What are the best methods to determine the optical purity of any organic compound (synthesised or naturally isolated)? If the sp. rotations of the enantiomers is known in literature, then by simple polarimetric determination, we can calculate the ee of the compound. However, if the compound is novel, then what are the best and cheap methods available for a synthetic chemist? I have heard about chiral-HPLC. Kindly elaborate and give a best answer you can think of.
Only option we can determined by HPLC using chiral column (OD, OD-H, AD, AD-H..etc). we have to optimize the separation using various column.
Dear Govindarasu,
Thanks for your answer. Can you please tell which Chiral HPLC columns are suitable for alcohols, phenols and for amine-containing compounds?
You can determine the optical purity by HPLC using a chiral column, such as Chiralcel OD, which is for the separation of secondary alcohols. You can check
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HPLC using a column with the chiral stationary phase is the most common and convenient method to determine the stereochemical composition of chiral compounds nowadays, but not the only one (although it is still the preferable method when it works).
Also, the terminology like optical purity and enantiomeric excess (ee) are not recommended to use any more; the more proper terms will be enantiomer ratio or diastereomer ratio.
Relatively cheap method would be to use chiral derivatization reagents (Mosher's acid etc.) to form diastereomers and analyze them with achiral GC/HPLC or with NMR.
HPLC wise chiral IA columns by Chiralpak are widely used since they can withstand any organic solvent. The most efficient way besides shifting reagents in NMR is probably to equip a GC with a chiral column, which will directly allow to identify and determine the relative ratios of all compounds present in the mixture (diastereomers, enantiomers and so on) in one go.
Derivatization wth chiral reagents followed by HPLC will exactly tell about the purity of chiral molecule and thereafter one can easily determine the enantiomer excess of the compound.
It depends quite a bit on what the level of optical purity you are measuring is. For example, if you want to precisely determine purities well in excess of 99% ee, in such cases derivatization-based methods are good only if the purity of your reagent is very, very close to 100% ee.
Chiral NMR-shift reagents used to be a common way to go, these days chiral HPLC is more popular (if you have a chromophore in your molecule, and if the absorption coefficient is sufficiently high - weak absorption leads to less precise determinations). Another thing is that unless your mixture is very pure (contains exclusively the two isomers, and nothing else) you will have to have the racemate, or at least another mixture of pure two isomers, for a retention time standard. I have found this part to be quite important in the practice of ee determination by chiral HPLC. Interestingly, sometimes the RT difference is so high that the other isomer would not even elute during the run, and that can lead to erroneous conclusions about a very high ee!
I cannot recommend optical rotation-based methods for precise ee determinations, unless you have very good equipment, have very reliable data for the standard, and have grooved-in your technique really nicely.
Hope this helps :-) .
The suggestions given by the above colleagues are correct. Follow them.
Thanks to all for sparing your time in giving the answers. Also, I have seen some literature where absolute configuration is confirmed via the observations of cotton effect in the circular-dichroism spectra. If anyone has information regarding this, I would request you to give your viewpoints. Thanks.
In general, the sign of Cotton effect in CD spectra is as good for determination of the absolute configuration as the sign of the optical rotation. It means you need to compare it with the authentic sample or rely on the literature data.
That being said, there are situations where the empirical rules are helpful (the octant rule for ketones, for example); also, the quantum mechanical calculations recently becoming much more reliable for predicting the observed Cotton effect, but you need to know very well what you are doing ...
Dr.Vladimir Khlebnikov' suggestion seems right only when you have authentic smple.If compound is unknown it will be difficult to decide the purity of optical isomers.It is true that CD spectra will tell the absolute configuration and sign of optical rotation of compound
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