A closely related method does not require conversion of enantiomers to diastereomers,
but relies on the fact that (in principle, at least) enantiomers have different
NMR spectra in a chiral solvent, or when mixed with a chiral molecule (in which
case transient diastereomeric species may form). In such cases, the peaks may be
separated enough to permit the proportions of enantiomers to be determined from
their intensities. Another variation, which gives better results in many cases, is to
use an achiral solvent but with the addition of a chiral lanthanide shift reagent such
as tris[3-trifluoroacetyl-d-camphorato]europium(III). Lanthanide shift reagents
have the property of spreading NMR peaks of compounds with which they can
form coordination compounds, for example, alcohols, carbonyl compounds, and
amines. Chiral lanthanide shift reagents shift the peaks of the two enantiomers of
many such compounds to different extents.
Another method, involving GC, is similar in principle to the NMR method. A
mixture of enantiomers whose purity is to be determined is converted by means of
an optically pure reagent into a mixture of two diastereomers. These diastereomers
are then separated by GC (p. 172) and the ratios determined from the peak areas.
A closely related method does not require conversion of enantiomers to diastereomers,
but relies on the fact that (in principle, at least) enantiomers have different
NMR spectra in a chiral solvent, or when mixed with a chiral molecule (in which
case transient diastereomeric species may form). In such cases, the peaks may be
separated enough to permit the proportions of enantiomers to be determined from
their intensities. Another variation, which gives better results in many cases, is to
use an achiral solvent but with the addition of a chiral lanthanide shift reagent such
as tris[3-trifluoroacetyl-d-camphorato]europium(III). Lanthanide shift reagents
have the property of spreading NMR peaks of compounds with which they can
form coordination compounds, for example, alcohols, carbonyl compounds, and
amines. Chiral lanthanide shift reagents shift the peaks of the two enantiomers of
many such compounds to different extents.
Another method, involving GC, is similar in principle to the NMR method. A
mixture of enantiomers whose purity is to be determined is converted by means of
an optically pure reagent into a mixture of two diastereomers. These diastereomers
are then separated by GC (p. 172) and the ratios determined from the peak areas.
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