Two Component Mixture Separation by Extraction

Preparation: Mayo, et al., 113-118 and 122-126, and McMurry 20.3, 25.6, 24.4. From a first semester experiment (CHEM 3012) you have learned that simple mixtures of organic compounds that are normally soluble in organic solvents (like ether) but insoluble in water can be separated by extraction using reagents such as HCl and NaOH. These reagents selectively react with organic (carboxylic) acid and organic base (amine) components in the mixture to form ions that are soluble in water. Examples of such reactions are:

Neutral compounds (organic compounds that are neither acids nor bases) remain in the ether solution. The amine salt or carboxylate ion in the water can be then physically separated from the neutral organic compounds by simply separating the water phase form the ether phase. Once it is clear that the separation is complete, the water soluble ions can be returned to their original form (carboxylic acid or amine).

The first step in the identification of each component of the two component mixture is to determine whether it is an acid+neutral or a base+neutral mixture. This will be determined by the test described below. Using the results of the test, a full separation on several grams of the mixture will first be designed and then carried out.

Following this separation you will then follow the procedure found in "Identification of Acids, Bases and Neutral Compounds".  These procedures are for the identification of each component of the mixture using Infrared, CMR and   PMR data, physical property tests, and chemical tests.  It is critical that your separations be clean and enough material recovered to adequately test and identify the unknowns.

Procedure: To make a preliminary examination of your mixture, add 2-3 drops of the mixture to each of three 15x125 mm test tubes containing (#1) 2 mL of 3 M HCl, (#2) 2 mL of 2.5 M (10%) NaOH and (#3) 2 mL of H₂O. Mix vigorously to facilitate reaction. Compare each test tube with the test tube that contains water only. Remember that the solvent used to prepare the acid (HCl) and base (NaOH) is already water.  It is not unexpected that some unknowns will be insoluble in ether (as you receive them from the stockroom).  Therefore, you may need to test both the solids and the solution components separately to "see" the results adequately.  Clearly record your results by sketching the three test tubes in the data/observations section of your notebook before and after mixing the contents of each.

Did part of the mixture react with or dissolve in the acid?

Did part of the mixture react with or dissolve in the base?

Should you expect any reaction or dissolving of any part of the mixture with pure water?

Do you have an acid+neutral or a base+neutral mixture?

Examine the equations above when you interpret your observations.  Remember that since your unknowns will have varying degrees of solubility in ether and water, you will need to adapt the procedure to the properties of your individual unknown.  Thus, it is imperative that you understand the chemistry taking place and not just rely on the procedure as a recipe for success.

Mixture Separation:  Prepare a schematic of the mixture separation that you will carry out to separate your mixture. Use the scheme provided in the previous experiment for the separation of benzoic acid, 4-aminobenzoate and 9-fluoreneone as a guide. Since this guide is for the separation of a 3-component mixture, you need only adapt part of this scheme in your case. Sketch simple drawings of your extraction steps and indicate the ion or compound you expect to find in each layer of the extraction for each step. Finally indicate the amounts of solvents and other reagents you expect to use for each step. Review this plan with the instructor before continuing.

For your separation, start with about 5g (~ 5 mL) of your unknown; if some of the unknown is not mixed, use both parts of it since you need to complete the separation with about equal amounts of both components. Dissolve this amount in 50 mL of ethyl ether, CH₃CH₂-O-CH₂CH₃ which will serve as the organic solvent and be called "the organic phase" (or "the organic layer").  Add more ether if necessary to dissolve all the unknown; if the final volume exceeds 75 mL use a 1 liter separatory funnel in the next step.

The organic layer is added to a separatory funnel (set up on a ring attached to a stand - check the valve first) and then add 15 mL of 3 M aqueous HCl or NaOH depending on your plan. Does the aqueous solution ("aqueous phase") you added float above or sink below the ether layer? Cap the funnel and shake the mixture well (for at least 5 sec) to allow the reaction to take place; invert the funnel and release pressure built up through the valve. Repeat the shaking for another 10 sec, release pressure again, and then set the funnel back on the ring. Remove the cap, and allow the layers to settle until they are well separated.
(Remember to make a drawing of the separatory funnel, clearly labeling each phase and the species dissolved in each layer.)

Transfer the aqueous phase to a 100 mL beaker labeled "carboxylate ion" or "amine salt" depending on your plan. Keep this beaker cooled in ice water. Because the initial reaction may not have been complete, another 15 mL of HCl or NaOH is added to the ether phase in the separatory funnel and the process is repeated; the aqueous phase is added to the "carboxylate ion" or "amine salt" beaker. The extraction process is repeated once more so that the "carboxylate ion" or "amine salt" beaker now contains about 45 mL of aqueous phase. The ether solution is now transferred from the separatory funnel into a flask labeled "neutral".

Ether dissolves small amounts of water.  Since the neutral component must be later tested without water, the ether phase must be "dried" - that is this small amount of water must be removed before the ether is evaporated. To the "neutral" flask add about half a spatula full of anhydrous Na₂SO₄ and swirl the mixture; if after 5 min. at least some of the solid Na₂SO₄ remains snowy after swirling, enough of it has been added. The Na₂SO₄ solids are removed by gravity filtration. Use a paper cone that fits up to the lip of a conical funnel (no suction is applied).  The ether solution is filtered into a pre-weighed (to the nearest 0.1g) flask The ether needs to be removed by evaporation before the end of the period; add a boiling splint and set the flask on the steambath. Be careful not to boil away your neutral compound!  Remove the splint when most of the ether is gone. The flask can be removed from the heat from time to time and tipped to determine if ether vapors are still pouring out. Once all the ether is removed, the flask is then re-weighed, stoppered, labeled and stored for future testing.

The "carboxylate ion" or "amine salt" beaker (still cold) will yield the other component of the mixture. First it must be neutralized with 6 M HCl or NaOH according to your plan. Add this to the beaker until the solution has made a definite pH shift to above 10 or below 4, depending on your plan. Note changes that occur as this pH is shifted. Write the reaction that has taken place as a result of the addition of HCl or NaOH.

(In order to retrieve the carboxylic acid or amine pure, without water, it is necessary to re-extract, that is to dissolve the carboxylic acid or amine in ether, separate the organic phase, dry and filter it and evaporate the ether in the same way as the neutral compound.) Therefore return the freshly neutralized aqueous solution to the separatory funnel, and rinse the beaker with at least 15 mL of ether and add it to the separatory funnel. Cap and shake the contents briefly, release the pressure as before, then shake again, release pressure, remove the top, allow the layers to settle. (Remember to draw the separatory funnel; label the phases and the species dissolved in each.)

Drain the aqueous layer back into the "carboxylate ion" or "amine salt" beaker, and transfer the ether phase into a clean flask labeled "carboxylic acid" or "amine". As before, re-extract the aqueous phase by returning it to the separation funnel. Rinse the beaker with another 15 ml of fresh ether and add it to the separation funnel, extract, and drain the aqueous layer back into the beaker. Combine the ether layer remaining in the separatory funnel with the other "carboxylic acid" or "amine". Extract a third time so that the final volume of "carboxylic acid" or "amine" now totals about 45 mL. As before, add Na₂SO₄, swirl, filter into a pre-weighed flask, and evaporate the ether. The re-weighed "carboxylic acid" or "amine" flask can now be labeled, stoppered and stored, or if time permits, you can begin the Identification of Acids, Bases and Neutral Compounds by running preliminary tests (melting/boiling point, infrared, NMR, etc.)

Be prepared to repeat this later if you run out of either component in future tests. You can scale up the extraction if necessary to obtain even larger amounts of separated components.

Conclusions: Make a rough accounting of the weights involved. Assuming you started with 5g, and the mixture is about 1:1 by weight, what percent of each component was actually isolated?  Comment on your ability to separate your mixture into pure components.

References:

"Mayo et al.": Mayo, D.W., Pike, R.M., Butcher, S.S. and Trumper, P.K. Microscale Techniques for the Organic Laboratory; Wiley: New York, 1991

"McMurry": McMurry, John, Organic Chemistry, 4th ed., Brooks/Cole Publishing Co.: Pacific Grove, CA, 1995.

Revised by SLP on 1/5/99.