Can anyone explain to me a simple procedure and solvent that would help me to obtain pure crystals? I am trying to isolate a pure compound that came off a silica column it was a solvent mixture of hexane and ethyl acetate and from the NMR and LC/MS/UV it is very close to pure, but I would like to recrystallize without using too high heat to dissolve it because I dont want to degrade it. What would be a good solvent? I hope the compound is a terpenoid.
What is the purity of compound based on NMR and LCMS? More the purity better the chances of recrystallization. What temperature your compound degrades (M.p (Some apparatus allows you see degradation pattern more accurately)? Once you identify the temperature you could see the possible solvent list on that range. As a thumb rule, you need try few solvent combination such that a) Your compound is moderately soluble in that mixture b) Insoluble on RT and soluble on heating. What is the Rf value in TLC (Guage the polarity, might help in choice of solvent).
A simple method for crystallization of pure compound could be performed as the following:
In a very narrow vial or container
1-Dissolve your compound in an organic solvent using the least possible amount
2-Add few drops of (1-3) of organic solvent where your compound isn't soluble( but it should be miscible with the first solvent)
3-Cover the top of the vial using aluminium foil
4-Make several but small opening in the foil using toothpick or needle .
5-leave the vial for evaporation for 24 hr ,you would notice the formation of crystals.
For your compound you may want to check for chloroform as the main solvent and few drops of petroleum ether
I advice you to see this paper
Analysis and Purification of Bioactive Natural Products: The AnaPurNa Study
J Nat Prod. 2012 Jun 22; 75(6): 1243–1255.
With the limited information you provided, and assuming that your compound is insoluble (or sparingly soluble) in n-heptane, I suggest the following procedure to recrystallize your sample:
Place approximately 200 mg of your sample in a 100 ml flat bottom round flask (24/40 female joint), add 20 ml of n-heptane and a stir bar. Adjust a reflux condenser, start the magnetic stirring and heat until it boils. After 5 to 7 minutes of boiling, observe if your sample completely or nearly dissolved (turbidity).
- If so, filter hot as fast as possible through filter paper; collect the filtrate in a 50 mL beaker, cover with aluminum foil and leave at room temperature for 3-5 hs. (or throughout the night). Collect the crystals formed by filtration, dry in a vacuum desiccator and take a melting point. The mother liquors are taken to refrigerator or freezer and left overnight. An additional crop of crystals is obtained. Verify purity by TLC and HPLC.
- In the event that all or most of your sample remains insoluble in boiling n-heptane, add ethyl acetate in small increments (eg, 1.0 ml each time) through the top of the condenser, and return to reflux for 3-5 minutes. Repeat the process until the sample dissolves completely. At this point, proceed as in the previous case (filter in hot through filter paper,…).
- Most compounds that crystallize after column chromatography can be recrystallized at the boiling temperature of n-heptane (98 ° C) without any alteration. Otherwise, you can use n-hexane (boiling point 68º C).
I advice you to adding heated solvent until further addition or agitation
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