CHE 240
Unit IV
Stereochemistry, Substitution
and Elimination Reactions
CHAPTER FIVE
Terrence P. Sherlock
Burlington County College
2004
Chiral Carbons
• Tetrahedral carbons with 4 different
attached groups are chiral.
• Its mirror image will be a different
compound (enantiomer).
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Mirror Planes of Symmetry
• If two groups are
the same, carbon
is achiral.
(animation)
• A molecule with an
internal mirror
plane cannot be
chiral.*
Caution! If there is no plane of symmetry,
molecule may be chiral or achiral. See if
mirror image can be superimposed. =>
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(R), (S) Nomenclature
• Different molecules (enantiomers) must
have different names.
• Usually only one enantiomer will be
biologically active.
O
OH
C
• Configuration around the
chiral carbon is specified H3C C H
NH2
with (R) and (S).
n atu ral al an i n e
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4
Cahn-Ingold-Prelog Rules
• Assign a priority number to each group
attached to the chiral carbon.
• Atom with highest atomic number
assigned the highest priority #1.
• In case of ties, look at the next atoms
along the chain.
• Double and triple bonds are treated like
bonds to duplicate atoms.
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Assign (R) or (S)
• Working in 3D, rotate molecule so that
lowest priority group is in back.
• Draw an arrow from highest to lowest
priority group.
• Clockwise = (R), Counterclockwise = (S)
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Properties of Enantiomers
•
•
•
•
Same boiling point, melting point, density
Same refractive index
Different direction of rotation in polarimeter
Different interaction with other chiral
molecules
– Enzymes
– Taste buds, scent
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Optical Activity
• Rotation of plane-polarized light
• Enantiomers rotate light in opposite directions,
but same number of degrees.
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Polarimetry
•
•
•
•
•
Use monochromatic light, usually sodium D
Movable polarizing filter to measure angle
Clockwise = dextrorotatory = d or (+)
Counterclockwise = levorotatory = l or (-)
Not related to (R) and (S)
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Biological Discrimination
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Racemic Mixtures
•
•
•
•
Equal quantities of d- and l- enantiomers.
Notation: (d,l) or ()
No optical activity.
The mixture may have different b.p. and m.p.
from the enantiomers!
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Racemic Products
If optically inactive reagents combine to
form a chiral molecule, a racemic
mixture of enantiomers is formed.
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Nonmobile Conformers
If the conformer is sterically hindered, it
may exist as enantiomers.
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Fischer Projections
• Flat drawing that represents a 3D molecule
• A chiral carbon is at the intersection of
horizontal and vertical lines.
• Horizontal lines are forward, out-of-plane.
• Vertical lines are behind the plane.
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Fischer Rules
• Carbon chain is on the vertical line.
• Highest oxidized carbon at top.
• Rotation of 180 in plane doesn’t
change molecule.
• Do not rotate 90!
• Do not turn over out of plane! =>
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Fischer Mirror Images
• Easy to draw, easy to find enantiomers,
easy to find internal mirror planes.
• Examples:
CH3
CH3
CH3
H
Cl
Cl
H
H
Cl
Cl
H
H
Cl
H
Cl
CH3
CH3
CH3
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Fischer (R) and (S)
• Lowest priority (usually H) comes forward, so
assignment rules are backwards!
• Clockwise 1-2-3 is (S) and counterclockwise
1-2-3 is (R).
• Example:
(S)
CH3
(S)
H
Cl
Cl
H
CH3
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Diastereomers
• Stereoisomers that are not mirror images.
• Geometric isomers (cis-trans)
• Molecules with 2 or more chiral carbons.
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Alkenes
Cis-trans isomers are not mirror images,
so these are diastereomers.
H
H
H
C C
C C
H3C
CH3
CH3
cis-2-bu te n e
H3C
H
trans-2-bu te n e
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Ring Compounds
• Cis-trans isomers possible.
• May also have enantiomers.
• Example: trans-1,3-dimethylcylohexane
CH3
CH3
H
H
H
=>
H
CH3
CH3
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Two or More Chiral Carbons
• Enantiomer? Diastereomer? Meso? Assign
(R) or (S) to each chiral carbon.
• Enantiomers have opposite configurations at
each corresponding chiral carbon.
• Diastereomers have some matching, some
opposite configurations.
• Meso compounds have internal mirror plane.
• Maximum number is 2n, where n = the
number of chiral carbons.
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Examples
COOH
COOH
H
HO
HO
OH
H
H
H
OH
COOH
COOH
(2S,3S)-tartaric aci d
(2R,3R)-tartaric aci d
COOH
H
OH
H
OH
COOH
(2R,3S)-tartaric acid
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Fischer-Rosanoff Convention
• Before 1951, only relative configurations
could be known.
• Sugars and amino acids with same relative
configuration as (+)-glyceraldehyde were
assigned D and same as (-)-glyceraldehyde
were assigned L.
• With X-ray crystallography, now know
absolute configurations: D is (R) and L is (S).
• No relationship to dextro- or levorotatory.
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D and L Assignments
CHO
H
*
CHO
OH
H
CH2OH
D-(+)-glyce ral de h yde
HO
H
COOH
H2N
H
OH
H
OH
=>
H *
OH
CH2CH2COOH
CH2OH
L-(+)-gl u tam i c aci d
D-(+)-gl u cose
*
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Properties of Diastereomers
• Diastereomers have different physical
properties: m.p., b.p.
• They can be separated easily.
• Enantiomers differ only in reaction with
other chiral molecules and the direction
in which polarized light is rotated.
• Enantiomers are difficult to separate.
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Resolution of Enantiomers
React a racemic mixture with a chiral compound to
form diastereomers, which can be separated.
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Chromatographic
Resolution of Enantiomers
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POWER POINT IMAGES FROM
“ORGANIC CHEMISTRY, 5TH EDITION”
L.G. WADE
ALL MATERIALS USED WITH PERMISSION OF AUTHOR
PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE
ORGANIC CHEMISTRY COURSE
BY:
ANNALICIA POEHLER STEFANIE LAYMAN
CALY MARTIN
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