Stereoisomers
•
Review:
– Structural Isomers: Compounds that have the same molecular formula, but differ in the structural arrangement
of atoms.
• Examples: 1-butene, 2-methyl-propene
– Geometric Isomers: Compounds that have the same molecular formula, the same connectivity between atoms,
but differ in the arrangement of groups attached to the double bond.
• Examples: cis-2-butene, trans-2-butene
•
Stereoisomers: Isomers with the same molecular formula, same connectivity of atoms, but different arrangements of
the atoms in 3-D space.
– Geometric Isomers are a form of stereoisomer.
– Enantiomers: Chiral compounds that are nonsuperimposable mirror images make up a pair of enantiomers.
Enantiomers behave the same in almost every aspect of physical and chemical properties. The only physical
property that differs for a pair on enantiomers is optical activity. The only difference in chemical properties is in
the way they react with other chiral compounds.
– Diastereomers: Stereoisomers that are not enantiomers. Diastereomers do not have the same chemical and
physical properties.
•
Chirality: Objects that have handedness are said to be chiral. They can not be superimposed on their mirror image.
Objects that can be superimposed on their mirror image are said to be achiral (without chirality).
•
If a molecule has an internal symmetry plane, it is achiral. A lack of an internal mirror plane indicates chirality. See
the examples below, and determine which are chiral and which are achiral:
2-D:
A
E
F
O
H
3-D:
H
C
H
H
H
P
H
Br
R
H
C
H3C
Q
C
H3 C
Br
Cl
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Optical Activity
•
Light travels in waves at all angles. Light can be plane polarized so that it travels only in one plane rather than at
all angles. The light will appear the same to you and I.
•
A chiral compound and its nonsuperimposable mirror image make up a pair of enantiomers. Enantiomers are
often referred to as optical isomers because they are optically active.
An optically active molecule will rotate plane polarized light either to the left (levorotatory) or the right
(dextrorotatory). Enantiomers rotate plane polarized light in the exact opposite direction.
If a molecule has two or more chiral centers, life gets more complicated due to the presence of meso compounds
and diastereomers. In an attempt to make life a little easier, we will always draw these molecules as Fischer
Projections.
•
•
CHO
CH2OH
CHO
CHO
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
CH2OH
CH2OH
enantiomers
CH2OH
CHO
H
OH
H
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
OH
CH2OH
CH2OH
CH2OH
CH2OH
diastereomers
meso compound; the mirror images are identical!
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The Significance of Enantiomers
Enantiomers react differently with chiral compounds. Life is made up of chiral compounds. Amino acids used for
biosynthesis in animals are always of the L conformation. We can obtain energy only from the D forms of
carbohydrates. Enantiomers can be the difference between life and death. Enantiomers are important to:
•
Pharmaceutical companies
•
Agriculture
•
Food additive industries
O
O
NH
N
O
O
O
H
HN
O
O
H
N
O
Thalidomide was used to treat pregnant women for morning sickness during the 1960's. The
enantiomer shown on the left was responsible for the therapeutic affect of the drug. Unfortunately, the
presence of the enantiomer shown on the right caused severe birth defects.
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Problems
Are the following pairs (1-3) Identical, Enantiomers, Diastereomers, Unrelated, and/or Meso
1.
CHO
CHO
H
OH
HO
HO
H
H
H
OH
H
OH
HO
H
H
OH
HO
H
CH2OH
CH2OH
2.
CH3
CH3
HO
H
H
H
OH
OH
H
OH
CH3
CH3
3.
CH3
CH3
HO
H
H
OH
HO
H
H
OH
CH3
CH3
4. Circle all compounds above that will be optically active.
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