racemisation n resolution
TRANSCRIPT
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RACEMISATIONAND
METHODS OF RESOLUTION
By: Sheikh Khalida Banu, K.J. Somaiya College of Sci. and Com.
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Introduction Racemisation is the process when
the enantiomer is converted into racemic modification
Resolution is the process when a racemic modification is separated into its constituent enantiomers.
Racemisation and Resolution are complementary to each other.
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Formation of racemic modifications
Racemic (mixture,
compound, solid
solution)
By synthesis
By mixingBy
racemisation
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Racemisation Enantiomers exist in 50:50
population. Thermodynamically favourable
process. Reversible process.
form form
+
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Mechanism of racemisation Involving carbanions Involving carbonium ions Involving free radicals Involving stable symmetrical intermediates Through rotation around bonds Configurational change in reactions
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Mechanism involving carbanionsOH
OOH
C
CHCH
CH
CHCH C
C
HH O -
H 2 O
OH
O
C
C
CHCH
CH
CH CH
C-
H
H + c a n re c o m bin e fro m e ithe r s id e s
+/-
(+) isomer + (-) isomer racemic mixture
carbanion
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Mechanism involving carbonium ions
Recombination of anion gives racemic mix.
R1
R2
C
R3
Cl R2
C+
R1
R3
-C l-
ac id
+/-
carbonium
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Mechanism involving free radical
Free radical has a near planar structure and if a chiral centre is converted into a free radical pair by homolytic cleavage of a bond, the recombination of pair leads to racemic product
R1
R2
C
R3
+ ClR2
C
R1
R3
ClCl C
R3
R1
R2
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Mechanism involving stable symmetrical intermediate
Enantiomers are interconverted through stable achiral intermediates and get racemised.
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Mechanism through rotation around bonds
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Configurational change in substitution reactions
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Methods of resolution By crystallisation method Through formation of diastereomers By chromatography Through equilibrium asymmetric
transformation Through kinetic asymmetric
transformation By biochemical transformation Through inclusion compounds
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Mechanical separation by crystallisation is based on solubility of enantiomers.
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Resolution through formation of diastereomers
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By chromatography
RESOLUTION IN FOUR WAYS : FORMATION OF DIASTEREOMERS AND
THEN SEPARATION DIRECT RESOLUTION USING CHIRAL
STATIONARY PHASE DIRECT RESOLUTION USING ACHIRAL
MOBILE PHASE DIRECT RESOLUTION USING ACHIRAL
STATIONARY PHASE MODIFIED BY CHIRAL REAGENT
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Resolution through equilibrium asymmetric transformation
In this, resolution involve 2 steps:1. Epimerisation of diastereotopic
species2. Precipitation of the predominant
epimer This combination of epimerisation and precipitation is known as second order asymmetric transformation.
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Resolution through kinetic asymmetric transformation
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Resolution by biochemical transformation
Chiral reagents are replaced by microorganisms or enzymes which are stereoselective in their reactions.
It is mainly used in resolution in amino acids.
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Through inclusion compounds
An inclusion compound is a complex in which one chemical compound (the "host") forms a cavity in which molecules of a second "guest" compound are located.
If the spaces in the host lattice are enclosed on all sides so that the guest species is ‘trapped’ as in a cage, the compound is known as a clathrate.
This method is of little practical use.
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