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6-3 Enantiomers Enantiomers: Nonsuperposable mirror images As an example of a molecule that exists as a pair of enantiomers, consider 2-butanol.
Citation preview
William Brown Thomas Poon
www.wiley.com/college/brown
Chapter SixChirality: The Handedness of
Molecules
6-2
Isomers In this chapter, we concentrate on enantiomers
and diastereomers.
6-3
Enantiomers• Enantiomers: Nonsuperposable mirror images• As an example of a molecule that exists as a pair
of enantiomers, consider 2-butanol.
OHC
H3C CH2CH3H
HOC
CH3H
CH3CH2Original molecule Mirror image
6-4
Enantiomers• One way to see that the mirror image of 2-butanol
is not superposable on the original is to rotate the mirror image.
Original molecule
OHC
H3C CH2CH3H
OHC
CH3H
CH3CH2
OHC
H3C HCH2CH3
Mirror image The mirror imagerotated by 180°
180°rotate the
mirror imageby 180° about
the C-OH bond
6-5
Enantiomers• Now try to fit one molecule on top of the other so
that all groups and bonds match exactly.
• The original and mirror image are not superposable.• They are different molecules with different
properties.• They are enantiomers (nonsuperposable mirror
images).
OHC
H3C CH2CH3H
OHC
H3C HCH2CH3
The original molecule
The mirror imageturned by 180°
6-6
Enantiomers• Objects that are not superposable on their
mirror images are chiral (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 stereocenter.
6-7
Enantiomers• If an object and its mirror image are
superposable, they are identical and there is no possibility of enantiomerism.• We say that such an object is achiral (without
chirality).• As an example of an achiral molecule, consider
2-propanol.• Notice that this molecule has no stereocenter.
Original molecule Mirror image
OHC
H3C CH3H
OHC
CH3H
H3C
6-8
Enantiomers• To see the relationship between the original and
its mirror image, rotate the mirror image by 120°.
• When we do this rotation, all atoms and bonds of the mirror image fit exactly on the original.
• This means that the original and its mirror image are the same molecule.
• They are just viewed from different perspectives.
Original molecule Mirror image
OHC
H3C CH3H
OHC
CH3H
H3C
OHC
H3C CH3
H
The mirror imagerotated by 120°
120° rotate by 120°about the
C-OH bond
6-9
Enantiomers• To summarize• An object that is nonsuperposable on its mirror
image is chiral (it shows 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 stereocenter.
• An object that is superposable on its mirror image is achiral (without chirality).
• Nonsuperposable mirror images are called enantiomers.
• Enantiomers, like gloves, always come in pairs.
6-10
Drawing Enantiomers• Following are four different representations for
one of the enantiomers of 2-butanol.
• Both (1) and (2) show all four groups bonded to the stereocenter and show the tetrahedral geometry.
• (3) is a more abbreviated line-angle formula; although we show the H here, we do not normally show them in line-angle formulas.
• (4) is the most abbreviated representation; you must remember that there is an H present on the stereocenter.
OHC
H3C CH2CH3H
OHC
H3C CH2CH3
HH OH OH
(1) (2) (3) (4)
6-11
Drawing Mirror Images• On the left is one enantiomer of 2-butanol.• On the right are two representations for its mirror
image (in this case, its enantiomer).OH OH OH
One enantiomerof 2-butanol
Alternative representationsfor its mirror image
6-12
Naming Enantiomers - R,S• 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 enantiomerof ibuprofen
6-13
The R,S System• To assign an R or S configuration:• Assign a priority from 1 (highest) to 4 (lowest) to
each group on the stereocenter; for priority rules, see Section 4.3C.
• Orient the stereocenter so that the group of lowest priority is facing away from you.
• Read the three groups projecting toward you in order from (1) to (3).
• If reading the groups is clockwise, the configuration is R (Latin, rectus, straight, correct).
• If reading the groups is counterclockwise, the configuration is S (Latin: sinister, left).
6-14
The R,S System • Problem: Assign an R or S configuration to each
stereocenter.
(a)H Cl
OHH
(b)
NH3+O-
O(c)
6-15
The R,S System• ProblemProblem: Assign an R or S configuration to the
enantiomers of ibuprofen.• Remember to add the H at the
stereocenter.COOH
CH3
HOOC
CH3
The active enantiomerThe inactive enantiomerof ibuprofen
6-16
Enantiomers & Diastereomers
• For a molecule with 1 stereocenter, 21 = 2 stereoisomers are possible.
• For a molecule with 2 stereocenters, a maximum of 22 = 4 stereoisomers are possible.
• For a molecule with nn stereocenters, a maximum of 2n stereoisomers are possible.
6-17
Enantiomers & Diastereomers• 2,3,4-Trihydroxybutanal
• Two stereocenters; 22 = 4 stereoisomers are possible.
HOCH2-CH-CH-CHOHOH
O* *
C
C
H OHCHO
OH
CH2OH
H
C
C
HHOCHO
HO
CH2OH
H H
CH2OH
HO
C
C
H OHCHO
C
C
HHOCHO
H
CH2OH
OH
A pair of enantiomers(Erythrose)
A pair of enantiomers(Threose)
6-18
Meso Compounds• Meso compound: an achiral compound
possessing two or more stereocenters.• Tartaric acid contains two stereocenters.• Two stereocenters; 2n = 4, but only three
stereoisomers exist, one meso compound and one pair of enantiomers.
C
C
H OH
COOH
OH
COOH
H
C
C
HHO
COOH
HO
COOH
H
C
C
H OH
COOH
H
COOH
HO
C
C
HHO
COOH
H
COOH
OH
A pair of enantiomersA meso compound(plane of symmetry)
6-19
Cyclic Molecules• 2-Methylcyclopentanol• 2 stereocenters; according to the 2n rule, a
maximum of 4 stereoisomers are possible.• How many actually exist? Answer 4, two pairs of
enantiomers.
cis-2-Methylcyclopentanol trans-2-Methylcyclopentanol
H3C
HO
CH3
OH
H3C
HO
CH3
OH
6-20
Cyclic Molecules• 1,2-Cyclopentanediol• 2 stereocenters = a maximum of 4 stereoisomers.• How many exist? Answer: three, one meso
compound (the cis isomer) and one pair of enantiomers (the trans isomer).
HO
HO
OH
OH
HO
HO
OH
OHcis-1,2-Cyclopentanediol trans-1,2-Cyclopentanediol
6-21
Cyclic Molecules• 4-Methylcyclohexanol• How many stereoisomers are possible?• Answer: two. The cis isomer (achiral) and the
trans isomer (also achiral).
OHH3C OHH3C
cis-4-Methylcyclohexanol trans-4-Methylcyclohexanol
6-22
Cyclic Molecules• 3-Methylcyclohexanol• 2 stereocenters = a maximum of 4 stereoisomers• How many exist? Answer: four, two pairs of
enantiomers.
trans-3-Methylcyclohexanolcis-3-Methylcyclohexanol
OH
CH3
HO
H3C
OH
CH3
HO
H3C
6-23
Cyclic Molecules• 2-Methylcyclohexanol• 2 stereocenters = a maximum of 4 stereoisomers• how many exist? Answer: four, two pairs of
enantiomers.
trans-2-Methylcyclohexanolcis-2-Methylcyclohexanol
OH
CH3
HO
H3C
OH
CH3
HO
H3C
6-24
Cyclic Molecules• 1,3-Cyclohexanediol• 2 stereocenters = a maximum of 4 stereoisomers• How many exist? Answer: three, the cis isomer
(meso) and the trans isomer (a pair of enantiomers).
trans-1,3-Cyclohexanediolcis-1,3-Cyclohexanediol
OH
OH
OH
OH
HO
HO
HO
HO
6-25
Three Or More Stereocenters• How many stereocenters are present in the
molecule on the left?• How many stereoisomers are possible?• One of the possible stereoisomers is menthol.• Assign an R or S configuration to each
stereocenter in menthol.
2-Isopropyl-5-methyl-cyclohexanol
OH
Menthol
OH
6-26
Three Or More Stereocenters• Cholesterol• On the left is the carbon skeleton of cholesterol.• How many stereocenters are present?• How many stereoisomers are possible?
HO HOThis is the stereoisomer found in
human metabolism
H
H
H
The carbon skeleton ofcholesterol
H
6-27
Optical Activity• 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: An instrument for measuring the ability of a compound to rotate the plane of plane-polarized light.
• Optically active: Showing that a compound rotates the plane of plane-polarized light.
6-28
Polarimeter• Figure 6.6 Schematic diagram of a polarimeter.
6-29
Optical Activity• Dextrorotatory: Clockwise rotation of the plane of
plane-polarized light.• Levorotatory: Counterclockwise rotation of the
plane of plane-polarized light.• Specific rotation: The observed rotation of an
optically active substance at a concentration of 1 g/100 mL in a sample tube 10 cm long; for a pure liquid, concentration is in g/mL (density).
DD
H3CC
OHH
COOH
CH3C
HOH
COOH
[]21 = -2.6°= +2.6°21[](R)-(-)-Lactatic acid(S)-(+)-Lactic acid
6-30
Chirality in the Biological World• 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.
• It’s a chiral world!
6-31
Chirality in Biomolecules• Enzymes (protein bio-catalysts) 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.
6-32
Chirality in the Biological World• Figure 6.7 Schematic diagram of the surface of
an enzyme capable of distinguishing between enantiomers.
6-33
Chirality in Biomolecules• 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, whereas its R enantiomer is inactive.
• The S enantiomer of naproxen is the active pain reliever, whereas its R enantiomer is a liver toxin!HOOC
CH3
HOOC
CH3
OCH3(S)-Ibuprofen (S)-Naproxen
6-34
Resolution• Racemic mixture: An equimolar mixture of two
enantiomers.• Because a racemic mixture contains equal
numbers of dextrorotatory and levorotatory molecules, its specific activity is zero.
• Resolution: The separation of a racemic mixture into its enantiomers.
6-35
Resolution• Enzymes as resolving agents.
H3CO
OEtH CH3
CO OCH3
C
HH3C
O
EtO
NaOH, H2O2. HCl, H2O
H3CO
C
H CH3
OH
O
Ethyl ester of (S)-naproxen Ethyl ester of (R)-naproxen(not affected by the esterase)
+
1. esterase
(S)-Naproxen
6-36
Chirality and the Handedness of Molecules
End Chapter 6End Chapter 6