Chemistry 2100
Lecture 8
Constitutional
IsomersStereoisomers ConformationalIsomers
GeometricIsomersEnantiomers Diastereomers
single bond rotation
chiral achiral
sameconnections
differentconnections
notmirrorimages
mirrorimages
Isomers
EnantiomersEnantiomers: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
Enantiomers
Original molecule
OHC
H3C CH2CH3H
OHC
CH3H
CH3CH2
OHC
H3C HCH2CH3
Mirror image The mirror imagerotated by 180°
180° rotate by 180°about the
C-OH bond
OHC
H3C CH2CH3H
OHC
H3C 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
EnantiomersObjects 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.
Drawing Enantiomers
thalidomide
N
O
ON
O
O
H
*
(CH3)2CHCH2 CH COOH
CH3
ibuprofen
*
NH2
CH CH3CH2
amphetamine
CH3
Enantiomers matter!
*
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 CentersFor 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.
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
CHO
CH2OH
OH
H
H
HO
D-threose
CHO
CH2OH
OH
OHH
H
D-erythrose
CHO
CH2OH
H
H
HO
HO
L-erythrose
CHO
CH2OH
OHH
HHO
L-threose
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 acid
lactic acid
CH3
H
C
O
COOH
H
CH3 C COOH
O[H]
(–)
(+)
H COOH
HO
CCH3
H CH3
HO
CCOOH
(–)
(+)
H COOH
HO
CCH3
H CH3
HO
CCOOH
HH
HH
HH
HH
COOH
COCH3
(–)
(+)
H COOH
HO
CCH3
H CH3
HO
CCOOH
HH
HH
HH
HH
COOH
COCH3
CH3CO
COOH
(–)
(+)
H COOH
HO
CCH3
H CH3
HO
CCOOH
HH
HH
HH
HH
COOH
COCH3
CH3CO
COOH
(–)
(+)
H COOH
HO
CCH3
H CH3
HO
CCOOH
HH
HH
HH
HH
COOH
COCH3
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
(+)
HCOOH
HO
CCH3
But, Biologically…
NAD H (+)
HCOOH
HO
CCH3
NAD H (+)
HCOOH
HO
CCH3
NAD H
COOHO
CH3
(+)
HCOOH
HO
CCH3
OCH3C
COOH
NAD H
COOHO
CH3
(+)
HCOOH
HO
CCH3
OCH3C
COOH
NAD H
COOHO
CH3
(+)
HCOOH
HO
CCH3
OCH3C
COOH
NAD H
COOHO
CH3
(+)
HCOOH
HO
CCH3
H+
OCH3C
COOH
NAD H
COOHO
CH3
(+)
HCOOH
HO
CCH3
H+
OCH3C
COOH
NAD H
COOHO
CH3
(+)
HCOOH
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