Chapter 5 Stereoisomers
I.
Introduction
A.
Mirror Images may not be Superimposable
1) Pair of gloves
2) Shoes
3) Your image in a mirror
4) Screws
B.
Isomer Types
1) Isomer = same molecular formula, different structure
a) Constitutional Isomers: different order of connectivity
b) Stereoisomers: same connectivity, different spatial orientation
i. Diastereomers: stereoisomers that are not mirror images
ii. Enantiomers: non-superimposable mirror images
2) Conformations = stereoisomers that can rapidly interconvert by rotation
II. Chirality = describes objects not superimposable with their mirror image
A) Products of Radical Bromination of Butane
1)
2)
3)
4)
5)
2 products are produced
Are they mirror images? Are they Superimposable?
Chiral, Enantiomeric products; each enantiomer is one of a pair
(Achiral = is superimposable with mirror image)
One carbon is bonded to 4 different: Me, Et, H, Br
a) These carbons are always chiral
b) Asymmetric carbon
A
A
c) Stereocenter
D
C
B
B
D
C
B)
Molecular Symmetry
1) Plane of Symmetry
a)
b)
c)
d)
2)
Bisects molecule to give mirror image halves
Chiral molecules have zero planes of symmetry
Methane has 6 planes of symmetry: Achiral
CHFClBr has no planes of symmetry: Chiral
H
H
H
When is Chiral not Chiral?
a) Rapid structural changes give an Average Symmetry
b) A given structure must be room temperature isolatable
c) Conformational changes
i. Butane conformations are chiral, but Butane is achiral
ii. cis-1,2-dimethylcyclohexane has achiral intermediate
H
III. Optical Activity
A) How do you tell Enantiomers Apart?
1) Most Physical Properties are identical: mp, bp, density, etc…
2) Enantiomers interact differently with plane polarized light
3)
Each enantiomer of a pair rotates the light in a different direction
a) Dextrorotatory = clockwise rotation (+)
b) Levorotatory = counterclockwise rotation (-)
4)
Descriptions: Optical Activity, Optically Active, Optical Isomers
B)
a 
Using a Polarimeter to calculate Optical Purity
to

1) a, the rotation angle, depends on cell length, T, solvent, etc…
a) L = length in dm (0.1 m) c = concentration in g/ml
b) Specific rotation can be compared; constant for given molecule
2)
3)
4)
5)
Pure enantiomer has maximum a (+23o or –23o)
Racemic Mixture = 50%/50% mixture of enantiomers has a = 0
Anywhere in between will have rotation between these values
Enantiomeric Excess = %Optical Purity = how much more of one
enantiomer than the other
 a


%Optical Purity   observed x100 
 a 

6)
Exercise 5-7 75% optical purity, [a] = 23.1o, what is [a]observed?
0.75 
7)
ao
 0.75a  a o  (0.75)( 23.1o )  17.33o
a
How much of each enantiomer is present? NOT 75%/25%
12.5% (+) / 12.5% (-) and 75% (+) Total = 87.5% (+), 12.5% (-)

a
lc
IV. R,S Rules
A) Absolute Configuration
1) +/- tell us the interaction of light, not the exact structure of enantiomers
A
D
C
2)
B)
A
B
B
D
C
X-Ray Crystallography gives us Absolute Configuration
a) Crystals are regularly arranged solid forms
b) They Diffract X-rays regularly, so we can tell what atom is where
c) X-Ray and Polarimetry lets us match +/- with a specific structure
d) Similar molecules usually have same +/- correlation
R/S Labels
1) Cahn-Ingold-Prelog System assigns name to each enantiomer
2) Arrange substituents with lowest priority in back
a) Clockwise arrangement high-to-low = R (rectus = right)
b) Counterclockwise = S (sinister = left)
R enantiomer
S enantiomer
3)
4)
Write the name: R-(+)-chloroflouorobromomethane
Racemic Mixture: R,S-(+/-)-chlorfluorobromomethane
5)
Rules for assigning priority (Try Exercise 5-8 thru 5-10)
a) Highest Atomic Number: Br > Cl > F > H
b) For equal atomic number, look at what is attached
i. Only look at one level at a time
ii. If second level identical, then go to third level
c) Treat double and triple bonds as 2 or 3 single bonds to that atom
H
a
c
ClCH2
CH3CH2
H
a
Cl,2H > C,2H > 3H
CH3
b
c
(CH3)2CH
CH3CH2
2C,H > C,2H > 3H
CH3
b
a
HOCH2CH2
CH3CH2
c
H
a
H
CH2CH2CH3
b
HC
C
CH3CH2
c
b
CH
CH2