Editing Enantioselective synthesis (section)

==Separation and analysis of enantiomers==
The two enantiomers of a molecule possess many of the same physical properties (e.g. [[melting point]], [[boiling point]], [[Chemical polarity|polarity]] etc.) and so behave identically to each other. As a result, they will migrate with an identical R<sub>f</sub> in [[thin layer chromatography]] and have identical retention times in [[High-performance liquid chromatography|HPLC]] and [[gas chromatography|GC]]. Their [[NMR]] and [[Infrared spectroscopy|IR]] spectra are identical.

This can make it very difficult to determine whether a process has produced a single enantiomer (and crucially which enantiomer it is) as well as making it hard to separate enantiomers from a reaction which has not been 100% enantioselective. Fortunately, enantiomers behave differently in the presence of other chiral materials and this can be exploited to allow their separation and analysis.

Enantiomers do not migrate identically on chiral chromatographic media, such as [[quartz]] or standard media that has been chirally modified. This forms the basis of [[chiral column chromatography]], which can be used on a small scale to allow analysis via [[gas chromatography|GC]] and [[High-performance liquid chromatography|HPLC]], or on a large scale to separate chirally impure materials. However this process can require large amount of chiral packing material which can be expensive. A common alternative is to use a [[chiral derivatizing agent]] to convert the enantiomers into a diastereomers, in much the same way as chiral auxiliaries. These have different physical properties and hence can be separated and analysed using conventional methods. Special chiral derivitizing agents known as 'chiral resolution agents' are used in the [[NMR spectroscopy of stereoisomers]], these typically involve coordination to chiral [[europium]] complexes such as [[EuFOD|Eu(fod)<sub>3</sub>]] and Eu(hfc)<sub>3</sub>.

The separation and analysis of component enantiomers of a racemic drugs or pharmaceutical substances are referred to as [[chiral analysis]].<ref>{{Cite book|last=Allenmark|first=Stig G.|title=Chromatographic enantioseparation : methods and applications|publisher=E. Horwood|year=1988|isbn=0-85312-988-6|location=Chichester, West Sussex, England|pages=64–66}}</ref> or [[enantioselective analysis]]. The most frequently employed technique to carry out chiral analysis involves separation science procedures, specifically chiral chromatographic methods.<ref>{{Cite book|last1=Snyder|first1=Lloyd R.|last2=Kirkland|first2=Joseph J.|last3=Glajch|first3=Joseph L.|date=1997-02-28|title=Practical HPLC Method Development|url=http://dx.doi.org/10.1002/9781118592014|doi=10.1002/9781118592014|isbn=978-1-118-59201-4}}</ref>

The [[enantiomeric excess]] of a substance can also be determined using certain optical methods. The oldest method for doing this is to use a [[polarimeter]] to compare the level of [[optical rotation]] in the product against a 'standard' of known composition. It is also possible to perform [[ultraviolet-visible spectroscopy of stereoisomers]] by exploiting the [[Cotton effect]].

One of the most accurate ways of determining the chirality of compound is to determine its [[absolute configuration]] by [[X-ray crystallography]]. However this is a labour-intensive process which requires that a suitable [[single crystal]] be grown.