term diastereomers. It is important to note that diastereomers in general possess
different physical properties. By contrast, enantiomers exhibit identical physical and
chemical properties except in two important respects:
• They rotate the plane of polarised light in opposite directions, though in equal
amounts. The isomer that rotates the plane to the left (counterclockwise) is called
the levo isomer which is indicated with (À), while the one that rotates the plane to
the right (clockwise) is called the dextro isomer and is designated (+).
• They react at different rates with other chiral compounds. These rates may be so
close together that the distinction is practically useless, or they may be so far apart
that one enantiomer undergoes the reaction at a convenient rate, while the other
does not react at all. This is the reason that many compounds are biologically
active while their enantiomers are not. However, enantiomers react at the same
rate as achiral compounds.
Furthermore, enantiomers may react at different rates with achiral molecules, if a
chiral catalyst is present, they may show different solubilities in a chiral solvent,
and they may exhibit different indexes of refraction or absorption spectra when
examined with circularly polarised light. In most cases, these differences are too
small to be useful and are often too small to be measured. In general, it can be
concluded that enantiomers possess identical properties in a symmetrical environment, but their properties may differ in an unsymmetrical environment. This
basic principle forms the background for enantioselective chromatography,
where chiral stationary phases are being used which form the asymmetric
environment, as required. Thus, diastereomeric complexes between the respective
enantiomers and the chiral stationary phase emerge that give rise to different
retention times, hence allowing an enantioselective separation. By contrast, diastereomers, which possess different physical properties, can also be separated on
achiral stationary phases.
The basic principles of chirality summarised above set the stage for an understanding of the different chromatographic characteristics of chiral and achiral environmental pollutants with one or more stereogenic centres. As the separation of
diastereomers can be separated on achiral stationary phases, the maximum number
of peaks to be expected will be identical with the number of diastereomers. The
separation of enantiomers requires an asymmetric environment and thus chiral
stationary phases. Then, the maximum number of peaks to be expected will be 2
n ,
provided that no meso-forms will be present and accordingly reduce this number.
An example for a chiral environmental pollutant that exhibits two asymmetric
centres and which could be successfully separated in its two diastereomeric
pairs of enantiomers (i.e. four peaks) by enantioselective capillary gas chromatography is represented by the musk compound HHCB (Fig. 1.9; for further details,
see Chap. 3.3).
14
1 Introduction
different physical properties. By contrast, enantiomers exhibit identical physical and
chemical properties except in two important respects:
• They rotate the plane of polarised light in opposite directions, though in equal
amounts. The isomer that rotates the plane to the left (counterclockwise) is called
the levo isomer which is indicated with (À), while the one that rotates the plane to
the right (clockwise) is called the dextro isomer and is designated (+).
• They react at different rates with other chiral compounds. These rates may be so
close together that the distinction is practically useless, or they may be so far apart
that one enantiomer undergoes the reaction at a convenient rate, while the other
does not react at all. This is the reason that many compounds are biologically
active while their enantiomers are not. However, enantiomers react at the same
rate as achiral compounds.
Furthermore, enantiomers may react at different rates with achiral molecules, if a
chiral catalyst is present, they may show different solubilities in a chiral solvent,
and they may exhibit different indexes of refraction or absorption spectra when
examined with circularly polarised light. In most cases, these differences are too
small to be useful and are often too small to be measured. In general, it can be
concluded that enantiomers possess identical properties in a symmetrical environment, but their properties may differ in an unsymmetrical environment. This
basic principle forms the background for enantioselective chromatography,
where chiral stationary phases are being used which form the asymmetric
environment, as required. Thus, diastereomeric complexes between the respective
enantiomers and the chiral stationary phase emerge that give rise to different
retention times, hence allowing an enantioselective separation. By contrast, diastereomers, which possess different physical properties, can also be separated on
achiral stationary phases.
The basic principles of chirality summarised above set the stage for an understanding of the different chromatographic characteristics of chiral and achiral environmental pollutants with one or more stereogenic centres. As the separation of
diastereomers can be separated on achiral stationary phases, the maximum number
of peaks to be expected will be identical with the number of diastereomers. The
separation of enantiomers requires an asymmetric environment and thus chiral
stationary phases. Then, the maximum number of peaks to be expected will be 2
n ,
provided that no meso-forms will be present and accordingly reduce this number.
An example for a chiral environmental pollutant that exhibits two asymmetric
centres and which could be successfully separated in its two diastereomeric
pairs of enantiomers (i.e. four peaks) by enantioselective capillary gas chromatography is represented by the musk compound HHCB (Fig. 1.9; for further details,
see Chap. 3.3).
14
1 Introduction