Purifying Organic Compounds

Purifying organic compounds in lab can be very challenging sometimes.  If you planning to use column purification you may end using lot of solvents and also your time.  Instead you can try some recrystallisation techniques for some large scale purifications.  These can also work for small scale but be wise in judging whether you want try RC’s for milligrams scale as sometimes column purification is best for milligrams and grams scale purification if you want to save time finding out right techniques unless you are developing a process though.

Below mentioned are some of the techniques which I do.  I just mentioned the rough idea and not a very detailed procedure here.

Idea 1: Try to dissolve your compound in acetone.  If it doesn't dissolve in acetone at rt then try to heat gently to make it dissolve.  Then bring the solution to rt and try to add water slowly (your compound should be water insoluble) and see whether your compounds precipitates out pure or not.

Try repeating this other water soluble solvents like Methanol, ethanol, IPA..etc

Idea 2: This is hot slurry techniques.  Make a stirrable slurry of your compound in solvents like ethyl acetate, MTBE, DCM (low boiling so be careful while heating)…etc..Stir the slurry under hot conditions and bring down the temperature to rt and filter.  You can try different temperatures for this slurry.  My idea is to try with the high temperature first and go to low temperature.

Hot filtration can also be done but I do not recommend it for large scale purifications and its risky.  For grams scale you can try hot filtration.

Idea 3:  dissolve your compound in more polar solvent like ethyl acetate or DCM and try to precipitate it with by adding a non-polar solvent like hexanes or heptanes an check for any precipitation.  See again its all about different combinations but a meaning full combination..:)  You can reverse this technique this way also.

Dissolve your compound in minimum amount of polar solvent and add this to your non-polar solvent slowly under stirring.  And check for precipitations.   This might work in some cases where the other mode of addition hasn’t worked.

Idea 4:  This is exactly a recrystallisation.  Dissolve your compound in a more polar solvent but in minimum volume under heating.  Basically what you do is take your compound and put in some polar solvent say like 5 or 8 volumes roughly.  Then start heating it until it dissolves or sometimes you need reflux for it to dissolve.  Once you get a solution of your compound, cool the solution to rt (or cool sometimes if you don’t see any ppt) and check for precipitation and filter if you get enough time precipitated.

 

Finally any of these techniques may or may not work for you.. Its all about trial and error.  Some times you end up getting oily product.  Don't worry repeat the same technique with few crystals of pure compound and see if it does any good.  Final word of suggestion be careful with hot RC.