The document discusses various methods for separating the two enantiomers within a racemic mixture. The most common method is conversion of the racemic mixture to diastereomers using a chiral resolving agent, followed by fractional crystallization to separate the diastereomers based on differences in solubility. Other methods include differential absorption on a chiral chromatography column, enzymatic reactions that preferentially target one enantiomer, and kinetic resolution that exploits small differences in reaction rates between enantiomers. The goal of these resolution techniques is to produce pure samples of the individual enantiomers.

1. RESOLUTION
OR
THE ANALYSIS AND SEPARATION OF ENANTIOMERS

2.  The process of separation of racemic form into
individual enantiomers is called resolution.
 Separating enantiomers is called resolution.
 A pair of enantiomers can be separated in several
ways, of which conversion to diastereomers and
separation of these by fractional crystallization is
the most often used. In this method and in some of
the others, both isomers can be recovered, but in
some methods it is necessary to destroy one.

3.  The following are various methods used for
resolution of racemic mixtures:
 Conversion to Diastereomers.
 Differential Absorption
 Chiral Recognition.
 Biochemical Processes
 Mechanical Separation
 Kinetic Resolution
 Deracemization

4.  To separate components of a racemate (reversibly) we make a
derivative of each with a chiral substance that is free of its
enantiomer (resolving agent).
 This gives diastereomers that are separated by theirdifferential
solubility.
 The resolving agent is then removed.
 Usually, fractional crystallizations must be used and the process is
long and tedious.
 Fortunately, naturally occurring optically active bases (mostly
alkaloids) are readily available. Among the most commonly used are
brucine, ephedrine, strychnine, and morphine.
 Once the two diastereomers have been separated, it is easy to
convert the salts back to the free acids and the recovered base can
be used again.
 Most resolution is done on carboxylic acids and often, when a
molecule does not contain a carboxyl group, it is converted to a
carboxylic acid before resolution is attempted.
 However, the principle of conversion to diastereomers is not
confined to carboxylic acids, and other functional groups may be
coupled to an optically active reagent.
 Racemic bases can be converted to diastereomeric salts with active acids.
 Alcohols can be converted to diastereomeric esters, aldehydes to
diastereomeric hydrazones, and so on.

5.  Using an Achiral amine doesn’t change the relationship
of the products Still can’t separate the Enantiomeric
Salts.

6.  Using a Chiral amine changes the relationship of
the products Now we can separate the
Diastereomeric Salts.

7.  2)
 When a racemic mixture is placed on a
chromatographic column, if the column consists
of chiral substances, then in principle the
enantiomers should move along the column at
different rates and should be separable without
having to be converted to diastereomers.
 This has been successfully accomplished with paper,
column, thinlayer, and gas and liquid chromatography.
 For example, racemic mandelic acid has been almost
completely resolved by column chromatography on
starch.
 Columns packed with chiral materials are now
commercially available and are capable of
separating the enantiomers of certain types of
compounds.

9.  Chiral Recognition
 In this host forms an inclusion compound with one
enantiomer of a racemic guest, but not the other. This is
called chiral recognition.
 One enantiomer fits into the chiral host cavity, the other
does not. More often, both diastereomers are formed, but
one forms more rapidly than the other, so that if the guest is
removed it is already partially resolved.
 An example is use of the chiral crown ether partially to
resolve the racemic amine salt.
 When an aqueous solution of 59 was mixed with a solution
of optically active 58 in chloroform, and the layers separated,
the chloroform layer contained about twice as much of the
complex between 58 and (R)-59 as of the diastereomeric
complex.
 Many other chiral crown ethers and cryptands have been

10.  Biochemical Processes
 Biological molecules may react at different rates
with the two enantiomers.
 For example, a certain bacterium may digest one
enantiomer, but not the other.
 Pig liver esterase has been used for the selective
cleavage of one enantiomeric ester.
 This method is limited, since it is necessary to find
the proper organism and since one of the
enantiomers is destroyed in the process.
 However, when the proper organism is found, the
method leads to a high extent of resolution since
biological processes are usually very
stereoselective.

11.  Mechanical Separation
 This is the method by which Pasteur proved that
racemic acid was actually a mixture of (+)- and (-)-
tartaric acids.
 In the case of racemic sodium ammonium tartrate, the
enantiomers crystallize separately: all the (+) molecules
going into one crystal and all the (-) into another.
 Since the crystals too are non-superimposable, their
appearance is not identical and a trained
crystallographer can separate them with tweezers.
 However, this is seldom a practical method, since few
compounds crystallize in this manner.
 Even sodium ammonium tartrate does so only when it
is crystallized <270C. A more useful variation of the
method, although still not very common, is the seeding
of a racemic solution with something that will cause
only one enantiomer to crystallize.

12.  Kinetic Resolution
 Since enantiomers react with chiral compounds at
different rates, it is sometimes possible to effect a
partial separation by stopping the reaction before
completion.
 A method has been developed to evaluate the
enantiomeric ratio of kinetic resolution using only the
extent of substrate conversion.
 When a racemic substrate reacts with a homochiral
reagent two reactions occur, one with each enantiomer
of the substrate. Each process has a transition state
which incorporates the same enantiomer of the reagent,
but opposite enantiomers of the substrate.
 The transition states for the two processes have
a diastereoisomeric relationship, and the respective
activation energies are different. Therefore the
reactions proceed at different rates.
 If the rates of the two reactions are sufficiently
different, the reaction can be used for separating the
enantiomeric reactants.

13.  For a kinetic resolution to be 100% efficient, the
faster reaction would have to be finished before the
slower reaction starts.

14.  Deracemization
 In this type of process, one enantiomer is converted
to the other, so that a racemic mixture is converted to
a pure enantiomer, or to a mixture enriched in one
enantiomer.
 This is not quite the same as the methods of
resolution previously mentioned, although an outside
optically active substance is required.
 To effect the deracemization two conditions are
necessary:
 (1) the enantiomers must complex differently with the optically
active substance; they must interconvert under the conditions
of the experiment.
 When racemic thioesters were placed in solution with a
specific optically active amide for 28 days, the solution
contained 89% of one enantiomer and 11% of the other.
 In this case, the presence of a base (EtN) was necessary for