Course Code: BP401T
Course Title: Pharmaceutical Organic Chemistry III
Enantiomers & Diastereomers
Session Objectives
By the end of this session, students will be able to:
➢ Explain configuration of enantiomers
➢ Classify Enantiomers and Diastereomers
➢ Assign D and L system to isomers
➢ Define meso compounds
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Optical activity
• A solution of optically active molecule (enantiomer) is placed in a sample tube,
plane-polarized light is passed through the tube and a rotation of the polarization
plane takes place
• The light then goes through a second polarizer called an analyser
• By rotating the analyser until the light passes through it, the new plane of
polarization can be found, and the extent of rotation that has taken place can be
measured
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Optical activity
• A mixture of enantiomers with the same amount of each is called a racemic mixture
• Racemic mixtures are optically inactive (i.e. they cancel each other out) and are
denoted by (±)
• Note: the amount of rotation depends on sample concentration and sample path
length
• To obtain a meaningful optical rotation data, we have to choose standard conditions
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Optical activity
• The specific rotation of a compound, designated as [α]D, is defined as the
observed rotation, when the sample path length l is 1 dm, the sample
concentration C is 1g/mL and light of 599.6 nm wavelength (the D line of a sodium
lamp, which is the yellow light emitted from common sodium lamps) is mostly
used
• As the specific rotation also depends on temperature, the temperature at which
the rotation is measured and denoted more precisely as
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Enantiomers can be described as (+) or (-)
• We can use the fact that two enantiomers rotate plane-polarized light in opposite
directions to assign each a label that doesn’t depend on knowing its configuration
• We call the enantiomer that rotates plane-polarized light to the right (gives a
positive rotation) the (+)-enantiomer (or the dextrorotatory enantiomer)
• And the enantiomer that rotates plane-polarized light to the left (gives a negative
rotation) the (–)-enantiomer (or the laevorotatory enantiomer)
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Enantiomers and Diastereomers
• In general molecule with n chiral centers, the maximum number of stereoisomers
possible is 2n
• That means for one chiral center its 2, for 2 its 4, for 3 its 8 and so forth
• Let us consider 2,3,4-trihydroxybutanal
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Enantiomers and Diastereomers
• In general molecule with n chiral centers, the maximum number of stereoisomers
possible is 2n
• Let us consider 2,3,4-trihydroxybutanal
• It contains two chiral centers and 4 stereoisomers are possible
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Enantiomers and Diastereomers
• Stereoisomers (a) and (b) are nonsuperposable mirror images and are, therefore, a
pair of enantiomers
• Stereoisomers (c) and (d) are also nonsuperposable mirror images and are a
second pair of enantiomers
• On naming, enantiomers (a) and (b) as (2R,3R)-erythrose and (2S,3S)-erythrose;
enantiomers (c) and (D) as (2R,3S)-threose and (2S,3R)-threose
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Enantiomers and Diastereomers
• Both belongs to the class of carbohydrates and erythrose is present
in erythrocytes (red blood cells)
• What is the relationship between (a) and (c), (a) and (d), (b) and (c),
(b) and (d) ??
• Answer is diastereomers
• Diastereomers are the stereoisomers that are not mirror images
• Molecules with at least two chiral centers can have diastereomers
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Meso compounds
• Molecules containing two or more chiral centers have special
symmetry properties that reduce the number of stereoisomers to
fewer than the maximum number predicted by the 2n rule
• For example, 2,3-dihydroxybutanedioic acid commonly called as
tartaric acid
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Meso compounds
• In tartaric acid, carbons 2 and 3 are chiral centers, number of stereoisomers
possible is 4
• (a) and (b) are nonsuperposable mirror images, enantiomers
• (c) and (d) are superposable mirror images, meso compounds
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Meso compounds
• Also (c) has plane of symmetry and is achiral
• A meso compound contains two or more chiral centers and is achiral
• To be a meso compound, molecule must also have chiral isomers
• Hence, tartaric acid has three stereosisomers, one pair of enantiomers and a meso
compound
• Enantiomers of tartaric acid will have same melting point, boiling point, solubility in
water and other common solvents, same value of pKa, and they undergo the same
acid-base reactions
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Meso compounds
• But differ in optical activity
• Diastereomers have different physical and chemical properties, even in achiral
environments
• Meso tartaric acid has different physical properties from those of the enantiomers
and can be separated from them by methods such as crystallization
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Physical properties of tartaric acid
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Summary
• Enantiomer that rotates plane-polarized light to the right (gives a positive
rotation) the (+)-enantiomer (or the dextro-rotatory enantiomer)
• Enantiomer that rotates plane-polarized light to the left (gives a negative
rotation) the (–)-enantiomer (or the laevo-rotatory enantiomer)
• The direction in which light is rotated is not dependent on whether a stereogenic
centre is R or S
• Optical activity does not tell us the actual configuration of an enantiomer
• In general molecule with n chiral centers, the maximum number of stereoisomers
possible is 2n
• Diastereomers are the stereoisomers that are not mirror images
• Molecules with at least two chiral centers can have diastereomers
• A meso compound contains two or more chiral centers and is achiral
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Disclaimer
All data and content provided in this presentation are
taken from the reference books, internet – websites
and links, for informational purposes only.
Thank You
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