Chirality
“chiral”
Greek for hands
chiral = non-superimposable mirror images
chiral = no plane of symmetry
C*
C with 4 different substituents
stereocenter
chiral C
Chirality
H
HOH2C C * CHO
OH
H
OHC C CH2OH
OH
enantiomers non-superimposable mirror images
no plane of symmetry
3-D Representations
glyceraldehyde
2,3-dihydroxypropanal
O
H H
C
HOH
C C * CHO
OH 2C
H OH H
CHO
H
*
vertical = into paper
OH
horizontal = out of paper
CH2OH
*
*
D-(+)-glyceraldehyde
L-(-)-glyceraldehyde
CHO
H
*
CHO
OH
CH2OH
HO
*
H
CH2OH
1. Most oxidized C on top
2. C* furthest from top OH on left = L enantiomer
OH on right = D enantiomer
Enantiomers
identical in most properties
differ in: 1.interaction with polarized light
2. interaction with chiral environments
Light interacts with molecules
when it passes through them
[interaction of electrical fields]
When light encounters mirror image
of molecule, interaction is reversed
Polarized light
One enantiomer - rotate light to the left (-)
Other enantiomer - rotate light to the right (+)
in 50/50 mix - no net rotation racemic mixture
one enantiomer - polarized light will be rotated
optical activity
Chirality
2. Enantiomers in chiral environments
hands chiral
mittens achiral - plane of symmetry
achiral environments
don’t distinguish between enantiomers
gloves
chiral - no plane of symmetry
chiral environments
do distinguish between enantiomers
Chirality
CH3
O
enantiomers
of carvone
Is there a C*
*
C
H3C
caraway seed
spearmint gum
CH2
carvone
Is there a plane of symmetry no
Chirality
CH3
CH3
O
H3C
O
*
*
C
C
CH2
(-) carvone
spearmint
rotates light to left
H2C
CH3
(+) carvone
caraway
rotates light to right
racemic mixture - optically inactive
taste buds are chiral environment
Formation of chiral compounds
catalyzed by chiral enzymes
alkene + H2O
alcohol
fumerase
+ H2O
chiral
*
fumaric acid
COOH
optically inactive
HO
* H
CH2CO2H
L- Malic acid
optically active
carboxylic acid + alcohol
ester + H2O
O-CH3
*
+ CH3OH
D-(-)- lactic acid
optically active
*
+ H2O
D- (+)- methyl lactate
optically active
ketone + mild reducing agent
+ NaBH4
H
pyruvic acid
optically inactive
* OH
2o alcohol
HO
* H
D-lactic acid
L-lactic acid
optically active optically active
racemic mixture
inactive
Compounds with more than 1 C*
H H H
O
H C C* C* C
H
HO OH OH
CHO
CHO
H
*
OH
HO
H
H
*
OH
HO
H
CH2OH
D-Erythrose
CH2OH
L-Erythrose
CHO
H
*
OH
H
*
OH
2 C*
A maximum of
2n steroisomers
enantiomers
CHO
HO
*
H
HO
*
H
CH2OH
CH2OH
L-Erythrose
D-Erythrose
CHO
CHO
HO
H
H
OH
CH2OH
D-Threose
enantiomers
diastereomers:
non-mirror image
stereoisomers
H
HO
OH
H
CH2OH
L-Threose
22
= 4 stereoisomers
I
II
COOH
H
HO
OH
H
COOH
HO
H
COOH
III
H H
O
C C* C * C
HO
OH
OH OH
O
H
OH
COOH
IV
COOH
COOH
H
OH
HO
H
H
OH
HO
H
COOH
COOH
I and II enantiomers
optically active
III and IV same
optically inactive
meso compound
plane of symmetry
superimposible
mirror images