By : James W Zubrick
Email: j.zubrick@hvcc.edu
The Road to Recovery—Back-Extraction
I’ve mentioned recovery of the four types of extractable materials, but that’s not all the work you’ll have to do to get the compounds in shape for further use.
1. If the recovered material is soluble in the aqueous recovery solution, you’ll have to do a back-extraction.
a. Find a solvent that dissolves your compound, and is not miscible in the aqueous recovery solution. This solvent should boil at a low temperature (<100°C), since you will have to remove it. Ethyl ether is a common choice. (Hazard! Very flammable).
b. Then you extract your compound BACK FROM THE AQUEOUS RECOVERY SOLUTION into this organic solvent.
c. Dry this organic solution with a drying agent (see Chapter 7, “Drying Agents”).
d. Now you can remove the organic solvent. Either distill the mixture or evaporate it, perhaps on a steam bath. All this is done away from flames and in a hood.
When you’re through removing the solvent and you’re product is not pure, clean it up. If your product is a liquid, you might distill it; if a solid, you might recrystallize it. Make sure it is clean.
2. If the recovered material is insoluble in the aqueous recovery solution, and it is a solid, collect the crystals on a Buchner funnel. If they are not pure, you should recrystallize them.
3. If the recovered material is insoluble in the aqueous recovery solution and it is a liquid, you can use your separatory funnel to separate the aqueous recovery solution from your liquid product. Then dry your liquid product and distill it if it is not clean. Or, you might just do a back-extraction as just described. This has the added advantage of getting out the small amount of liquid product that dissolves in the aqueous recovery solution and increases your yield. Remember to dry the back-extracted solution before you remove the organic solvent. Then distill your liquid compound if it is not clean.
A SAMPLE EXTRACTION
I think the only way I can bring this out is to use a typical example. This may ruin a few lab quizzes, but if it helps, it helps.
Say you have to separate a mixture of benzoic acid (1), phenol (2), p-tolui-dine (4-Methylanaline) (3), and anisole (methoxybenzene) (4) by extraction. The numbers refer to the class of compound, as previously listed. We’re assuming that none of the compounds react with any of the others and that you know we’re using all four types as indicated. Phenol and 4-methylanaline are corrosive toxic poisons and if you get near these compounds in lab, be very careful. When they are used as an example on these pages, however, you are quite safe. Here’s a sequence of tactics.
1. Dissolve the mixture in ether. Ether is insoluble in the water solutions you will extract into. Ether happens to dissolve all four compounds. Aren’t you lucky? You bet! It takes lots of hard work to come up with the “typical student example.”
2. Extract the ether solution with 10% HC1. This converts only compound 3, the basic p-toluidine, into the hydrochloride salt, which dissolves in the 10% HC1 layer. You have just extracted a base with an acid solution. Save this solution for later.
3. Now extract the ether solution with sat’d sodium bicarbonate solution.
Careful! Boy will this fizz! Remember to swirl the contents and release the pressure. The weak base converts only compound 1, the benzoic acid, to a salt, which dissolves in the sat’d bicarbonate solution. Save this for later.
4. Now extract the ether solution with the 10% NaOH solution. This converts the compound 2, weak acid, phenol, to a salt, which dissolves in the 10% NaOH layer. Save this for later. If you do this step before step 3, that is, extract with 10% NaOH solution before the sodium bicarbonate solution, both the weak acid, phenol, and the strong acid, benzoic acid, will be pulled out into the sodium hydroxide. Ha-Ha. This is the usual kicker they put in lab quizzes, and people always forget it.
5. The only thing left is the neutral organic compound dissolve in the ether. Just drain this into a flask.
So, now we have four flasks with four solutions with one component in each. They are separated. You may ask, “How do we get these back?”
1. The basic compound (3). Add ammonium hydroxide until the solution turns basic (test with litmus or pH paper). The p-toluidine, or organic base (3), is regenerated.
2. The strong acid or the weak acid (1,2). A bonus. Add dilute HC1 until the solution turns acidic to an indicator paper. Do it to the other solution. Both acids will be regenerated.
3. The neutral compound (4). It’s in the ether. If you evaporate the ether (No flames!), the compound should come back.
Now, when you recover these compounds, sometimes they don’t come back in such good shape. You will have to do more work.
1. Addition of HC1 to the benzoic acid extract will produce huge amounts of white crystals. Get out the Buchner funnel and have a field day! Collect all you want. But they won’t be in the best of shape. Recrystallize them. (Note: This compound is insoluble in the aqueous recovery solution.)
2. The phenol extract is a different matter. You see, phenol is soluble in water, and it doesn’t come back well at all. So, get some fresh ether, extract the phenol from HC1 solution to the ether and evaporate the ether. Sounds crazy, no? No. Remember, I called this a back-extraction and you’ll have to do this more often than you would like to believe. (Note: This compound is soluble in the aqueous recovery solution.)
3. The p-toluidine should return after the addition of ammonium hydroxide. Recyrstallize it from ethanol so it also looks respectable again.
4. The neutral anisole happens to be a liquid (B.P. 155°C), and you’ll have to take care when you evaporate the ether so as not to lose much anisole. Of course, you shouldn’t expect to see any crystals. Now this neutral anisole liquid that comes back after you’ve evaporated the ether (no flames!) will probably be contaminated with a little bit of all of the other compounds that started out in the ether. You will have to purify this liquid, probably by a simple distillation.
You may or may not have to do all of this with the other solutions, or with any other solution you ever extract in your life. You must choose. Art over science. As confusing as this is, I have simplified it a lot. Usually you have to extract these solutions more than once, and the separation is not as clean as you’d like. Not 100%, but pretty good. If you are still confused, see your instructor.
PERFORMING AN EXTRACTION OR WASHING
1. Suspend a sep funnel in an iron ring.
2. Remove the stopper.
3. Make sure the stopcock is closed! You don’t really want to scrape your product off the benchtop.
4. Add the solution to be extracted or washed. Less than half full, please. Add the extraction or washing solvent. An equal volume is usually enough. The funnel is funnel shaped and the equal volumes won’t look equal.
5. Replace the stopper.
6. Remove the sep funnel from the iron ring. Hold stopper and stopcock tightly. Pressure may build up during the next step and blow your product out onto the floor.
7. Invert the sep funnel (Fig. 56).
Point the stem up away from everyone— up into the back of a hood if at all possible!
Make sure the liquid has drained down away from the stopcock, then slowly open the stopcock. You may hear a woosh, possibly a pffffft, as the pressure is released. This is due to the high vapor pressure of some solvents, or to a gas evolved from a reaction during the mixing. This can cause big trouble when you are told to neutralize acid, by washing with sodium carbonate or sodium bicarbonate solutions.
8. CLOSE THE STOPCOCK!
9. Shake the funnel gently, invert it, open the stopcock again.
Fig. 56 Holding a sep funnel so as not to get stuff all over.
10. Repeat the steps 8 and 9 until no more gas escapes.
11. If you see that you might get an emulsion — a fog of particles—with this gentle inversion, do NOT shake the funnel vigorously. You might have to continue the rocking and inverting motions 30 to 100 times, as needed, to get a separation. Check with your instructor and with the hints on breaking up emulsions (see “Extraction Hints,” following). Otherwise, shake the funnel vigorously about 10 times, to get good distribution of the solvents and solutes. Really shake it.
12. Put the sep funnel back in the iron ring.
13. Remove the glass stopper. Otherwise the funnel won’t drain and you’ll waste your time just standing there.
14. Open the stopcock and let the bottom layer drain off into a flask.
15. Close the stopcock, swirl the funnel gently, then wait to see if any more of the bottom layer forms. If so, collect it. If not, assume you got it all in the flask.
16. Let the remaining layer out into another flask.
To extract any layer again, return that layer to the sep funnel, add fresh extraction or washing solvent, and repeat this procedure starting from step 5.
Never, never, never, never, ever throw away any layer, until you are absolutely sure you’ll never need it again. Not very much of your product can be recovered from the sink trapl
EXTRACTION HINTS
1. Several smaller washings or extractions are better than one big one.
2. Extracting or washing a layer twice, perhaps thrice, is usually enough. Diminishing returns set in after that.
3. Sometimes you’ll have to find out which layer is the water layer. This is so simple, it confounds everyone. Add 2-4 drops of each layer to a test tube containing 1 ml of water. Shake the tube. If the stuff doesn’t dissolve in the water, it’s not an aqueous (water) layer. The stuff may sink to the bottom, float on the top, do both, or even turn the water cloudy! It will not, however, dissolve.
4. If only the top layer is being extracted or washed, it does not have to be removed from the funnel, ever. Just drain off the bottom layer, then add more fresh extraction or washing solvent. Ask your instructor about this.
5. You can combine the extracts of a multiple extraction, if they have the same material in them.
6. If you have to wash your organic compound with water, and the organic is slightly soluble in water, try washing with saturated salt solution. The theory is that if all that salt dissolved in the water, what room is there for your organic product? This point is a favorite of quizmakers, and should be remembered. It’s the same thing that happens when you add salt to reduce the solubility of your compound during a crystallization (see “Salting-Out”).
7. If you get an emulsion, you have not two distinct layers, but a kind of a fog of particles. Sometimes you can break up the charge on the suspended droplets by adding a little salt, or some acid or base. Or add ethanol. Or add salt. Or stir the solutions slowly with a glass rod. Or gravity filter the entire contents of your separatory funnel through filter paper. Or laugh. Or cry. Emulsion-breaking is a bit of an art. Careful with the acids and bases though. They can react with your product and destroy it.
8. If you decide to add salt to a sep funnel, don’t add so much that it clogs up the stopcock! For the same reason, keep drying agents out of sep funnels.
9. Sometimes some material comes out, or will not dissolve in the two liquid layers, and hangs in there in the interface. It may be that there’s not enough liquid to dissolve this material. One cure is to add more fresh solvent of one layer or the other. The solid may dissolve. If there’s no room to add more, you may have to remove both (yes, both) layers from the funnel, and try to dissolve this solid in either of the solvents. It can be confusing. If the material does not redissolve, then it is a new compound and should be saved for analysis. You should see your instructor for that one.
