Chapter 15: Chirality Chiral Stereoisomers Enantiomers R or S Enantiomers R or S Diastereomers D or...
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Transcript of Chapter 15: Chirality Chiral Stereoisomers Enantiomers R or S Enantiomers R or S Diastereomers D or...
Chapter 15: Chirality
Chiral Stereoisomers Chiral Stereoisomers
EnantiomersR or S
EnantiomersR or S
DiastereomersD or L
DiastereomersD or L
Chapter 15: Chirality
Objects that are nonsuperposable on their mirror images are chiralchiral (from the Greek: cheir, hand).• They show handedness.
The most common cause of enantiomerism in organic molecules is the presence of a carbon with four different groups bonded to it.• A carbon with four different groups bonded to it is called a
stereocenterstereocenter.
Chapter 15: Chirality
Enantiomers:Enantiomers: Nonsuperposable mirror images.• As an example of a molecule that exists as a pair of enantiomers,
consider 2-butanol.
OH
CH3C CH2CH3
H
HO
CCH3
HCH3CH2
Original molecule Mirror image
Chapter 15: Chirality
• To summarize;• Objects that are nonsuperposable on their mirror images are
chiralchiral (they show handedness).• The most common cause of chirality among organic molecules is
the presence of a carbon with four different groups bonded to it.
• We call a carbon with four different groups bonded to it a stereocenterstereocenter.
• Objects that are superposable on their mirror images are achiralachiral (without chirality).
• Nonsuperposable mirror images are called enantiomersenantiomers. • Enantiomers always come in pairs.
The R,S system
Because enantiomers are different compounds, each must have a different name.• Here are the enantiomers of the over-the-counter drug
ibuprofen.
• The R,S system is a way to distinguish between enantiomers without having to draw them and point to one or the other.
COOH
H CH3
HOOC
H3C H
The active enantiomerThe inactive enantiomer
Chapter 15: Chirality
-OH-NH2
-CHO
-CH2OH
-CH2CH3
-CH2H-H
-SH
-COHO
-CH2NH2
-CNH2
O
-I-Br
-Cl
Atom orGroup
oxygen (8)nitrogen (7)
carbon to oxygen, oxygen, then hydrogen (6 —> 8, 8, 1)carbon to oxygen (6 —> 8)
carbon to carbon (6 —> 6)carbon to hydrogen (6 —> 1)hydrogen (1)
sulfur (16)
Reason for Priority: First Point of Difference(Atomic numbers)
carbon to oxygen, oxygen, then oxygen (6 —> 8, 8, 8)
carbon to nitrogen (6 —> 7)
carbon to oxygen, oxygen, then nitrogen (6 —> 8, 8, 7)
bromine (35)
chlorine (17)
iodine (53)
Step 1:Order groups by
Priority.
Higher Atomicnumber
=Higher Priority
Chapter 15: Chirality
Example:Example: Assign priorities to the groups in each set.
-CH2OH -CH2CH2OH-CH2CH2OH -CH2NH2(a) (b)and and
-CH2OH -CH2CH2COH
O
-CH2NH2 -CH2COH
O
and(c) (d)and
Chapter 15: Chirality
Step 2:Orient the molecule in space so that the group of lowest priority (4) is directed away from you. The three groups of higher priority (1-3) then project toward you.
2
3
14
Chapter 15: Chirality
Step 3:Follow the three groups projecting toward you in order from highest (1) to lowest (3) priority.
2
3
14
Steering Right = R configuration Steering Left = S configuration
2
1
34
SR
Chapter 15: Chirality
Example:Example: Assign an R or S configuration to each stereocenter.
OH
CH3C CH2CH3
H
H3C COOHC
HH2N
(a)
(b)
2-Butanol
Alanine
Chapter 15: Chirality
For a molecule with nn stereocenters, the maximum number of possible stereoisomers is 22nn.• We have already verified that, for a molecule with one
stereocenter, 21 = 2 stereoisomers (one pair of enantiomers) are possible.
• For a molecule with two stereocenters, a maximum of 22 = 4 stereoisomers (two pair of enantiomers) are possible.
• For a molecule with three stereocenters, a maximum of 23 = 8 stereoisomers (four pairs of enantiomers) are possible, and so forth.
Diastereomers:Diastereomers: Stereoisomers that are not mirror images. (N>=2)
Chapter 15: Chirality
Example:Example: Mark all stereocenters in each molecule and tell how many stereoisomers are possible for each.
CH3
CH3
OH
OH
CH2=CHCHCH2CH3
OH
NH2
OHHO
HO
COOH
NH2
OH
NH2
OH
O
(a) (b) (c)
(d) (e) (f)
Chapter 15: Chirality
• Ordinary light:Ordinary light: Light waves vibrating in all planes perpendicular to its direction of propagation.
• Plane-polarized light:Plane-polarized light: Light waves vibrating only in parallel planes.
• Polarimeter:Polarimeter: An instrument for measuring the ability of a compound to rotate the plane of plane-polarized light (See next slide).
• Optically active:Optically active: Showing that a compound is capable rotating the plane of plane-polarized light.
Chapter 15: Chirality
Figure 15.6 Schematic diagram of a polarimeter with its sample tube containing a solution of an optically active compound.
Chapter 15: Chirality
• Dextrorotatory:Dextrorotatory: Clockwise rotation of the plane of plane-polarized light. Indicated by (+ or D).
• Levorotatory:Levorotatory: Counterclockwise rotation of the plane of plane-polarized light. Indicated by (- or L).
• Specific rotation:Specific rotation: The observed rotation of an optically active substance at a concentration of 1 g/mL in a sample tube 10 cm long.
DD
H3CC
OHH
COOH
CH3
C
HOH
COOH
[]21 = -2.6°= +2.6°21
[]
(R)-(-)-Lactatic acid(S)-(+)-Lactic acid
Chapter 15: Chirality
Except for inorganic salts and a few low-molecular-weight organic substances, the molecules in living systems, both plant and animal, are chiral.• Although these molecules can exist as a number of
stereoisomers, almost invariably only one stereoisomer is found in nature.
• Instances do occur in which more than one stereoisomer is found, but these rarely exist together in the same biological system.
Chapter 15: Chirality
How an enzyme distinguishes between a molecule and its enantiomer.
Figure 15.7 A schematic diagram of an enzyme surface that can interact with (R)-glyceraldehyde at three binding sites but with (S)-glyceraldehyde at only two of the three sites.
Chapter 15: Chirality
Enzymes (protein biocatalysts) all have many stereocenters.• An example is chymotrypsin, an enzyme in the intestines of
animals that catalyzes the digestion of proteins.• Chymotrypsin has 251 stereocenters.• The maximum number of stereoisomers possible is 2251!• Only one of these stereoisomers is produced and used by any
given organism.• Because enzymes are chiral substances, most either produce or
react with only substances that match their stereochemical requirements.
Chapter 15: Chirality
• Because interactions between molecules in living systems take place in a chiral environment, a molecule and its enantiomer or one of its diastereomers elicit different physiological responses.
• As we have seen, (S)-ibuprofen is active as a pain and fever reliever, while its R enantiomer is inactive.
• The S enantiomer of naproxen is the active pain reliever, but its R enantiomer is a liver toxin!
HOOC
H3C H
HOOC
H3C H
OCH3(S)-Ibuprofen (S)-Naproxen