1
Stereochemistry
2
Handedness Some things have a “handedness,”
that is look at your right and left hand. They look alike, but are not the same. They are mirror images.
3
Nobel Prize - 2001
Their research deals with the fact that many molecules appear in two forms that are mirror images of each other, just like the left and right hands.
4
The mirror imageof a chiral object isdifferent and will notsuperimpose on the original object.
Objects which are chiral have a sense of “handedness” and exist in two forms.
Chirality
5
Mirror Image
6
HCl
HCl Are these two
structures identical?
mirror
7
HCl
HCl
Stereoisomers
8
Stereoisomers
9
Stereoisomers
10
Stereoisomers
11
Stereoisomers
12
Stereoisomers
13
enantiomersenantiomers
Stereoisomers that are nonidentical mirror images are called enantiomers.enantiomers.
Stereoisomers
14
Visualize, visualize ….
C C
Br
F
H
Cl
BrH
CC
Br
F
H
Cl
BrH
CC
H
Br
F
Cl
BrH
15
Visualize, visualize …
C C
Br
F
H
Cl
BrH
CC
Br
F
H
Cl
BrH
Cl
HBr
F
H Br
Cl
BrH
F
Br H
16
C
CH3
BrF
H
C
CH3
FBr
H
enantiomer
1..3..5…etc interchanges = enantiomer
2..4..6...etc interchanges = original compound
ODD:
EVEN:
Visualize, visualize …
17
C C
CH3
BrF
H CH3
H
FBr
C C C C
CH3
BrF
H CH3
Br
FH CH3
H
BrF
H
CH3 BrF
ENANTIOMER ENANTIOMERSAME
12
3
YOU CAN USEINTERCHANGES
Are these identical or are they enantiomers?
18
Isomers Isomers: different compounds with the
same molecular formula Constitutional isomers: isomers with a
different connectivity Stereoisomers: isomers with the same
molecular formula, the same connectivity but a different orientation of their atoms in space that cannot be interconverted by rotation about a single bond
19
Chirality Mirror image: the reflection of an object in
a mirror Objects that are not superposable on their
mirror images are said to be chiral, that is, they show handedness
Objects that are superposable on their mirror images are said to be achiral, that is, they do not show handedness. An achiral object has at least one element of symmetry
20
Chirality A molecule
cannot be chiral if it has a plane of symmetry.
21
Chirality A plane of symmetry is a plane that
cuts through an object in such a way that one half of the object is an exact mirror image of the other half.
A molecule that has a plane of symmetry must be identical to its mirror image and therefore must be nonchiral, or achiral.
22
BrFH
Cl stereocenterThis is one type of ….
…. others are possible
A stereogenic carbon is tetrahedral and has four different groups attached.
Stereogenic Carbons
23ClClBr
F F
BrCl Cl
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
24
ClCl
Br
H
H
Br
center of symmetry
Elements of Symmetry Center of symmetry: a point so
situated that identical components of the object are located equidistant on opposite sides and equidistant from the point along any axis passing through the point
25
Two identical groups renders a tetrahedral carbon achiral.
F
BrClCl
ClClBr
FThe plane of the paper is a plane of symmetry
Achiral
26
Cl
ClBr
F F
Br
Cl Cl
plane ofsymmetry
side view edge view
Two Views of the Plane of Symmetry
27
Symmetry Plane
C
COOHHH
COOH
C
CH3
H
COOH
OH
Symmetry plane No symmetry plane
achiral chiral
28
CONSTITUTIONAL ISOMERS
Isomers with a differentorder of attachment ofthe atoms in their molecules
STEREOISOMERSIsomers with the same orderof attachment, but a differentconfiguration (3D arrangement)of groups on one or more of the atoms
ISOMERSDifferent compoundswith the same molecular formula
cis/trans ISOMERS
ENANTIOMERSStereoisomers whose molecules are non-superimposible mirrorimages of each other
DIASTEREOMERSStereoisomers whose molecules are not mirror images of each other
each isomer could
double bond or ring
both can apply
have stereoisomers
with a ring
TYPES OF ISOMERISMTYPES OF ISOMERISM(geometric)
29
Enantiomers Enantiomers: stereoisomers that
are nonsuperposable mirror images; refers to the relationship between pairs of objects
30
BrFH
Cl Cl
HBrF
HFBr
Cl
rotate
this moleculeis chiral
note that the fluorineand bromine have beeninterchanged in theenantiomer
do interchanges in class
Enantiomers
31
Enantiomers Lactic acid
C
C
HOCH3
H
OHO
C
C
OHH3CH
O OH
32
Enantiomers 1,2-propanediol
CH3CHCH2OH
OH
C
OH
H
CH2OHH3C C
OH
H
HOH2C CH3
33
Enantiomers 3-Chlorocyclohexene
Cl
H
Cl
H
Cl
34
CH3CH2CH2CH2CH2 C CH2CH2CH2CH3
Br
H
35
HO CH3
CH3
CH3
H
H
36
CH3
O
O
carvone nootkatonespearment oil grapefruit oil
37
CH3
O
HCH3C
CH2
CH3
O
HCH3C
CH2
(R)-carvonecaraway and dill seed oils
(S)-carvonespearmint oil
Carvone
38
LimoneneCH3
HCH3C
CH2
CH3
HCH3C
CH2
(R)-limoneneodor of oranges
(S)-limoneneodor of lemons
39
Chiral Drugs Most pharmaceutical drugs are
chiral thalidomide
N
N
O
O O
O
H
H
N
N
O
O O
O
HH
40
Optically Active Refers to molecules that interact
with plane-polarized light
Jean Baptiste Biot French Physicist - 1815
He discovered that some natural substances (glucose, nicotine, sucrose) rotate the plane of plane-polarized light and that others did not.
41
Optical Activity
incidentpolarizedlight
transmittedlight (rotated)
sample cell
angle ofrotation,
(usually quartz)
a solution of the substance to beexamined is placed inside the cell
42
Plane Polarized Light Ordinary light: consists of waves vibrating
in all planes perpendicular to its direction of propagation
Plane polarized light: consists of waves vibrating only in one plane
Plane polarized light is an equal mixture of left and right-circularly polarized light. These two forms are nonsuperposable mirror images and, therefore, enantiomers.
43
Plane-Polarized Light Beam
.
unpolarizedbeam
wavelength
=c
frequency ( n )
c = speed of light
polarized beam
Sine wavesare not alignedin the sameplane.
NOT PLANE-POLARIZED
ENDVIEWSIDE
VIEW
44
Plane Polarized Light Because of its handedness, circularly
polarized light reacts one way with a stereocenter with R-handedness, and differently with its enantiomer
The net effect of the interaction of plane polarized light with a chiral compound is that the plane of polarization is rotated
Polarimeter: a device for measuring the extent of rotation of plane polarized light
45
Optical Activity optical activity - ability of certain molecules
to rotate plane polarized light
detected using a polarimeter
46
Polarimeter
47
l(dam)
sample cell
Na vapor lamp
polarizer analyzer
Polarimeter
48
Optical Activity Observed rotation: the number of
degrees, , through which a compound rotates the plane of polarized light
Dextrorotatory (+): rotation of the plane of polarized light to the right
Levorotatory (-): rotation of the plane of polarized light to the left
49
Optical Activity Specific rotation:
Observed rotation of the plane of polarized light when a sample is placed in a tube 1.0 dam in length and at a concentration of 1g/mL.
= observed rotation
c = concentration ( g/mL )
l = length of cell ( dm )
D = yellow light from sodium lamp
T = temperature ( Celsius )
cl
TD
50
Optical Activity For a pair of enantiomers, the
value of the specific rotation of each is the same, but opposite in sign
[]25D
-13.52 +13.52D25
[]
(R)-(-)-2-Butanol(S)-(+)-2-Butanol
C
OH
CH3CH3CH2
HC
HO
H3C CH2CH3
H
51
Discovery of EnantiomersLouis Pasteur Recrystallized
tartaric acid Two different kinds
of crystals that were mirror images.
Each type of crystal rotated light in opposite directions.
52
Discovery of Enantiomers
“There is no doubt that in dextro tartaric acid there exists an assymetric arrangement having a nonsuperimposible image.”
C
C
COO-Na+
COO-Na+
H
HO H
OH
53
H COOHH
OHOH
HOOCHOOC HH COOH
OHOH
H HCOOH
OHOH
HOOC
meso
enantiomers
(as a minor component)ALSO FOUND
more about thiscompound later
H COOHH
OHOH
HOOCHOOC HH COOH
OHOH
H HCOOH
OHOH
HOOC
meso
enantiomers(+)-tartaric acid (-)-tartaric acid
[]D = 0
meso -tartaric acid
Tartaric Acid
54
R,S Convention Priority rules (Cahn, Ingold, Prelog)
Each atom bonded to the stereocenter is assigned a priority, based on atomic number. The higher the atomic number, the higher the priority
Increasing Priority
H CH3 NH2 OH SH Cl Br I
1 6 7 8 16 17 35 53
55
R,S Convention If priority cannot be assigned on the
basis of the atoms bonded to the stereocenter, look to the next set of atoms. Priority is assigned at the first point of difference.
CH2 H CH2 CH3 CH2 NH2 CH2 OH1 6 7 8
Increasing Priority
56
R,S Convention Atoms participating in a double or
triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds
C
H
O C
H
O
O
C
57
Naming Enantiomers
1. Locate the stereocenter2. Assign a priority to each
substituent from 1 (highest) to 4 (lowest)
3. Orient the molecule so that the group of lowest priority (4) is directed away from you
58
Naming Enantiomers
4. Read the three groups projecting toward you in order from highest (1) to lowest priority (3)
5. If reading is clockwise, configuration is R (from the Latin rectus). If it is counterclockwise, configuration is S (from the Latin sinister).
59
2clockwise counterclockwise
(rectus) (sinister)
view with substituentof lowestpriority inback
1 2
4
3
C C
1
4
3
R S
R, S Convention
60
I
C
BrCl
F
I
C
ClBrF
1
2
3
4
RR SS
1
32
4
Enantiomers
Bromochlorofluoroiodomethane
61
Priorities
1. -OH2. -COOH3. -CH3
4. -H (R)-(-)-lactic acid
C
HHO COOH
CH3
C
H
CH3
HOOC OH
(S)-(+)-lactic acid
62
You try it!
1. Br2. COOH3. CH3
4. H
CH3
Br
H
COOH
HH3C
Br
COOH
63
Diastereoisomer Enantiomers: opposite
configurations at all stereogenic centers.
Diastereomers: Stereoisomers that are not mirror images of each other. Different configuration at some locations.
64
Diastereoisomer Stereoisomers
that are not mirror images of each other. Different configuration at some locations.
COOH
C
C
H
HCH3
NH2
OH
COOH
C
C
H
H OHCH3
H2N
65
Two Stereocenters
diasteromers
entaiomers
entaiomers
Br Cl
H3CH
HCH3
Cl Br
HH3C
CH3
H
Cl Br
HH3C
HCH3
Br Cl
HH3C
HCH3
66
Diastereomers Threonine: 2 pairs
of enantiomers
COOH
C
C
H
H OHCH3
H2N
COOH
C
C
H
HCH3
NH2
OH
2R, 3S 2S, 3R
2R, 3R 2S, 3S
COOH
C
C
H
HHOH3C
NH2
COOH
C
C
H
HH3C
H2N
HO
2R,3R 2S,3S 2R,3S & 2S,3R2S,3S 2R,3R 2R,3S & 2S,3R2R,3S 2S,3R 2R,3R & 2S,3S2S,3R 2R,3S 2R,3R & 2S,3S
67
Enantiomers & Diastereomers For a molecule with 1 stereocenter,
2 stereoisomers are possible For a molecule with 2 stereocenters,
a maximum of 4 stereoisomers are possible
For a molecule with n stereocenters, a maximum of 2n stereoisomers are possible
2n-1 pairs of enantiomers
68
Enantiomers & Diastereomers For tartaric acid, the three possible
stereoisomers are one meso compound and a pair of enantiomers.
Meso compound: an achiral compound possessing two or more stereocenters.
69
Symmetry Plane 2R, 3S and 2S, 3R
are identical Molecule has a
plane of symmetry perpendicular to C-C and is therefore achira
COOH
C
C
H
HO HCOOH
OH
2R, 3S 2S, 3R
2R, 3R 2S, 3S
COOH
C
C
H
OHHCOOH
OHCOOH
C
C
H
HO HCOOH
HO
COOH
C
C
H
OHCOOH
HO
H
70
Symmetry Plane 2R, 3S and 2S, 3R
are identical Molecule has a
plane of symmetry perpendicular to C-C and is therefore achira
One meso compound and a pair of enantiomers
COOH
C
C
H
HO HCOOH
OH
2R, 3S 2S, 3R
2R, 3R 2S, 3S
COOH
C
C
H
OHHCOOH
OHCOOH
C
C
H
HO HCOOH
HO
COOH
C
C
H
OHCOOH
HO
H
Mirror image is identical
71
H CH3
CH3 H
Cl Br
CH3 HH CH3
Br Cl
H HCH3 CH3
Br Cl
CH3CHCHCH3
Cl Br
H HCH3 CH3
Cl Br
enantiomers 1
enantiomers 2
diastereomers
S R RS
S S R R
mirror
2-Bromo-3-chlorobutane
72
H HCH3 CH3
Cl Cl
CH3CHCHCH3
Cl Cl
H HCH3 CH3
Cl Cl
CH3 HH CH3
Cl Cl
H CH3
CH3 H
Cl Cl
meso
enantiomers
diastereomers
S R
S S R R
mirror image is identical
2,3-Dichlorobutane
73
Meso Meso compounds are achiral by virtue
of a symmetry plane, but contain a stereogenic center.
Cl Cl
HH3C
HCH3
Cl Cl
HH3C
HCH3
plane of symmmetry mirror
74
Racemic Mixture Racemic mixture (d,l;): an
equimolar mixture (50:50) of two enantiomers because a racemic mixture contains
equal numbers of dextrorotatory and levorotatory molecules, its specific activity is zero.
75
Properties of Stereoisomers Enantiomers have identical
physical (except for ) and chemical properties.
Diastereomers are different compounds and have different physical and chemical properties
Meso-tartaric acid, for example, has different physical and chemical properties from its enantiomers
76
Tartaric Acid(-) - tartaric acid
[]D = -12.0o
mp 168 - 170o
solubility of 1 g 0.75 mL H2O
1.7 mL methanol 250 mL ether
insoluble CHCl3 d = 1.758 g/mL
(+) - tartaric acid[]D = +12.0o
mp 168 - 170o
solubility of 1 g 0.75 mL H2O
1.7 mL methanol 250 mL ether
insoluble CHCl3 d = 1.758 g/mL
meso - tartaric acid[]D = 0o solubility of 1 gmp 140o 0.94 mL H2Od = 1.666 g/mL insoluble CHCl3
77
H OH
CH3
CH2CH3Fischer Projections Fischer projection: a two-
dimensional representation showing the configuration of a stereocenter horizontal lines represent bonds
projecting forward vertical lines represent bonds
projecting to the rear the only atom in the plane of the paper
is the stereocenter
78
Fischer Projections
(R)-lactic acid
C
COOH
HOH CH3
COOH
CH3
H OH
How?
79
Fischer Projections
C
COOH
HOH CH3
COOH
CH3
H OH
80
Fischer Projections
COOH
CH3
H OH
COOH
CH3
H OH
81
Fischer Projections
1. Orient the stereocenter so that bonds projecting away from you are vertical and bonds projecting toward you are horizontal
2. Flatten it to two dimensions
(S)-2-Butanol (3-D formula)
(1) (2)
(S)-2-Butanol (Fischer projection)
CH3CH2
HCH3
OH
C
CH3
CH2CH3
OHH HC OH
CH3
CH2CH3
82
Assigning R,S Configuration Lowest priority group goes to the
top. View rest of projection. A curved arrow from highest to
lowest priority groups. Clockwise - R (rectus) Counterclockwise - S (sinister)
83
Assigning R,S Configuration
1
2
3
4
H
OH
H3C COOH
s-lactic acid
84
Rules of Motion
Can rotate 180°, but not 90° because 90° disobeys the Fischer projection. Same groups go in and out of plane
CH3
HO H
COOH
CH3
COOH
HO H180
COOH
H OH
CH3
COOH
CH3
H OH= =
85
Rules of Motion
Can rotate 180°, but not 90° because 90° disobeys the Fischer projection. Different groups go in and out of
plane This generates an enantiomeric
structure
H
H3C COOH
OH
H
OH
H3C COOH90
COOH
H OH
CH3
COOH
CH3
H OH= =
(R)-lactic acid (S)-lactic acid
86
Rules of Motion
One group can be held steady and the others rotated.
H
COOH
HO CH3same as
CH3
COOH
H OH
87
Rules of Motion To determine if two Fischer
projections represent the same enantiomer carry out allowed motions. C2H5
CH3
HO H
OH
C2H5
H CH3
H
OH
H3C C2H5
A B C
88
Rules of Motion By performing two allowed
movements on B, we are able to generate projection A. Therefore, they are identical.
H
CH3
HO
CH2CH3
CH2CH3
HHO
CH3
HO
H
CH3
CH2CH3
CH3
B A
CH3CH2
C2H5
CH3
HO H
OH
C2H5
H CH3
H
OH
H3C C2H5
A B C
89
Rules of Motion Perform one of the two allowed
motions to place the group with lowest priority at the top of the Fischer projection.
180
H
CH3
OH
CH2CH3
C not A
CH3H
OH
CH2CH3
CH2CH3
HH3C
OH
OH90
C2H5
CH3
HO H
OH
C2H5
H CH3
H
OH
H3C C2H5
A B C
90
Priorities
1. NH2
2. COOH3. CH3
4. H
S - stereochemistry
CH3
H
HOOC NH2
H
CH3
HOOC NH2
CH3
H2N H
HOOC
CH3
H2N H
HOOC
CH3
HOOC NH2
HCH3
91
Stereochemistry of Reactions
CH3CH2CH CH2ether
CH3CH2CHCH3
Br
achiral chiral
H Br
92
Addition of HBr
Br-
Br-
CCH3CH2
Br
HCH3
CCH3CH2
Br
HCH3
CH3CH2 C
CH3
H
CH3CH2CH CH2ether
H Br
CH3CH2 C
CH3
H
93
Addition of Br2
Cis
Racemic mixture Achiral bromonium ion
C CH H
CH3 CH3Br
Br
C CH H
CH3 CH3
Br2C C
H H
CH3 CH3
Br
Br-
aC C
H H
CH3 CH3Br
Br
ab
ba
b
94
Addition of Br2
Trans
Symmetry plane, therefore meso Models are superimposible
C C
Br
Br
H
H3C
CH3
H
ba
C C
BrH
H3C
CH3
HBr
a
Br-
C CH
CH3
Br
H
CH3Br2
C CH
CH3 H
CH3
b
a b
95
Addition of Br2
C C
Br
Br
H
H3C
CH3
H
C C
BrH
H3C
CH3
HBr
C C
BrH3C
H
CH3
H
Br
96
Addition of HBr to a Chiral Alkene
CH3 HHBr
C CH3
CH3 H HBr
-
CH3
CH3 H Br H
CH3
CH3 H BrH
2S,4R 2R,4R
97
Addition of HBr to a Chiral Alkene Chiral intermediate is not attacked
equally from top and bottom because of steric reasons. Therefore, a mixture of product is formed in unequal amounts.
98
Chirality in Substituted Cyclohexanes Symmetry plane No stereogenic centers 1,4-disubstituted Only cis & trans diastereomers
CH3
CH3
99
1,3-disubstituted Cis Symmetry plane Meso compound
CH3
CH3
CH3
CH3
100
1,3-disubstituted Trans No symmetry plane Therefore enantiomers
CH3
CH3
CH3
CH3
101
1,2-disubstituted Trans Enantiomers
CH3
CH3
CH3
CH3
102
1,2-disubstituted Cis Meso
CH3
CH3
CH3
CH3
103
Br Cl BrCl
Cl
Br Br
Cl
enantiomers
enantiomersdiastereomers
cis
trans
1-Bromo-2-chlorocyclohexane
104
ClBr BrCl
Cl
Br Br
Cl
enantiomers
enantiomers
cis
trans
diastereomers
R S SR
R R S S
1-Bromo-2-chlorocyclopropane
105
BrBr BrBr
Br
Br Br
Br
mirror image identical
meso
enantiomers
diastereomers
cis
trans
1,2-Dibromocyclopropane
106
(S)-ibuprofen
C
H
CH3
COOH