Extraction is a technique used to separate compounds based upon their different solubilities in two solvents that do not mix. Most commonly, one of the solvents will be water and the other will be an immiscible organic solvent (often methylene chloride, diethyl ether, or ethyl acetate). In general, very non-polar compounds will partion to the organic solvent and very polar compounds and salts will partion to the aqueous phase. Since the two solvents do not mix, they can be separated in a separatory funnel providing a very quick and easy way to separate compounds. Whether the water layer is on the top or bottom depends on the density of the other solvent (methylene chloride is heavier than water and goes to the bottom of the separtory funnel and diethyl ether and ethyl acetate are lighter than water and stay on the top). This techique when coupled with acid-base chemistry provides a very powerful method for separating organic acids, bases and neutral compounds from one another. By treating your organic layer with either an aqueuous acid or base solution one can ensure that organic bases or acids are converted to their corresponding conjugate acids or bases respectively. These compounds will be charged and thus favor the water layer allowing them to be separated from other organic compounds. To see a general scheme for separating organic acids, bases and neutral compounds go here.


ext_app ext_vent ext2

Note the use of the 2 inch ring to support the funnel. Also, the top yellow and bottom clear liquids are immiscible.

Bottom arrow: Note that the stopper is firmly held in place.
Top arrow: Turn the stopcock to vent the funnel.

While the lower layer is drained from the funnel, it is necessary to remove the stopper.

In the left pannel, a separatory funnel is shown containing two immiscible liquids. This sample is actually the mixture of the organic compound benzil, which is yellow, in diethyl ether and water. Since diethyl ether is less dense than water, it is easy to see that the organic compound is dissolved in the organic solvent (like dissolves like). Notice that the separatory funnel is placed in a securely clamped 2 inch ring. To be sure that the compounds are completely partioned, the separatory funnel is shaken with a stopper on top and with the bottom stop-cock closed. The center pannel illustrates the mixing/venting process. When mixing be sure that the separatory funnel is corked and that the stopcock is closed. Shake the flask and then with the bottom of the flask pointing upward and not toward yourself or your neigbor, open the stopcock to vent the pressure in the flask. Be sure to vent the separatory funnel often to avoid pressure build-up. Place the flask back in the ring stack and allow the layers to separate again. Place a receiving flask under the funnel, remove the stopper from the top and open the stopcock and allow the bottom layer to flow out of the funnel. Close the stopcock and you have now separated the two layers. One should always keep both layers until the desired compound is isolated and one should also carefully label the layers that they have separated. In general, you will have to "dry" the organic layer by placing it over magnesium sulfate or sodium sulfate.

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