Pasteur was the first documented person to separate stereoisomers.
He did so by noticing that crystals of tartaric acid had either
a left-handed crystal or a right handed crystal, and then
he used a microscope and tweezers to separate the crystals
from each other. The discovery of stereoisomerism in tartaric
acid crystals, like Fleming's discovery of penicillin, was
serendipitous, since very few stereoisomers form separate
crystals, and tartaric acid only forms such separate crystals
at cold temperature (it is speculated that it was a cold day
when Pasteur synthesized the tartaric acid, and that he had
recrystallized it on his cold windowsill).
are enantiomers currently separated? Even if one could use
Pasteur's method of separation, one would be hard-pressed
to find workers as patient as Pasteur, willing to spend their
days under a microscope separating the different crystal forms
with a pair of tweezers.
that are enantiomers of each other have the same boiling points,
refractive indices, reactivities, melting points, and solubilities.
So if a mixture of enantiomers is obtained, how can they be
separated? They can't be recrystallized, since they have the
same solubility properties; they can't be distilled, since
they have the same boiling points; they can't be run on regular
achiral silica chromatography columns since they have the
same attractions to the stationary phase. Because of their
difficulty in separation, a great deal of current study today
is done to generate catalysts that form only a single stereoisomer
of a compound. There are, however, techniques that can be
employed to separate enantiomers.
Conversion to diastereomers
to separate enantiomers is to chemically convert them into
species that can be separated: diastereomers. Diastereomers,
unlike enantiomers, have entirely different physical properties--boiling
points, melting points, NMR shifts, solubilities--and they
can be separated by conventional means such as chromatography
was desired to separate a mixture of an R and S carboxylic
acid, for example, this mixture could be reacted with a single
enantiomer of a chiral amine to make the diastereomic ammonium
salts that could then be separated. Once the diastereomic
salts have been separated, mineral acid can reprotonate the
carboxylic acid to reform the original enantiomers. This is
a general, three step, technique for separating enantiomers:
the enantiomers with a single enantiomer of another compound
to form diastereomers
the diastereomers by conventional means (chromatography,
the original enantiomers, now separated
amine used in these reactions with carboxylic acids is S-Brucine,
an alkaloid found in only its S enantiomer. S-Brucine is used
because it is commercially available, although in theory any
amine that is purely one enantiomer should work just as well.
reactions that form diastereomers from various functional
groups have been described in the literature, although this
one with carboxylic acids is particularly effective because
it involves a noncovalent modification, and the reactions
are quick and performed in high yields.
Another technique for separating enantiomers is chiral chromatography.
While enantiomers cannot be distinguished in achiral environments,
such as a solvent system or by normal silica gel chromatography,
they can be distinguished in chiral environments, such
as in the active site of an enzyme,
or in a chiral stationary phase of a column. In a chiral column,
achiral silica gel (SiO2)is converted into a chiral
stationary phase by a reaction with a chiral molecule. Once
the enantiomers that need to be separated are run down the
column, one enantiomer will "stick" to the stationary
phase better than the other, and there will be separation
(of course, a disadvantage is that chiral silica gel is much
more expensive than standard silica gel).
hypothetical example of an interaction between a chiral stationary
phase (left) with an enantiomer of a biphenyl derivative (right),
there is a three-point interaction, with the carboxy groups
aligning with the amino groups and the aromatics lining up
with each other to form pi stacking interactions. The enantiomer
of this biphenyl would not be able to have all three of these
interactions because its groups would not be aligned correctly,
and, consequently, it would stick less to the chiral stationary
phase and filter off the column first.
Click here for the animation
of chiral flash chromatography (40kb, opens in a popup window).
A diagram of chiral column chromatography: the enantiomer
of the biphenyl that can form the three-point interaction
with the stationary phase (red band) sticks better and filters
off the column after its enantiomer (green band).