Ch' 5 Stereochemistry

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Ch. 5 Stereochemistry
Alleve (naproxen)
What is the relationship of the two structures?
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Types of Isomers
Stereoisomers (different arrangement of
atoms in space)
Enantiomers (non superimposable mirror
images)
Diastereomers (not mirror images)
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Normal light oscillates in a plane perpendicular
to its direction of motion
It can be thought of consisting of two planes of
oscillation
A polaroid filter removes one of the oscillating
planes
The result is plane polarized light
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Two aligned polaroids transmit plane polarized
light
Two polaroids rotated 90 degrees transmit no light
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Some substances rotate plane polarized light
These substances contain only one isomer of a
pair of chiral molecules
Chirality means the same thing as handedness.
Some things come in right and left handed
forms Other things come in only one form
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Some objects are not chiral
Shovels, socks, pencils
Some objects are chiral
Screws, shoes, keyboards
Chiral objects come in pairs. The two items in
the pair are not the same. They are mirror
images of each other.
All objects have mirror images (except vampires).
Objects that are chiral are different from their
mirror images
Achiral objects have symmetry, chiral ones do not
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Achiral operations with achiral objects
Digging with post holers, shooting a free throw,
picking up a pencil
Chiral operations with achiral objects
Digging with a shovel, shooting a layup, writing
with a pencil
Achiral operations with chiral objects
Pushing a screw, picking up a ball glove
Chiral operations with chiral objects
Inserting a screw, catching a ball thrown to your
side
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Chirality
Identical or enantiomers?
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Chirality
Identical or enantiomers?
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Chirality
Identical or enantiomers?
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Requirements for Chirality
Stereogenic center (aka chiral center) This is
usually a carbon with four different groups
attached.
In order to be chiral, a molecule must contain at
least one stereogenic center
Three possible cases
1. No stereogenic centers the molecule is not
chiral
2. One stereogenic center the molecule is
chiral
3. Two or more stereogenic centers the
molecule is chiral UNLESS it contains a plane of
symmetry
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Which of the following molecules is chiral?
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Which of the following are chiral?
A) 2, 3, 4, 5 B) 2, 3 C) 4, 5 D) 3, 4, 5 E)
1, 3, 4
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How many stereogenic centers are present in the
following compound?
3-azido-3-deoxythymidine (AZT)
A) 0 B) 1 C) 2 D) 3 E) 4
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Each of the compounds in an chiral pair is
called an enantiomer of the other.
One enantiomer rotates plane polarized light
clockwise and is Referred to as dextrorotatory
(d or ) the other enantiomer rotates plane
polarized light counterclockwise by exactly the
same amount And is called levorotatory (l or -).
A 5050 mixture of the two enantiomers is
called a racemic mixture. A racemic mixture does
not rotate the plane of polarized light because
the two enantiomers cancel each other out.
A 7525 mixture of the enantiomers has ½ of the
rotation that one pure enantiomer would have (50
racemic mixture, 50 excess of the other
enantiomer. This is said to have an optical
purity or enantiomeric excess of 50.
Enantiomers behave the same except when the
operation they are undergoing is an achiral one.
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Labeling Stereogenic Centers
How do we give each of these a name that
distinguishes it from its enantiomer.
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Labeling Stereogenic Centers
Stereogenic centers are labeled as R or S using
the Cahn-Ingold-Prelog system
1. Assign a priority to the four atoms directly
bonded to the stereogenic center. The highest
atomic number gets the highest priority (1).
Lowest atomic number gets the lowest priority (4).
2. In case two atoms are tied, look at the atoms
bonded to them. The atom with the highest atomic
number atom bonded to it gets the higher
priority.
3. Treat multiple bonds as the equivalent number
of single bonds.
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Labeling Stereogenic Centers
4. Orient the molecule so the lowest priority
group is pointing away from you. 5. Draw a
circle going from 1 to 2 to 3.
If the circle goes clockwise, the stereogenic
center has the R configuration
If the circle goes counterclockwise, the
stereogenic center has the S configuration
(R)-2-chlorobutane
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Compounds with gt1 Stereogenic Center
For n stereogenic centers, the maximum number of
stereoisomers is 2n
3-chloro-4-fluoro-3,4-dimethylhexane
4 stereoisomers maximum
R S
S R
S S
R R
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Which of the following are diastereomers?
1 2 3
4
A) 1 and 4 B) 1 and 3 C) 2 and 4 D) None
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Compounds with gt1 Stereogenic Center
Meso compound an achiral compound that has
stereogenic centers




A compound with only 1 stereogenic center cant
be meso
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Which of the following is a meso compound?
A B
C
D E
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Properties of Stereoisomers
Diastereomers have different physical and
chemical characteristics (melting point, boiling
point, NMR spectra etc.)
Enantiomers have identical physical and chemical
characteristics. The can be differentiated by the
way in which they interact with chiral substances
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Optical Activity
Chiral molecules rotate plane polarized light
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Optical Activity
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Optical Activity
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Optical Activity
Enantiomers rotate plane polarized light the same
amount, but in opposite directions
A compound can be either- Dextrorotatory (d)
rotation is in clockwise direction
OR Levorotatory (l) rotation is in
counterclockwise direction
This has no relationship to stereogenic centers
being R or S
Racemic mixture 50/50 mixture of enantiomers,
not optically active
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Chiral Drugs
Enantiomers have identical properties in an
achiral environment
Ibuprofen
Thalidomide
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