We are doing chiral separation using Supercritical fluid chormatography, during separation chiral purity is fine but after distillation we are facing racemisation of our compound.
Without seeing the chemical structure it is difficult to know but a clue may be in the term "distillation". If this is at elevated temperature, it may be sufficient to racemise the material. Otherwise the racemisation may be due to presence of small amounts of acid or base which could catalyse the process.
Thank you sir,
we are frequantly facing same problem, i will try to isolate the product without distillation. if you have any idea to prevent the racemisation so please share.
Usually the major component of the mobile phase in super-critical fluid chromatography is CO2 - and considering that at normal conditions CO2 is a gas, its removal should be fairly easy (one of the advantages of this technique).
So, most likely some additives are used which can be more difficult to remove, and it would be good to know what they are to answer this question better. As a general recommendation, vacuum distillation (using rotary evaporator) of lyophilization can be used.
Another question, could the enantiomers that you separate racemize at room temperature ?
For better chromatography we are using 0.1% of Diethylamine, but as John sir said small amount of acid or base will catalyzed the racemisation so we also tried 100 % methenol as a co-solvent (without any pH change).
No at room temperature it is stable, we do chiral purity analysis of fraction(in solution form) which is 100% but after distillation we are not achieving purity.
Pranav,
Both diethylamine and methanol are low-boiling point substances (bp 55C and 65C, accordingly), so usually their removal in vacuum can be done at room temperature. At the same time, diethylamine is a strong base (pKa ~11) and also strong nucleophile, so it's presence could be a reason for racemization in your case, as Dr. Hollertone mentioned.
I would try enantiomers separation without diethylamine to clarify this issue. Another option would be using a column with different chiral stationary phase that does not require diethylamine additive.
Good luck,
Vladimir
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