Chemistry 2100
Chapter 15
Constitutional
IsomersStereoisomers
ConformationalIsomers
GeometricIsomers
Enantiomers Diastereomers
single bond rotation
chiral achiral
sameconnections
differentconnections
notmirrorimages
mirrorimages
Isomers
EnantiomersEnantiomers: 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
Enantiomers
Original molecule
OH
CH3C
CH2CH3
H
OH
CCH3
HCH3CH2
OH
CH3C
HCH2CH3
Mirror image The mirror imagerotated by 180°
180° rotate by 180°about the
C-OH bond
OH
CH3C
CH2CH3
H
OH
CH3C
HCH2CH3
The original molecule
The mirror imageturned by 180°
chiral
asymmetric
glycine
NH2H
C
COOH
CH3H2 NH CH3
C
COOH
alanine
plane of symmetry
achiral
NH2
HC
COOH
CH3H2 NH CH3
C
COOH
glycine
H H
glycine
Enantiomers
Objects that are nonsuperposable 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.
Drawing Enantiomers
thalidomide
N
O
ON
O
O
H
*
(CH3)2CHCH2 CH COOH
CH3
ibuprofen
*
NH2
CH CH3CH2
amphetamine
CH3
Enantiomers matter!
*
The R,S SystemBecause 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.
The R,S System
The first step in assigning an R or S configuration to a stereocenter is to arrange the groups on the stereocenter in order of priority.– Priority is based on atomic number.– The higher the atomic number, the
higher the priority.
R,S Priority of Some Groups
The R,S System
• To assign an R or S configuration:1. Assign a priority from 1 (highest) to 4
(lowest) to each group bonded to the stereocenter.
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.
3. Read the three groups projecting toward you in order from highest (1) to lowest (3) priority.
4. If reading the groups 1-2-3 is clockwise, the configuration is R. If reading them is counterclockwise, the configuration is S.
Properties of Enantiomers
Optical activity
plane-polarized light
plane-polarized light
plane-polarized light
plane-polarized light
plane-polarized light
Fig. 15-6, p. 435
(±) or (dl) optically inactive raceme
levorotatory (l) (–) rotation "left-hand isomer"
dextrorotatory (d) (+) rotation "right-hand isomer"
Assigning Rotation
Multiple Chiral Centers
For a molecule with n stereocenters, the maximum number of possible stereoisomers is 2n.– 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.
2R,3S 2S,3R 2R,3R 2S,3S
C
C
CHO
CH2OH
OHH
HHO
L-threose
C
C
CHO
CH2OH
OH
OHH
H
D-erythrose DC B A
C
C
CHO
CH2OH
OH
H
H
HO
D-threose
C
C
CHO
CH2OH
H
H
HO
HO
L-erythrose
The Four Stereoisomers of a Simple sugar
C
C
Fischer Projections
Threose and Erythrose, Fischer-ized
W X Y Z
meso
H HO HO H
COOH
COOH
COOH
COOH
OH
OHH
H
COOH
COOH
OHH
HHO
COOH
COOH
OH
H
H
HO
Tartaric Acid
meso
COOH
COOH H H OH
OH
W X Y Z
COOH
COOH
OH
OHH
H
COOH
COOH
OHH
HHO
COOH
COOH
OH
H
H
HO
COOH
COOH H H OH
OH
meso
W X Y Z
COOH
COOH
OH
OHH
H
COOH
COOH
OHH
HHO
COOH
COOH
OH
H
H
HO
Meso form
**
sodium ammonium tartrate
O
OH
CHCNa O CH C
OH
O NH4
O
**
sodium ammonium tartrate
O
OH
CHCNa O CH C
OH
O NH4
O
Stereochemical Reactions
CH3
H
C
O
COOH
H
CH3 C COOH
O[H] *
pyruvic acid lactic acid
*
pyruvic acidlactic acid
CH3
H
C
O
COOH
H
CH3 C COOH
O[H]
(–)
(+)
H
COOH
HO
CCH3
HCH3
HO
CCOOH
(–)
(+)
H
COOH
HO
CCH3
HCH3
HO
CCOOH
HH
HH HH
HH
COOH
CO
CH3
(–)
(+)
H
COOH
HO
CCH3
HCH3
HO
CCOOH
HH
HH HH
HH
COOH
CO
CH3
CH3CO
COOH
(–)
(+)
H
COOH
HO
CCH3
HCH3
HO
CCOOH
HH
HH HH
HH
COOH
CO
CH3
CH3CO
COOH
(–)
(+)
H
COOH
HO
CCH3
HCH3
HO
CCOOH
HH
HH HH
HH
COOH
CO
CH3
CH3CO
COOH
(–)
(+) H
CH3
HO
CCOOH
HH
HH HH
HH
H
COOH
HO
CCH3
COOHCO
CH3
CH3CO
COOH
(–)
(+) H
CH3
HO
CCOOH
HH
HH HH
HH
H
COOH
HO
CCH3
COOHCO
CH3
CH3CO
COOH
(–)
(+) H
CH3
HO
CCOOH
H
COOH
HO
CCH3
HH HH
HH
HH
(+)
H
COOH
HO
CCH3
But, Biologically…
NAD H (+)
H
COOH
HO
CCH3
NAD H (+)
H
COOH
HO
CCH3
NAD H
COOH
OCH3
(+)
H
COOH
HO
CCH3
O
CH3CCOOH
NAD H
COOH
OCH3
(+)
H
COOH
HO
CCH3
O
CH3CCOOH
NAD H
COOH
OCH3
(+)
H
COOH
HO
CCH3
O
CH3CCOOH
NAD H
COOH
OCH3
(+)
H
COOH
HO
CCH3
H+
O
CH3CCOOH
NAD H
COOH
OCH3
(+)
H
COOH
HO
CCH3
H+
O
CH3CCOOH
NAD H
COOH
OCH3
(+)
H
COOH
HO
CCH3
Chirality of 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, 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