By : James W Zubrick
Email: j.zubrick@hvcc.edu
Extraction is one of the more complex operations you’ll do in the organic chemistry lab. For this reason, I’ll go over it especially slowly and carefully. Another term you’ll see used simultaneously with extraction is washing. That’s because extraction and washing are really the same operation, but each leads to a different end. How else to put this?
Let’s make some soup. Put the vegetables, fresh from the store, in a pot. You run cold water in and over them to clean them and throw this water down the drain. Later, you run water in and over them to cook them. You keep this water—it’s the soup.
Both operations are similar. Vegetables in a pot in contact with water the first time is a wash. You remove unwanted dirt. You washed with water. The second time, vegetables in a pot in contact with water is an extraction. You’ve extracted essences of the vegetables into water. Very similar operations; very different ends.
To put it a little differently, You would extract good material from an impure matrix. You would wash the impurities from good material.
The vegetable soup preparation is a solid-liquid extraction. So is coffee making. You extract some component(s) of a solid directly into the solvent. You might do a solid-liquid extraction in lab as a separate experiment; liquid-liquid extractions are routine. They are so common that if you are told to do an extraction or a washing, it is assumed, you will use two liquids — two INSOLUBLE liquids—and a separatory funnel. The separatory funnel, called a sep funnel by those in the know, is a special funnel that you can separate liquids in. You might look at the section on separatory funnels (later in this chapter) right now, then come back later.
Two insoluble liquids in a separatory funnel will form layers; one liquid will float on top of the other. You usually have compounds dissolved in these layers, and either the compound you want is extracted from one to the other, or junk you don’t want is washed from one layer to the other.
Making the soup, you have no difficulty deciding what to keep or what to throw away. First you throw the water away; later you keep it. But this can change. In a sep funnel, the layer you want to keep one time may not be the layer you want to keep the next time. Yet, if you throw one layer away prematurely, you are doomed.
NEVER-EVER LAND
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 trap!
I’m using a word processor, so I can copy this warning over and over again, but let’s not get carried away. One more time, WAKE UP OUT THERE!
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 trap!
STARTING AN EXTRACTION
To do any extraction, you’ll need two liquids, or solutions. They must be insoluble in each other. Insoluble here has a practical definition:
When mixed together, the two liquids form two layers.
One liquid will float on top of the other. A good example is ether and water. Handbooks say that ether is slightly soluble in water. When ether and water are mixed, yes, some of the ether dissolves; most of the ether just floats on top of the water.
Really soluble or miscible liquid pairs are no good for extraction and washing. When you mix them, they will not form two layers! In fact, they’ll mix in all proportions. A good example of this is acetone and water. What kinds of problems can this cause? Well, for one, you cannot perform any extraction with two liquids that are miscible.
Let’s try it. A mixture of say, some mineral acid (is HC1 all right?) and an organic liquid, “Compound A,” needs to have that acid washed out of it. You dissolve the compound A-acid mixture in some acetone. It goes into the sep funnel, and you now add water to wash out the acid.
Acetone is miscible in water. No layers form! You lose!
Back to the lab bench. Empty the funnel. Start over. This time, having called yourself several colorful names because you should have read this section thoroughly in the first place, you dissolve the Compound A-acid mixture in ether and put it into the sep funnel. Add water, and two layers form! Now you can wash the acid from the organic layer to the water layer. The water layer can be thrown away.
Note that the acid went into the water, then the water was thrown out! So we call this a wash. If the water layer had been saved, we’d say the acid had been extracted into the water layer. It may not make sense, but that’s how it is. Review:
1. You must have two insoluble liquid layers to perform an extraction.
2. Solids must be dissolved in a solvent, and that solvent must be insoluble in the other extracting or washing liquid.
3. If you are washing or extracting an organic liquid, dissolve it into another liquid, just like a solid, before extracting or washing it.
So these terms, extraction and washing are related. Here are a few examples.
1. Extract with ether. Throw ether together with the solution of product and pull out only the product into the ether.
2. Wash with 10% NaOH. Throw 10%NaC)H together with the solution of product and pull out everything but product into the NaOH.
3. You can even extract with 10% NaOH.
4. You can even wash with ether.
So extraction is pulling out what you want from all else! Washing is pulling out all else from what you want.
And please note—you AL WA YS do the pulling from ONE LA YER INTO ANOTHER. That’s also two immiscible liquids.
You’ll have to actually do a few of these things before you get the hang of it, but bear with me. When your head stops hurting, reread this section.
DUTCH UNCLE ADVICE
Just before I go on to the separatory funnel, I’d like to comment on a few questions I keep hearing when people do washings and extractions.
1. “Which layer is the water layer ? “ Look at both layers in the funnel and get an idea of how big they are in relation to one another. Now add water to the funnel. Watch where the water goes. Watch which layer grows. Water to water. That’s how you find the water (aqueous) layer. Don’t rely on odor or color. Enough ether dissolves in water to give the water layer the odor of an ether layer; just enough of a highly colored substance in the wrong layer can mislead you.
2. “How come I got three layers?” Sometimes, when you pour fresh water or some other solvent into the funnel, you get a small amount hanging at the top, and it looks like there are three different layers. Yes, it looks as if there are three different layers, but there are not three different layers. Only two layers, where part of one has lost its way. Usually, this mysterious third layer looks just like its parent, and you might gently swirl the funnel and its contents to reunite the family.
3. “What’s the density of sodium hydroxide?” You’ve just done a wash with 5 -10% sodium hydroxide solution, you’ve just read something about finding various layers in the funnel by their densities, and, by this question, you’ve just shown that you’ve missed the point. Most wash solutions are 5 to 10% active ingredient dissolved in water. This means they are 90 to 95% water. Looking up the density of the solid reagents then, is a waste of time. The density of these solutions is very close to that of water. (10% NaOH has a specific gravity of 1.1089.)
4. “I’ve washed this organic compound six times with sodium bicarbonate solution so why’s it not basic yet?” This involves finding the pH of the organic layer. I’ll give it away right now. You cannot find the pH of an organic layer. Not directly. You find the pH of the aqueous layer that’s been in contact with the organic layer. If the aqueous layer is on the top, dip a glass rod into it and touch the glass rod to your test paper. If the aqueous layer is on the bottom and your sep funnel in a ring, let a drop of the aqueous layer out of the funnel to hang on the outlet tip. Transfer the drop to your test paper. Warning. Be sure you are testing the aqueous layer. Some organics are very tenacious and can get onto your glass rod. The organic layer may WET the test paper, but without water any color you see doesn’t mean much.
THE SEPARATORY FUNNEL
Before going on to some practical examples, you might want to know more about where all this washing and extracting is carried out. I’ve mentioned that it’s a special funnel called a separatory funnel (Fig. 53) and that you can impress your friends by calling it a sep funnel. Here are a few things you should know.
The Stopper
At the top of the sep funnel is a T glass stopper. There is a number, commonly T 22, possibly T 19/22, on the stopper head. Make sure this number is on the head and that it is the same as the number marked on the funnel. If this stopper is not so marked, you may find the product leaking over your shoes when you turn the sep funnel upside down. Try not to grease this stopper unless you plan to saute your product. Unfortunately, these stoppers tend to get stuck in the funnel. The way out is to be sure you don’t get the ground glass surfaces wet with product. How? Pour solutions into the sep funnel as carefully as you might empty a shotglass of Scotch into the soda. Maybe use a funnel. To confuse matters, I’ll suggest you use a light coating of grease. Unfortunately, my idea of light and your idea of light may be different.
Consult your instructor!
This is the time-honored favorite of separatory funnel makers everywhere. There is a notch at the small end that contains either a rubber ring or a metal clip, but not both! There are two purposes for the ring.
Fig. S3 Garden variety separatory funnels.
1. To keep the stopcock from falling out entirely. Unfortunately, the rubber rings are not aware of this and the stopcock falls out anyway.
2. To provide a sideways pressure, pulling the stopcock in, so that it will not leak. Names and addresses of individuals whose stopcocks could not possibly leak and did so anyway will be provided on request. So provide a little sideways pressure of your own.
When you grease a glass stopcock (and you must), do it very carefully so that the film of grease does not spread into the area of the stopper that comes in contact with any of your compound (Fig. 54).
In wide use today, the Teflon stopcock (Fig. 55) requires no grease and will not freeze up! The glass surrounding the stopcock is not ground glass and cannot be used in funnels that require ground glass stopcocks! The Teflon stopcocks are infinitely easier to take care of. There is a Teflon washer, a rubber ring, and, finally, a Teflon nut, placed on the threads of the stopcock. This nut holds the whole thing on. Any leakage at this stopcock results from
1. A loose Teflon nut. Tighten it.
2. A missing Teflon washer or rubber ring. Have it replaced.
Fig. 54 The infamous glass stopcock.
Fig. 55 Extreme close up of teflon stopcock popping ritual.
3. An attempt to place the wrong size or taper Teflon stopcock into the funnel. This is extremely rare. Get a new funnel.
Emergency stopcock warning!
Teflon may not stick, but it sure can flow! If the stopcock is extremely tight, the Teflon will bond itself to all the nooks and crannies in the glass in interesting ways. When you’re through, always loosen the Teflon nut and “pop the stopcock” by pulling on the handle. The stopcock should be loose enough to spin freely when spun with one finger—then remember to tighten it again before you use it.
It seems to me that I’m the only one that reads the little plastic bags that hold the stopcock parts. Right on the bags it shows that after the stopcock goes in, the Teflon washer goes on the stem first, followed by the rubber ring, and then the Teflon nut (Fig. 55). So why do I find most of these things put together incorrectly?
THE STEM
The stem on a sep funnel can either be straight or have a ground glass joint on the end (Fig. 53). The ground glass joint fits the other jointware you may have and can be used that way as an addition funnel to add liquids or solutions into a setup (see “Addition and Reflux”). You can use this type of separatory funnel as a sep funnel. You can’t, however, use the straight-stem separatory funnel as an addition funnel without some help; remember, straight glass tubes don’t fit ground glass joints (see “The Adapter With Lots of Names”).
WASHING AND EXTRACTING VARIOUS THINGS
Now, getting back to extractions, there are really only four classes of compounds that are commonly handled in undergraduate extractions or washings.
1. Strong Acids. The mineral acids, and organic acids (e.g., benzoic acid). You usually extract these into sodium bicarbonate solution or wash them with it.
2. Really weak acids. Usually phenols, or substituted phenols. Here, you’d use a sodium hydroxide solution for washing or extraction. You need a strong base to work with these weak acids.
3. Organic bases. Any organic amine (aniline, triethylamine, etc.). As you use bases to work with acids, use a dilute acid (5 to 10% HC1, say) to extract or wash these bases.
4. Neutral compounds. All else, by these definitions (e.g., amides, ethers, alcohols, hydrocarbons).
HOW TO EXTRACT AND WASH WHAT
Here are some practical examples of washings and extractions, covering various types and mixtures and separations and broken down into the four classifications listed above.
1. A strong organic acid. Extract into sat’d (saturated) sodium bicarbonate solution.
CAUTION! Foaming and fizzing and spitting and all sorts of carrying on.) The weak base turns the strong acid into a salt, and the salt dissolves in the water-bicarbonate solution. Because of all the fizzing, you’ll have to be very careful. Pressure can build up and blow the stopper out of the funnel. Invert the funnel. Point the stem A WA Y FROM EVERYONE up and toward the BACK OF THE HOOD—and open the stopcock to vent or “burp” the funnel.
a. To recover the acid, add conc. (concentrated) HC1 until the solution is acidic. Use pH or litmus paper to make sure. Yes, the solution really fizzes and bubbles. You should use a large beaker so material isn’t thrown onto the floor if there’s too much foam.
b. To wash out the strong acid, just throw the solution of bicarbonate away.
2. A weakly acidic organic acid. Extract into 10% NaOH-water solution. The strong base is needed to rip the protons out of weak acids (they don’t want to give them up) and turn them into salts; Then they’ll go into the NaOH-water layer.
a. To recover the acid, add conc. HC1 until the solution of base is acid when tested with pH or litmus paper.
b. To wash out the weak acid, just throw this NaOH-water solution away.
3. An organic base. Extract with 10% HC1-water solution. The strong acid turns the base into a salt (This turning the whatever into a salt that dissolves in the water solution should be pretty familiar to you by now. Think about it.). Then the salt goes into the water layer.
a. To recover the base, add ammonium hydroxide to the water solution until the solution is basic to pH or litmus paper. Note that this is the reverse of the treatment given to organic acids.
b. To wash out an organic base, or any base, wash as above and throw out the solution.
4. A neutral organic. If you’ve extracted strong acids first, then weak acids, then bases, there are only neutral compound(s) left. If possible, just remove the solvent that now contains only your neutral compound. If you have more than one neutral compound, you may want to extract one from the other(s). You’ll have to find two different immiscible organic liquids, and one liquid must dissolve ONLY the neutral organic compound you want! A tall order. You must count on one neutral organic compound being more soluble in one layer than in the other. Usually the separation is not clean—not complete. And you have to do more work.
What’s “more work”? That depends on the results of your extraction.
