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Separation of Stereoisomers

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Separation of Stereoisomers
Resolution of Racemic Mixtures
The separation of a racemic mixture into the individual
enantiomerically pure enantiomers is called resolution
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Since diastereomers have different physical properties, they can
be separated by conventional physical techniques
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All
methods for separating or characterizing enantiomers are based
on this principle
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Weaker, non-covalent complexes are
often discriminating enough to allow separation of enantiomers
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There are many variations on this theme, and this traditional
approach is still very commonly used, especially for large scale,

industrial applications
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These are:
(a) Formation of diastereomeric salts with an enantiopure
resolving agent
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(c) Use of chiral stationary phases for chromatographic
resolution of racemic mixtures
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Resolution of Racemic Mixtures
Diastereomeric Salt Formation
Louis Pasteur, in his pioneering work, was able to isolate the
stereoisomers of tartaric acid because they crystallize from
solution as crystals with differing symmetry and shape
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This was a very lucky outcome that
transformed the study of stereochemistry in a profound way
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The diastereomers are
separated by fractional crystallization
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Note that the lactic acid would be soluble in the organic layer,
while the ammonium salt would be in the water layer
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This is achieved by treating the (S)-1-phenylethyl ammonium
chloride salt with a base such as sodium hydroxide to regenerate
and recover the chiral amine
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Resolution of Racemic Mixtures
Formation of Diastereomeric Compounds
An equally effective strategy of resolving racemic mixtures is by

reacting with an enantiopure resolving

agent leading to

formation of covalently bonded diastereomeric compounds
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Resolution of Racemic Mixtures
Formation of Diastereomeric Esters of Menthol
(-)-Menthol, a chief component of peppermint oil, can be readily
used as a chiral resolving agent
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These are incorporated in gas chromatography and liquid
chromatography systems
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Resolution of Racemic Mixtures
Chromatography on Chiral Stationary Phases

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Resolution of Racemic Mixtures
Chiral Stationary Phases
Chiral chromatography exploits
the development of transient,
diastereomeric

interactions

between the enantiomers and
the chiral stationary phase

Retention time
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Resolution of Racemic Mixtures
Enzymatic Resolution
In enzymatic or kinetic resolution, there is preferential reaction
of just one enantiomer resulting in an enantioenriched sample
of the less reactive enantiomer
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e
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It is denoted by the symbol ee
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Consider the following example:
If a mixture contains 95% of one enantiomer and 5% of the
other, the enantiomeric excess is 95% -5% = 90%
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Resolution of Racemic Mixtures
Calculating Enantiomeric Excess (Assignment)
The enantiomeric excess can also be calculated if the specific
rotation [α] of a mixture and the specific rotation [α] of a pure
enantiomer are known
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A sample of mandelic acid analysed in a polarimeter gave an
observed specific rotation of -75 degrees
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(iii) Calculate the percentage of each enantiomer in the mixture
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As the specific rotation of the mixture is negative, (R)-mandelic
acid is the dominant one
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7 %
Let the % of R-enantiomer be R, that of the S-enantiomer be S,
then R + S = 100, while R – S = enantiomeric excess
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7
2R – 100 = 48
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7
The major R-enantiomer = 74
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6%
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