Chapter 7 The Types of Isomers Constitutional
Configurationaland conformational
Isomers are compounds which have the same
molecular formula, but differ in the way the
atoms are arranged.
There are three types of isomers constitutional
（构造）, configurational（构型） and
conformational(构象）.
configurational（构型） -- Optical isomers
Optical
isomers
(configurational
isomerism)are
configurational isomers which have the ability to rotate
plane-polarized light clockwise or counterclockwise.
They have identical chemical and physical properties
(apart from their effects on plane-polarized light), but can
have different biological properties.
Pasteur’s Discovery of Enantiomers
(1849)
• Louis Pasteur discovered that sodium
ammonium salts of tartaric acid crystallize
into right handed and left handed forms
• The optical rotations of equal concentrations
of these forms have opposite optical rotations
• The solutions contain mirror image isomers,
called enantiomers and they crystallized in
distinctly different shapes – such an event is
rare
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Optical Activity
• Light restricted to pass through a plane is
plane-polarized
• Plane-polarized light that passes through
solutions of achiral compounds remains in
that plane
• Solutions of chiral compounds rotate planepolarized light and the molecules are said to
be optically active
• Phenomenon discovered by Biot in the early
19th century
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Optical Activity
• Light passes through a plane polarizer
• Plane polarized light is rotated in solutions of
•
•
•
•
•
optically active compounds
Measured with polarimeter
Rotation, in degrees, is []
Clockwise rotation is called dextrorotatory
（右旋的）
Anti-clockwise is levorotatory（左旋的）
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Asymmetric molecules
Lactic acid exists as two nonsuperimposable mirror images
because it is asymmetric – in other words it lacks symmetry.
Asymmetric molecules can also be termed as chiral, the
ability of molecules to exist as two optical isomers is
called chirality. Molecules containing a single axis of
symmetry can also be chiral.
Asymmetric carbon centers
A simple method of identifying most chiral molecules involves
identifying what are known as asymmetric carbon centers.
This works for most chiral molecules, but it is important to
realize that it is not foolproof and that there are several cases
where it will not work. For example, some chiral molecules
have no asymmetric carbon centers, and some molecules
having more than one asymmetric carbon center are not chiral.
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Examples of Enantiomers
•
Molecules that have one carbon with 4 different substituents
have a nonsuperimposable mirror image – enantiomer
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• The flask has a mirror plane, or plane of symmetry
• There is no mirror plane for a hand
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Polarimeter (schematic)
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Specific Rotation and Molecules
•
•
Characteristic property of a compound that is optically active – the
compound must be chiral
The specific rotation of the enantiomer is equal in magnitude but
opposite in sign (or direction).
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Sequence Rules (IUPAC)
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•
•
Assign each group priority
according to the Cahn-IngoldPrelog scheme With the
lowest priority group pointing
away, look at remaining 3
groups in a plane
Clockwise is designated R
(from Latin for “right”)
Counterclockwise is
designated S (from Latin
word for “left”)
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Configuration at Chirality Center
• Lowest priority group is pointed away and direction of
higher 3 is clockwise, or right turn
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Examples of Applying Sequence
Rules
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Racemic Mixtures and Their Resolution
• A 50:50 mixture of two chiral compounds that
are mirror images does not rotate light –
called a racemic mixture (named for
“racemic acid” that was the double salt of (+)
and (-) tartaric acid
• The pure compounds need to be separated
or resolved from the mixture (called a
racemate)
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CH3
CH2
OH
ethanol, b.p. = 78°C
CH3
CH2
NH2
ethyl amine, b.p. 17°C
=>