How to work-up reaction using separatory funnel?

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>> Monday, June 18, 2012

separatory funnel, liquid-liquid extraction
As you know, to get a pure compound is not an easy task. Here, I am going to write about the work-up process (liquid-liquid extraction) using separatory funnel, one of the methods for the purification of crude compound. I hope it will be helpful for you. In this method, two immiscible solvents, aqueous and organic solvent (ethyl acetate, methylene chloride etc) are used which allows the removal of water soluble impurities present in the reaction mixture. 

Work-up procedure

Transfer the reaction mixture completely into the separatory funnel using the appropriate organic solvent. "Be cautious to close the stopcock". After that pour distilled water into the separatory funnel. Hold the stopper tightly and shake the reaction mixture gently. "Be sure to release the pressure- hold the stopper tightly, invert the funnel pointing outlet upwards and open the stopcock (See the video below)".
I want to share my bitter experience here. Once, while doing work-up (during shaking), I forgot to release the pressure. As soon as I kept the separatory funnel to stand, solvent splashed out of the separatory funnel through the stopper. Oh gosh! You can imagine the situation.:(
After gentle shaking, stand the separatory funnel for some time to see the clear separation of two layers i.e. organic and aqueous layer. Now comes the little trick. You need to determine the organic layer, where your desired compound remains. It can be done simply by comparing the density of two immiscible solvents used. For instance, ethyl acetate has lower density whereas methylene chloride has higher density than water. When you use ethyl acetate as organic solvent, it will remain at the upper layer. In case of methylene chloride it will remain at the lower layer. The organic layer is washed 2 or 3 times with water followed by saturated NaCl (brine). Check the TLC of all the washing to confirm the absence of your desired compound. After confirmation, you can discard the aqueous washing and collect the organic extract. Organic extract is then dried over magnesium sulfate or sodium sulfate. Remove the drying agent by filtration to get the clear organic extract. Evaporate the organic solvent using the rotary evaporator to get the concentrated product which can be further purified by recrsytallization or column chromatography, if necessary.

Things to remember while doing workup

  • Make sure to close the stopcock before transferring the reaction mixture into separatory funnel. It is one of the common mistakes to lose the compound. Keeping flask under separatory funnel will be a good idea.
  • Always remember to release the pressure while shaking the solvent mixture.
  • Sometime, there may not be clear separation of two layers i.e emulsion formation, in such case try to gently swirl the mixture and wait. If it does not work, add little saturated NaCl solution.
  • Make sure to differentiate the layers (see the density).
  • Check the TLC for all aqueous washings before discarding.
  • Sometime there may be formation of uniform black layers. In such case, you can add ice lump which will float in water (at the line between water and organic solvent).
  • For more troubleshooting, you may refer here and here.
Here is the diagrammatic illustration of work-up process.
work-up, separatory funnel, purification

And if you are still confused, please check out the video on how to do work-up.

If you have any queries, please leave them in the comment section below.


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