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Constitutional
IsomersStereoisomers
ConformationalIsomers
GeometricIsomers
Enantiomers 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.
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°
Enantiomers
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.
thalidomide
N
O
ON
O
O
H
*
(CH3)2CHCH2 CH COOH
CH3
ibuprofen
*
NH2
CH CH3CH2
amphetamine
CH3
Enantiomers matter!
*
(±) 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 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
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
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
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