their mirror images
• Result whenever a tetrahedral carbon is bonded to four
different substituents CHXYZ (one need not be H)
• Lactic acid (2-hydroxypropanoic acid) has four different
groups (-H, -OH, -CH3, -CO2H) bonded to the central
carbon atoms and exists as a pair of enantiomers
Enantiomers and the Tetrahedral Carbon
Enantiomers of lactic acid
•
•
(+)-lactic acid
• Occurs in muscle tissue
• Found in sour milk
(-)-lactic acid
• Found in sour milk
Enantiomers and the Tetrahedral Carbon
A molecule of (+)-lactic acid can not superimpose on a
molecule of (-)-lactic acid
Regardless of how the molecules are oriented, they are not
identical
•
•
When the –H and –OH substituents match up, the –CO2H and
the CH3 substituents do not
When –CO2H and the CH3 match up, -H and –OH do not
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5.2
The Reason for Handedness in Molecules:
Chirality
Chiral
• From the Greek cheir meaning “hand”
• Molecules that are not identical to their mirror images, and
thus exist in two enantiomeric forms
• A molecule is not chiral if it has a plane of symmetry
Plane of symmetry
• A plane that cuts through the middle of an object (or
molecule) so that one half of the object is a mirror image of
the other half
The Reason for Handedness in Molecules:
Chirality
A laboratory flask has a
plane of symmetry
•
One half of the flask
is a mirror image of
the other half
b) A hand does not have a
plane of symmetry
•
One half of the hand
is not a mirror image
of the other half
a)
The Reason for Handedness in Molecules:
Chirality
Achiral
• A molecule that has a plane of
symmetry in any of its possible
conformations must be
identical to its mirror image
•
Propanoic acid, CH3CH2CO2H
•
Has a plane of symmetry
and so must be achiral
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The Reason for Handedness in Molecules:
Chirality
•
Lactic Acid
•
Has no plane of
symmetry in any
conformation and is
chiral
The Reason for Handedness in Molecules:
Chirality
Chirality center
• Most common cause of chirality in an organic molecule is
the presence of a carbon atom bonded to four different
groups
•
•
The central carbon atom in 5-bromodecane
Chirality is a property of the entire molecule
The Reason for Handedness in Molecules:
Chirality
Methylcyclohexane
•
Achiral because there is no
carbon atom in the molecule
that is bonded to four
different groups
• Has a plane of symmetry
passing through the methyl
group and through C1 and
C4 of the ring
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The Reason for Handedness in Molecules:
Chirality
2-Methylcyclohexanone
•
Chiral because C2 is bonded
to four different groups: a –
CH3 group, an –H atom, a
–COCH2– ring bond (C1) and
a –CH2CH2– ring bond (C3)
• Has no plane of symmetry
The Reason for Handedness in Molecules:
Chirality
• Note: Carbons in –CH2, –CH3, C=O, C=C, and C≡C groups
cannot be chirality centers
• * denotes a chirality center
Worked Example 5.1
Drawing the Three Dimensional Structure of a
Chiral Molecule
Draw the structure of a chiral alcohol.
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5.3 Optical Activity
Stereochemistry
• Study originated in the early 19th century during the
investigations by the French physicist Jean-Baptiste Biot
into the nature of plane-polarized light
•
A beam of ordinary light consists of electromagnetic waves
that oscillate in an infinite number of planes at right angles to
the direction of light travel
Optically active organic substances
• Biot observed that when a beam of plane-polarized light
passes through a solution of certain organic molecules,
the plane of polarization is rotated
Optical Activity
Polarimeter
• Measures the amount (angle) of rotation
•
•
•
•
•
A solution of optically active organic molecules is placed in a
sample tube
Plane-polarized light is passed through the tube
Rotation of the polarization plane occurs
Light goes through a second polarizer called the analyzer
• The new plane of polarization and degree of rotation can
be found by rotating the analyzer until the light passes
through it
Angle of rotation is denoted and is expressed in degrees
Optical Activity
Rotation
•
•
The amount of rotation observed in a polarimetry experiment
depends on the number of optically active molecules
Number of optically active molecules depends on sample
concentration and sample pathlength
• the pathlength is the length of the sample tube
Assigning direction of rotation
•
Levorotatory molecules
•
•
•
Optically active molecules that rotate polarized light to the left
(counterclockwise)
Given the symbol (-) as in (-)-morphine
Dextrorotatory molecules
•
•
Optically active a molecules that rotate polarized light to the right
(clockwise)
Given the symbol (+) as in (+)-sucrose
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Optical Activity
The specific rotation, [ ]D
•
•
Optical rotation expression under standard conditions
The observed rotation when light of 589.6 nanometer (nm; 1
nm = 10-9 m) wavelength is used with a sample pathlength l of 1
decimeter (dm; 1 dm = 10cm) and a sample concentration C of 1
g/mL
•
Light of 589.6 nm, sodium D line, is the yellow light emitted from
common sodium lamps
[ ]D
Observedrotation (degrees)
Pathlength, l (dm) Concentration, c (g/cm3 )
l c
Optical Activity
When optical rotation data are expressed in the standard way the
specific rotation, [ ]D , is a physical constant characteristic of a
given optically active compound
•
•
(+)-lactic acid has [ ]D = +3.82
(-)-lactic acid has [ ]D = -3.82
• Two enantiomers rotate the plane-polarized light to exactly the
same extent but in opposite directions
Worked Example 5.2
Calculating an Optical Rotation
A 1.20 g sample of cocaine, [ ]D = -16, was dissolved in 7.50
mL of chloroform and placed in a sample tube having a
pathlength of 5.00 cm. What was the observed rotation?
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Worked Example 5.2
Calculating an Optical Rotation
Strategy
Since [ ]D
Then
l c
l c [ ]D
where [ ]D = -16
l = 5.00 cm = 0.500 dm
and C = 1.20 g/7.50 mL = 0.160 g/mL
Worked Example 5.2
Calculating an Optical Rotation
Solution
l c [ ]D
= (0.500) x (0.160) x (-16) = -1.3º
5.4 Pasteur’s Discovery of Enantiomers
Louis Pasteur discovered
enantiomers in 1848 when he
began his study of crystalline
tartaric acid salts derived
from wine
• He observed that two distinct
kinds of crystals precipitated
from a concentrated solution
of ammonium tartrate
• The two kinds of crystals
were mirror images
• Pasteur separated the
crystals into piles of “lefthanded” crystals and “righthanded” crystals
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Pasteur’s Discovery of Enantiomers
Solution of ammonium tartrate
• The original mixture, a 50 : 50 mixture of right and left,
was optically inactive
• Solutions of crystals from each of the sorted piles
were optically active
•
Their specific rotations were equal in amount but
opposite in sign
• Enantiomers, also called optical isomers
• Have identical physical properties, such as melting and
boiling point
• Differ in the direction in which their solutions rotate
plane-polarized light
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