stereochemistr y chirality - cook.chem.ndsu.nodak.edu
TRANSCRIPT
STEREOCHEMISTRYCHIRALITY
A
CB C
D
A
CBC
D
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Chirality - Stereoisomers
Chirality
‣ Enantiomers - molecules that are not superimposable on their mirror image.
‣ Molecules that can exist as enantiomers are called Chiral Molecules.
‣ Molecules that have a plane of symmetry are not chiral (achiral).
‣ Stereogenic Carbons are those that have four different groups attached.
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‣ Carbohydrates / Sugars
‣ DNA
‣ Amino Acids / Proteins
Biomolecules are Chiral
OH
HO
H
HO
H
HOHH OH
OH
Glucose
H2NO
OHR
Chirality in Smell and Taste
‣ Mirror image molecules (enantiomers) interact with chiral receptors in our nose differently.
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O
(R)-(-)-Carvone (S)-(+)-Carvonespearmint oil caraway seed oil
O
(R)-(+)-Limonene (S)-(-)-Limoneneorange citrus turpentine/Lemon
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Chiral Molecules in Enzymes
O
ONH
N
O
O
NOH
H
Me
HO
The Case of Thalidomide
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NH
O O
N
O
O
*stereogeniccarbon
(R)-thalidomidebeneficial sedative
NH
O O
N
O
O
*stereogeniccarbon
(S)-thalidomidecauses birth defects
Tartaric Acid
‣ 1849 - Louis Pasteur separated mirror image crystals of sodium ammonium tartrate.
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*HO
O
OH
OHOH
O
* *O
O
OH
OHO
O
*Na NH4
Louis Pasteur
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“I found...that the tartrate was hemihedral..., but strange to say the paratartrate was hemihedral also. Only the hemihedral faces which in the tartrate were all turned the same way, were, in the paratartrates inclined sometimes to the right and sometimes to the left.... I carefully separated the crystals that were hemihedral to the right from those hemihedral to the left, and examined their solutions separately in the polarizing apparatus. I then saw with no less surprise than pleasure that the crystals hemihedral to the right deviated the plane of polarization to the right, and that those hemihedral to the left deviated it to theleft; and when I took an equal weight ofeach of the two kinds of crystals, themixed solution was indifferent towardsthe light in consequence of theneutralization of the two equal andopposite individual deviations.”
Optical Activity
‣ Enantiomers have identical physical properties
‣ mp, bp, spectroscopic, absorptions, etc.
‣ BUT they are different when interacting with plane-polarized light
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light travels in waves but oscillates in all three dimensions along the path of travel
polarizing filter
Light is polarized and oscillates in a single dimension
chiralsample
when the polarized light passes through a chiral sample, it is bent out of the original plane
chiralsampleenantiomer
Note that the enantiomer will bend the light in the opposite direction
Optical Activity
‣ Amount of rotation depends on how many molecules the light interacts with.
‣ Light at 589 nm (Sodium D line)
‣ Standardized for unit cell and concentration
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observed rotation (°)cell length (dm) x conc. (g/mL)[α]D =
Racemates
‣ Racemic Mixture - 50/50 mixture of enantiomers. Shows NO optical activity.
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“...when I took an equal weight of each of the two kinds of crystals, the mixed solution was indifferent towards the light in consequence of the neutralization of the two equal and opposite individual deviations.”
light travels in waves but oscillates in all three dimensions along the path of travel
polarizing filter
Light is polarized and oscillates in a single dimension
chiralsample
when the polarized light passes through a chiral sample, it is bent out of the original plane
chiralsampleenantiomer
Note that the enantiomer will bend the light in the opposite direction
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CIP Rules for R & S Configuration
Cl Cl
(R)-2-chlorobutane (S)-2-chlorobutane
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H H41
2 3
4
‣ Rectus - Right, Sinister - Left
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R & S Configuration
HONH2
OH sphingosine
HO RNH2
OH
S R
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R & S Configuration
H2N
CH3
O
OHNH O
OH
alanine proline
S S
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R & S Configuration
glucoseO
HO
HO
OHOH
OH
SR
SR
S
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Diastereomers
‣ Diastereomers - stereoisomers that are not mirror images
STEREOISOMERS
DIASTEREOMERS ENANTIOMERS
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cis/trans Diastereomers
‣ Although not optically active (not chiral), cis/trans isomers are a form of diastereomer. They differ only in their arrangement in 3-dimensions but are not mirror images.
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configurational Diastereomers
OH
OH
O
HO
O
HO
HO
HO
O
OH
O
OH
ENANTIOMERS
S R
S R
DIASTEREOMERSDIASTEREOMERS
OH
OH
O
HO
O
HOS
R
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MESO compounds
‣ Meso Compounds are molecules that contain stereogenic carbons, but are not chiral. Their mirror images are identical. This occurs when there is a plane of symmetry in the molecule.
OH
OH
O
HO
O
HO
HO
HO
O
OH
O
OH
S R
SR
IDENTICAL
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ISOMERS
ISOMERS(same number and kind of atoms, but are different molecules)
Constitutional(bonded differently)
Stereoisomers(Only different in 3-dimensional arrangement)
Enantiomers(mirror images)
Diastereomers(not mirror images)
Configurational cis/trans