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Properties of Enantiomers
Achiral Properties
Any pair of enantiomers are physically and chemically
indistinguishable by most techniques in achiral environments
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This phenomenon

is

called optical activity
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Chiral molecules are thus optically active
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The source light passes through a polarizer and then is
detected at an analyzer
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Enantiomers
Measurement of Optical Rotation
Optical rotation a is measured in degrees
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The optical rotation a is not very useful for direct comparative
purposes since it depends on the path length the light
traverses, the concentration of the analyte, the analyte itself
and the wavelength of the light
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Enantiomers
Calculation of Specific Rotation
Specific rotation [α] is a standardized physical constant for the
degree that a solution rotates plane-polarized light
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The temperature is maintained at 25

oC

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Enantiomers
Sample Calculation of Specific Rotation
The optical rotation of a solution of 0
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58o
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Remember to include its sign (+ or -)
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497g / 5 mL = 0
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58 / (0
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1o
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Enantiomers
Stereochemical Terminology vs Optical Activity
Enantiomerically pure or enantiopure: Comprises of exclusively
one enantiomer
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Enantiomers
Stereochemical Terminology vs Optical Activity
Dextrorotatory (+): an optically active compound that rotates
plane polarized light in a clockwise direction
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Levorotatory (-): an optically active compound that rotates
plane polarized light in a counterclockwise direction
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Enantiomers
Stereochemical Terminology vs Optical Activity
D/L vs d/l Notations
The D/L notation is unrelated to (+)/(−) or d/l; it does not
indicate which enantiomer is dextrorotatory and which is
levorotatory
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The dextrorotatory isomer of
glyceraldehyde is, in fact, the D- isomer
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Isolated from Sweet and Bitter almonds

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Enantiomers
Chiral Properties: Optical Activity and Aroma

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Enantiomers
Chiral Properties: Biological Activity
Stereochemistry is important in biological systems because
most body reactions are stereospecific
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Other
configurations of the same chemical may not elicit a favorable
response or be toxic
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Enantiomers
Biological Activity: The Tragic Case of Thalidomide
Thalidomide was once hailed as a "wonder drug" that provided
a "safe, sound sleep"
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However, thalidomide later became notorious as the killer and
12:09 PMdisabler of thousands of babies
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The S enantiomer cannot interact with the same site due to the
different arrangement of atoms (3D shape)
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Enantiomers
Chiral Properties: Taste
Although there are individual variations between enantiomers,
there are now well-recognized taste differences between

enantiomers of many compounds
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Enantiomers
Chiral Properties: Kinetics of Enantiomers
Enantiomers react at different rates with chiral reagents
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The enzyme typically binds only one enantiomer since as

expected “only the right hand fits the right-hand glove”
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The family
tree below illustrates these relationships
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Diastereomers can be divided into two categories:
(a) Configurational Diastereomers: These are stereoisomers of a
chiral molecule, but ones that are not enantiomers
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Diastereomers
Configurational Diastereomers
Configurational diastereoisomerism occurs in molecules that
have more than one chiral center
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Diastereomers
Configurational Diastereomers (Exceptions)
Occasionally, exceptions occur
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Diastereomers
Meso Compound
A meso compound or meso isomer is an achiral member of a
set of stereoisomers, at least two of which are chiral
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Although a meso compound passes the tests of same molecular
formula, same connectivity of atoms, having a chiral centre and

mirror image, It fails the test of non-superimposability since it is
superimposable on its mirror image
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(b) cis-1,2-disubstituted cycloalkanes are meso if the two
substituents are identical
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Diastereomers
Assignment
Determine which of the following compounds are chiral and
which ones are not
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Diastereomers
Properties of Configurational Diastereomers
Configurational diastereomers have different chiral and achiral
properties in any environment and are thus different
compounds
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Rates of reactions with any reagents
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Biological properties

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Diastereomers
Properties of Configurational Diastereomers
Compare the listed properties of the two diastereomers of
tartaric acid (dextrotartaric and mesotartaric acid) and note the
differences in magnitude
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Molecular Structure Relationships
The Roadmap
There are various categories of isomeric relationships that exist
between compounds of the same chemical formula
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Diastereoisomers
Cis-Trans Diastereomers (Geometric Isomers)
Cis-Trans diastereomers or geometric isomers usually arise
when there is restricted rotation in a molecule; commonly at a
carbon-carbon double bond
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Diastereoisomers
Assigning Configuration of Geometric Isomers
By applying the CIP rules, the configuration of a double bond is
specified with a prefix (E or Z) based on whether the high
priority substituents are on opposite or same side of the double
bond
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If, by contrast they are on opposite sides, then the stereoisomer
is assigned an E or Entgegen configuration
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Note that for fumaric and maleic acid, the stereochemistry of
the compounds is implied in their names
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Diastereoisomers
Physical Properties of Geometric Isomers
Cis-trans diastereomers are different compounds
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For
example, the cis and trans isomers of butenedioic acid show

large differences in their melting points
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Whereas maleic
acid readily undergoes dehydration, fumaric acid is resistant to
dehydration
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Triprolidine, an antihistamine drug used to combat symptoms
associated with allergies and provides general relief for flu-like
symptoms, is more effective in the E-configuration

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