Resolution of Enantiomers Part II
• Diastereomeric Salt Formation
– Separation of enantiomers from a racemic mixture
– Enantiomers are temporarily converted into a mixture
of diastereomers
• Diastereomers have different physical properties
• Do this by combining enantiomers with a resolving
agent
– Resolving agent
» Chiral compound
» Enantiomerically pure
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• By this method a racemic mixture of an amine
may be resolved with a chiral carboxylic acid
Racemic Mixture
Resolving Agent
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• Two diastereomeric salts are formed
• These salts have different physical properties
• The (S,R,R)-diastereomer is less soluble in
methanol and can be selectively crystallized
6.8 Enantiomeric Resolution
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• Each pure enantiomer may then be recovered
by decomposition of the salt with base
6.8 Enantiomeric Resolution
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Meso Compounds
• Meso compounds:
molecules that have two
or more chiral carbons
yet are achiral
– not optically active
– Possess an internal
mirror plane
Stereoisomers of 2,3-Butanediol
Stereoisomers of 2,3-Butanediol
• An internal plane of symmetry is present
6.7 Meso Compounds
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Stereoisomers of 2,3-Butanediol
• Two of the stereoisomers are the same
• Hence, there are only three stereoisomers of
2,3-butanediol
6.7 Meso Compounds
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Stereoisomers of 2,3-Butanediol
• Relationship among the 2,3-butanediol
stereoisomers
6.7 Meso Compounds
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Meso Compounds
• A molecule with n stereocenters, can exist as
2n stereoisomers
• This number is reduced if a meso compound is
present among the possibilities
• 2,3-Butanediol possesses two stereocenters =
2n = 22 = 4 possible stereoisomers
6.7 Meso Compounds
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Problems
• Which of the following structures represent
meso compounds?
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