Contributed by:
The highlights are:
1. Definitions
2. Plane-Polarised Light
3. Specific rotation
4. Chirality
5. Enantiomers
6. Fischer Projections
7. Racemic mixtures
8. Meso compounds
1.
Stereochemistry
Stereochemistry refers to the
3-dimensional properties and
reactions of molecules. It has its
own language and terms that need
to be learned in order to fully
communicate and understand the
3.
Definitions
• Stereoisomers – compounds with the same
connectivity, different arrangement in space
• Enantiomers – stereoisomers that are non-
superimposible mirror images; only properties
that differ are direction (+ or -) of optical
rotation
• Diastereomers – stereoisomers that are not
mirror images; different compounds with
different physical properties
4.
More Definitions
• Asymmetric center – sp3 carbon with 4
different groups attached
• Optical activity – the ability to rotate the plane
of plane –polarized light
• Chiral compound – a compound that is
optically active (achiral compound will not
rotate light)
• Polarimeter – device that measures the
optical rotation of the chiral compound
6.
Plane-Polarized Light through
an Achiral Compound
7.
Plane-Polarized Light through
a Chiral Compound
8.
Polarimeter Measures
Optical Rotation
9.
Specific Rotation, [α]
[α] = α / cl
= observed rotation
c = concentration in g/mL
l = length of tube in dm
Dextrorotary designated as d or (+), clockwise
rotation
Levorotary designated as l or (-), counter-
clockwise rotation
10.
Chirality Center
Carbon has four different groups attached
11.
Enantiomers
nonsuperimposible mirror images
mirror
plane
OH OH
H CH3 CH3 H
HO2C CO 2H
(S)(+) lactic acid (R)(-) lactic acid
from muscle tissue
o
from milk o
[] = +13.5 [] = -13.5
12.
Enantiomeric Excess
(Optical Purity)
observed rotation
rotation of pure enantiomerx 100 = enantiomeric excess (e.e.)
o
observed rotation = +109
109.0
e.e. = 123.0 x 100
= 88.6% e.e.
H H
88.6% (+)
(S)-(-) Limonene (R)(+) Limonene 11.4% racemic
o o
[= [] = +123.0 actually 94.3% (+)
from lemons from oranges
13.
Absolute Configuration
Use Cahn, Ingold, Prelog priorities
Place the lowest priority group back
(focus down C - 4 bond)
draw arrow from 1-2-3
1 1
clockwise counterclockwise
4 2 4
2 3
3
(R) (S)
14.
Assign Priority to each Group
on Asymmetric Center
I I 1
rotate
Cl F 4 H Cl 2
H F3
focus down C-4 bond
15.
Lactic Acid
1 1
OH OH
4 4
H CH3 CH3 H
HO2C CO2H
3 3 2
2
(S) (R)
16.
Fischer Projections
OH OH OH
H CO 2H H CO 2H
H CH3
HO 2C CH3 CH3
Horizontal bonds approach you (wedge bonds)
Vertical bonds move away (dashed bonds)
17.
Assigning Absolute
Configuration to Fischer
Projections
OH OH OH
H CO2H H CO2H
H CH3
HO2C CH3 CH3
rotate
(S) (S) (S)
18.
Rotation of the Projection 90o
Reverses Absolute Configuration
OH o H CH3 o
90 o
90 CO2H
90
H CO2H CH3 OH HO2C H HO CH3
CH3 CO2H OH H
(S) (R) (S) (R)
19.
Diastereomers
Stereoisomers That Are Not Mirror Images
H OH H OH
3 3
2
2 CO2H CO2H
H Br Br H
(2S,3S) (2S,3R)
same stereochemistry at C2 (S)
opposite stereochemistry at C3
20.
Fischer Projections with 2
Chiral Centers
CO2H CO2H
2 2
H OH H OH
3 3
Br H H Br
CH3 CH3
(2S,3S) (2S,3R)
21.
Identical, Enantiomers or
Diastereomers?
H CH2CH3
a) Br CH3 & H CH3
CH2CH3 Br
CH3
HO H
b) CH3 H NH2
H C C &
CH3 NH2 H OH
CH3
22.
Tartaric Acids
R,R S,S
CO2H CO2H
H OH HO H
HO H H OH
CO2H CO2H
R,S S,R
CO2H CO2H
H OH HO H
H OH HO H
CO2H CO2H
23.
Racemic Mixture
R,R S,S
CO2H CO2H
H OH HO H
HO H H OH
CO2H CO2H
Racemic Mixture (Racemate): 50/50 mixture of enantiomers
(R,R) Tartaric acid (S,S) Tartaric Acid (+/-) Tartaric acid
o
m.p. C 168-170 168-170 210-212
[] (degrees) - 12 + 12 0
(g/mL) 1.7598 1.7598 1.7723
24.
Meso Compound
Internal Plane of Symmetry
Optically Inactive
R,S S,R
CO2H CO2H
H OH HO H
H OH HO H
CO2H CO2H
o
rotate 180
superimposible
25.
2,3,4-trichlorohexane
How many stereoisomers?
Cl
* * *
Cl Cl
3 asymmetric centers
2n, n= # asymmetric centers (3)
8 stereoisomers
26.
n = 3; 2n = 8
CH3 CH3 CH3 CH3
S H Cl Cl H R H Cl Cl H
S H Cl Cl H R H Cl Cl H
RH Cl Cl H S Cl H H Cl
CH2CH3 CH2CH3 CH2CH3 CH2CH3
CH3 CH3 CH3 CH3
H Cl Cl H Cl H H Cl
Cl H H Cl H Cl Cl H
H Cl Cl H H Cl Cl H
CH2CH3 CH2CH3 CH2CH3 CH2CH3
27.
A Carbohydrate
CHO
H OH R
HO H S
H OH R
H OH R
CH2OH
(+) D-Glucose
28.
Internal Planes of Symmetry
Both are Meso
CH3 CH3 CH3 CH3
CH3 CH3 CH3 CH3
29.
Asymmetric Centers on Rings
Br Br
Br Br
Br Br
A B
1(R),2(S) cis 1,2-dibromocyclohexane
nonsuperimposible but A flips into B Meso
30.
Allenes can be Chiral
H H
H H
C C C C C C
Cl Cl
CH3 CH3
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