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Chirality and Enantiomers Gloves v. Socks Chiral
Exhibits handedness Non-superimposable images in mirror test
Achiral Does not exhibit handedness Identical or superimposable images in
mirror test
3
Enantiomers A pair of molecules related as
nonsuperimposable mirror images Look at 2-chloropropane
Chiral? Superimposable mirror images?
Now, look at 2-chlorobutane Superimposable mirror images? Chiral?
4
Stereogenic Centers C atom with 4 different groups attached
to it Marked with an asterisk (*) Indicates existence of enantiomers Lacks plane of symmetry
5
Configurations and R-S Convention Arrangement of groups around
stereogenic center Enantniomers…opposite configurations Need for identifying each enantiomer
Cahn-Ingold-Prelog (CIP) system…otherwise known as the R-S system
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R-S Convention Four groups attached to stereogenic
center are placed in priority order (abcd)
Stereogenic center is then observed from the view point opposite the lowest priority group, d If remaining groups form clockwise array,
then it’s R If counterclockwise, then it’s S
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R-S Convention—Assigning Priority of Groups Rule 1
Atoms directly attached to stereogenic C are ranking according to At#...>At# = higher priority
H always has lowest priority If H is a group, then you’ll look down the
C-H bond to view the molecule, otherwise look down the C-lowest priority bond
8
R-S Convention—Assigning Priority of Groups Rule 2
Used if Rule 1 does not allow you to reach a decision…two groups are identical
Work outward from the stereogenic center until a decision is reached
Ethyl has higher priority than methyl Saturated cyclic cpds follow same rules
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R-S Convention—Assigning Priority of Groups Rule 3
Required to handle unsaturated bonds and aromatic rings
Multiple bonds are treated as if the atoms involved bonded singly to each other
vinyl group, CH=CH2
CH CH2
C C
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E-Z Convention of Cis-Trans Isomers Eliminates ambiguity Prioritize each group attached to
double-bonded C, and do same for other C in double bond
E (entgegen)—if both higher priority groups on opposite side of double bond
Z (zusammen)—if high priority groups are on same side of double bond
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Polarized Light and Optical Activity = observed rotation depends on
Molecular structure Concentration Length of sample tube Wavelength of light Temperature
Dextrorotatory (+) Levorotatory ()
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Calculating Specific Rotation…
= observed rotation l = length of tube in dm c = concentration of soln in g/mL t = temperature of soln…usually RT (298K) = wavelength of light (typically 589.3
nm, that of sodium vapor’s D-line )
[] = t
l x c
[] t
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What Makes a Molecule Optical Active Achiral molecules cancel polarity…
1 molecule rotates light in one direction, while mirror image molecule rotates light in opposite direction…cancelling activity
Chiral molecule solns consisting of a single type of enantiomer (R or S, but not both) are optically active…a 1:1 mixture of R&S enantiomers are known as racemic mixtures and are optically inactive
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Properties of Enantiomers
Baseball Ball Bat Glove
Identical achiral properties (mp, bp, D) Different chiral properties
Direction of rotation of plane –polarized light
NOT degrees of rotation
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Lactic Acid Enantiomers
(R)-()-lactic acid
3.33 (H2O)
mp = 53°C
[] 25°C
D
(S)-(+)-lactic acid
+3.33 (H2O)
mp = 53°C
[] 25°C
D
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Enantiomers in Biology
Neither R nor S dictates () or (+) Direction can determine rxn that will
occur Lactic acid dehydrogenase reacts with
(+)-lactic acid to make pyruvic acid, but not with ()-lactic acid
()-adrenalin, cardiac stimulant; (+)-adrenalin is ineffective
One could be toxic, the other harmless
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Fischer Projections Replaces dashed and solid wedges to 2-
dimensions Stereogenic C is intersection of horizontal
and vertical Horizontal represent groups connected to
stereogenic C above (toward you) plane of page
Vertical represent groups connected to stereogenic C below (away from you) plane of page
If lowest priority group is not on vertical arm, rotate three of four groups to make it so
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Compounds with More Than One Stereogenic Center
Biochemistry, carbohydrate chemistry Consider 2-bromo-3-chlorobutane
2 stereogenic centers Each center could be R or S Four isomers: (2R,3R), (2S,3S), (2R,3S),
(2S,3R)
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Compounds with More Than One Stereogenic Center
(2R,3R) (2S,3S) (2R,3S) (2S,3R)
-2-bromo-3-chlorobutane suffix for allRemember to rotate 3 of 4 groups to get lowest
priority on vertical axis
Br
C
C
CH3
CH3
H
H
Cl
H
C
C
CH3
CH3
Cl
Br
H
Br
C
C
CH3
CH3
Cl
H
H
H
C
C
CH3
CH3
H
Br
Cl
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Compounds with More Than One Stereogenic Center
(2R,3R) & (2S,3S) enantiomers (2R,3S) & (2S,3R) also enantiomers of
each other So, how are (2R,3R) & (2R,3S) related to
each other
Br
C
C
CH3
CH3
H
H
Cl
Br
C
C
CH3
CH3
Cl
H
H
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Diastereomers
(2R,3R) & (2R,3S) are stereoisomers, but not enantiomers
Diastereomers—stereoisomers that are not mirror images of each other May differ in all properties, including
achiral properties…mp, bp, D, rotation, degrees of rotation…could be everything
2 different cpds
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Calculating Number of Stereoisomers
As number of stereogenic centers (n) increases, so does number of stereoisomers
Every time a stereogenic center is added, R and S configurations are possible Rule: 2n = number of stereoisomers What is the maximum number of
enantiomers equation?
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Meso Compouds
Occur when diastereomers are achiral and therefore identical isomers
Optically inactive due to achirality Consider 2,3-dichlorobutane
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Meso Compounds
enantiomers, chiral identical, achiral, meso
Cl
C
C
CH3
CH3
H
H
Cl
H
C
C
CH3
CH3
Cl
Cl
H
Cl
C
C
CH3
CH3
Cl
H
H
H
C
C
CH3
CH3
H
Cl
Cl
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Meso Compouds
Plane of symmetry Represent a single cpd Same 4 groups attached Superimposable mirror images Identical Achiral Cl
C
C
CH3
CH3
Cl
H
H
H
C
C
CH3
CH3
H
Cl
Cl
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A Research Project on Meso Cpds and Racemic Mixtures
Conduct independent research on Louis Pasteur’s work with tartaric acid.
Learn all that you can about his work Be prepared to discuss such work on
examination
30
Combinations of Stereoisomers Consider cis-2-butene and trans-2-
butene, also known as Z-2-butene and E-2-butene
32
Stereochemistry and Rxns
Achiral reactants chiral product (both enantiomers are formed at equal rates and in equal concentrations—racemic mixture)
Chiral and achiral reactants new stereogenic center (diastereomers, different rates, different concentrations)
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Achiral Reactants Chiral Product
CH3CH2CH=CH2 + H+ CH3CH2CHCH3
CH3CH2CHCH3
2-butyl cation
+ Br -
Br
38
Achiral Reactants Chiral Product
Br C
H CH3CH2 CH3
CH3CH2—C
CH3
CH3CH2
CBr
(S)-2-bromobutane
H
HCH3
Br -
(R)-2-bromobutane
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Achiral Reactant + Chiral Reactant Write the rxn of 3-chloro-1-butene with
HBr.
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CH3CHCH=CH2 + HBr
Cl
+
Achiral Reactant + Chiral Reactant
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Cl
CH
C
CH2
CH3
H Cl
C
C
CH3
CH3
H
H
Br
Cl
C
C
CH3
CH3
Br
H
H
HBrR R R
SR +
(R)-3-chloro-1-butene (2R,3R)-2-bromo-3-chlorobutane (2S,3R)-2-bromo-3-chlorobutane
The Cl group remains R; the new stereogenic center can be either R or S.
Resolving Racemic Mixtures Separation of racemic mixture’s
enantiomers is resolution Convert to diastereomers
By letting enantiomers react w/chiral reagent Pasteur was 1st to resolve racemic mixture
Convert separate diastereomers back to enantiomers
Same concept as specificity in biological rxns—enzymes usually chiral reagent
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