1. Stereoisomerism

1
Chapter 3
Stereochemistry
1. Stereoisomerism 2. Chirality 3. Naming
stereocenters - R/S configuration 4. Acyclic
Molecules with 2 or more stereocenters 5. Cyclic
Molecules with 2 or more stereocenters 6.
Properties of Stereocenters 7. Optical
activity 8. Separation of Enantiomers,
Resolution 9. Significance of Chirality in the
biological world
20

c
a
b
2
ISOMERISM same molecular formula, but different.
constitutional connectivity different
stereoisomers - same connectivity different 3D
orientation
conformational isomers chair/boat
3
diastereomers
4
enantiomers
5
Chirality
- right and left-handed substances - 2 spatial
arrangements of atoms - analyze reflections
(mirrors) same (achiral), different (chiral).
plane
mirror
chiral -non-superposable, different arrangements
in space
6
Elements of Symmetry
Plane of symmetry an imaginary plane passing
through an object dividing it such that one half
is the mirror image of the other half.
7
Elements of Symmetry
Conformations of 2,3-butanediol
.
If symmetry element is present, substance is
achiral
meso or R,S (later)
8
Elements of Symmetry
Center of symmetry a point situated so identical
components of the object are located equidistant
on opposite sides.
9
Chirality
superposable mirror images - same compound
- a plane of symmetry - achiral
10
Chirality-sp3 or tetrahedral center with 4
different groups chiral molecule
Cl
Cl
C
C
H
H
F
F
Br
Br
Different, non-superposable mirror images, ?
enantiomers
11
Tetrahedral - 4 different substituents (sp3)
enantiomers
- an atom that interchange of 2 groups gives a
stereoisomer
12
Enantiomers
Lactic acid
How do we distinguish chiral molecules?
R/S
13
R,S Convention

• Priority rules
• Assign priority to each atom bonded to
  stereocenter.
• higher atomic number
• higher priority

(6)
(1)
-CH3
-H
14
R,S Convention
2. If same atoms bonded to the stereocenter, look
to next set of atoms. Priority to the first
point of difference.
Z stereocenter, 4 gps.
15
R/S absolute configuration of chiral
compounds Rules
1. Identify stereogenic center (C - 4 different
gps.) 2. Assign priority to groups (C - 1, 2,
3, 4)
4. If 1gt2gt3 clockwise R counterclockwise S
R
16
R S configuration
enantiomers
17
R,S Convention
3. Atoms in double (triple) bond viewed as
equivalent number of single bonds (C is a
stereogenic center)
18
priorities and assigning R/S
19
Assign R/S to the stereogenic center of the ester
S
20
Assign R or S to carvone
spearmint
21
Enantiomers Diastereomers

• For a molecule with n stereocenters, a maximum of
  2n stereoisomers might be possible.

For a molecule with 1 stereocenter, 21 2
stereoisomers are possible.
For a molecule with 2 stereocenters, a MAXIMUM of
22 4 stereoisomers might exist.
22
2n
256 (ignore sugar)
23
earlier
RS
R - R R - S S - R S - S
24
Molecules with more than 1 stereocenter

22 4
trans-1-bromo-2-chlorocyclopropane

relative stereochemistry - both are trans
absolute stereochemistry - each is unique,
enantiomers
(1R,2R)-1-bromo-2-chlorocyclopropane
(1S,2S)-1-bromo-2-chlorocyclopropane
25
Molecules with more than 1 stereocenter

22 4
trans-1-bromo-2-chlorocyclopropane

cis-1-bromo-2-chlorocyclopropane
(1S,2R)-1-bromo-2-chlorocyclopropane
(1R,2S)-1-bromo-2-chlorocyclopropane
(1R,2R)-1-bromo-2-chlorocyclopropane
(1S,2S)-1-bromo-2-chlorocyclopropane
26
Molecules with more than 1 stereocenter
(1S,2R)-1-bromo-2-chlorocyclopropane
(1R,2S)-1-bromo-2-chlorocyclopropane
(1R,2R)-1-bromo-2-chlorocyclopropane
(1S,2S)-1-bromo-2-chlorocyclopropane
27
Molecules with more than 1 stereocenter
(1S,2R)-1-bromo-2-chlorocyclopropane
(1R,2S)-1-bromo-2-chlorocyclopropane
(1R,2R)-1-bromo-2-chlorocyclopropane
(1S,2S)-1-bromo-2-chlorocyclopropane
28
2 or more stereocenters with symmetry leads to
a meso isomer, superposable mirror images.
Consider 2,3-dibromobutane
S
S
R
R
R
S
R
S
29
2 or more stereocenters with symmetry leads to
a meso isomer, superposable mirror images.
Consider 2,3-dibromobutane
S
R
R
S
meso isomer
S
R
S
R
only 3 realized
part2