chirality
enantiomers: non superimposible mirror images
Me
Me
Br
Et
Br
Et
H
H
“stereogenic center”
formalism:
wedge (forward)
dashed (back)
Not all mirror images are chiral. These are the same (not chiral)
enantiomers: non superimposible mirror images
Br
Et
H
H
Me
Me
Me
Me
Et
Et
Br
Et
Et
Et
H
H
How to tell if molecules are chiral? Clue: many
(but not all*) chiral carbons have 4 different
things attached
chiral
Me
OH
R1
Br
Cl
Et
R3
R2
R4
N
achiral
Et
R1
R3
R2
OH
R1
NH2
enantiomers
have same mp, bp, NMR, IR, etc
* exceptions to be discussed
When plane-polarized light interacts with a chiral molecule, the
plane of polarization of the light is rotated to the left or right.
This effect is called optical rotation and the molecule is said to be
optically active.
Optical activity is measured using a polarimeter. This device
contains a light source, a polarizer to produce the plane-polarized
light, a sample cell, and an analyzer to determine the amount of
rotation.
Cahn Ingold Prelog rules
prioritize by:
1) Highest atomic #
2) then by isotope atomic weight
3) then by substituents, one atom at a time
4) if atoms are equal, go to first point of difference
Higher priority: this carbon
is connected to Br,C,H,H
Br
a
c D
H b
d
this carbon
is connected to C,C,H,H
b
CH3
cD
H
d
a
CH3
Br
b
c D
H
d
a
this carbon
is connected to C,C,H,H
c D
H
d
this carbon
is connected to C,C,C,H
Higher priority: this carbon
is connected to C,C,C,H
Cl
b
a
Cl
Higher priority: this carbon
is connected to Cl, C, H, H
5) multiple bonds
View the molecule with the lowest priority group in the back
~
b
c
H3C
H
d
H3C
H
a
H
C C
~
b
H3C
H
a
C
C
R
c
d
O
opposite enantiomer:
C
C
O
O
~
H3C
H
b
H
(Latin, rectus)
b
b
d
counterclockwise
S
c
O
a
c
H3C
H
d
clockwise
c
H
b
c
H3C
H
d
a
a
b
O
a
c
a
(Latin, sinister-left)
O
H
O
Chiral molecules are not necessarily optically active
CH3
b
H
Br
a Br
c
CH3
c
CH3
b
C3H7
H
d
Bra
R
=0
same rules for alkenes
higher priority
higher priority
(E)
Br
CH3
I
Br
higher priority
(Z)
Pile of Latvian Mittens
(50% Left, 50% Right)
higher priority
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What happens if an enantiomer interacts with another
element of chirality?
L + R
L + L
Racemates are not optically active
CH3
H
CH3
CH3
H
H
Br
CH3
Br
CH3
CH3
CH3
H
=0
Br
H
Br
Br
Br
Br
diastereomers
H
H
CH3
H
Br
What happens if an enantiomer interacts with another
element of chirality?
C3H7
C2H5
H3C
Cl
F
Br
R R
C3H7
C2H5
H3C
diastereomeric pairs up the nose
Cl
Br
F
R S
diastereomers
have different mp, bp, NMR, IR, etc
O
R-carvone
spearment
O
S-carvone
caraway
diastereomers
Meso compounds contain an internal mirror plane (or center of symmetry) which divides the two
halves of the molecule which are mirror images of each other. NOT OPTICALLY ACTIVE!
easy to see in ring compounds
R
meso compounds
R
Br
R I
Br
S I
easy to see in acyclic compounds if we are consistent in our drawings
R
R
Br
R I
Br
S I
Vitruvian man = meso man. He has symmetrically substituted elements of chirality, equal and
opposite
meso compounds
recognizing planes and centers of symmetry
meso compounds
easy to see in ring compounds
R
A
A
A
C
B
C
B
plane of symmetry
A
C
B
R
A
C
B
S
B
C
C
B
A
center of symmetry
A
C
B
R
B
C
A
S
in general, look for R & S stereocenters in the
same molecule attached to exactly the same thing
Me
S Me
Me
S
Me R
easy to see in acyclic compounds if we are consistent in our drawings
R
s
Me
R Me
Me
S
Me R
CH3
Br (R)
Cl
H