In the
previous tutorial, we learned about filtration, a technique that allows us to separate solid
and liquid components of a mixture.
But what if the components we wish to separate are both in solution?
There is nothing about particle size that will allow us to do so.
Fortunately there are separation techniques in chemistry that take advantage of factors
other than particle size in order to separate components.
These focus on properties such as solubility, polarity, and reactivity.
The technique we will go over in this tutorial is called extraction.
Extraction is a technique that uses a two-phase system to separate mixtures.
The most common form is a liquid-liquid extraction, and in this context the two-phase system will
be an organic phase and an aqueous phase.
So in performing this technique, we will separate a desired compound from a mixture by moving
it from one solvent to another.
To perform this technique, we will be taking the solution and adding a new solvent that
dissolves and isolates one compound from the mixture.
We must use a solvent that is immiscible with the first solvent, so that separate layers
will form, as this makes it quite easy to separate them.
To do this, we’ll need a separatory funnel.
This is a tool that is used to separate two immiscible liquids of different densities.
Once again, in most cases, the two liquids will be an organic solvent, plus whatever
compounds are dissolved in it, and then water, plus whatever compounds are dissolved in it.
It is usually the case that the aqueous layer has a greater density than the organic layer,
so it goes to the bottom while the organic solvent stays on top, as gravity will pull
the denser layer down with greater force.
However, some organic solvents are more dense than water and will thus occupy the bottom
layer, so it is important to consult a table of densities to be sure as to which layer
is which.
When in doubt, if you want to identify which layer is which, add a bit of water.
Whichever layer grows, that’s the aqueous layer.
Now let’s get more familiar with the intricacies of the separatory funnel.
As we can see here, the funnel has a stopper at the top and a stopcock at the bottom.
This middle region is where our separation will take place.
To get this ready, you might have to add a little bit of grease to the stopper and/or
stopcock since they tend to get stuck.
To set up for the extraction, we’ll start with a ring stand attached to a support like
this.
We’ll then place our separatory funnel in the ring and make sure everything is secure.
To verify that the stopcock works, add some water.
Make sure there are no leaks when the valve is closed.
Open the valve to see how the water flows.
Once satisfied, remove the water.
We are now ready to get started.
So let’s say this is a reaction mixture that we just got from performing a chemical
reaction in some organic solvent.
Most of the time you’ll need to quench a reaction once it’s finished by adding water.
This means to stop a reaction mixture from reacting further, often with dilute weak acid
or base to neutralize any ionic species.
This means that after any chemical reaction, you will need to perform an extraction to
separate your organic compounds from whatever is in the aqueous layer.
In this particular case, rather than a reaction mixture, we simply have a mixture containing
benzoic acid and 2-methoxynaphthalene dissolved in diethyl ether, and our goal is to obtain
each of these two compounds in a pure form.
These are both soluble in diethyl ether but insoluble in water.
However, if we add aqueous base to this mixture, benzoic acid will deprotonate, becoming the
benzoate anion, which has a formal negative charge and will thus become water soluble,
and move into the aqueous phase.
Since 2-methoxynaphthalene will not react, that will remain in the organic phase, and
the two compounds will have separated.
With our strategy understood, we can begin performing the technique.
Making sure that the stopcock is closed, let’s transfer our mixture to the separatory funnel
using a conical glass funnel.
This is our organic layer.
To make sure we have transferred everything, rinse the beaker that held the mixture with
ether and transfer that as well.
Had this been the product of a chemical reaction it would already have been quenched and thus
have formed organic and aqueous layers, but since we are just doing an isolated extraction,
let’s go ahead and perform the necessary acid-base reaction right in the separatory
funnel.
To separate benzoic acid from the mixture, we will add 30 milliliters of 1 molar sodium
hydroxide to the separatory funnel.
This will pass through the organic layer, react with benzoic acid on the way to form
sodium benzoate, and drag that down to the newly forming aqueous layer on the bottom.
To make sure the reaction goes to completion, we need to mix the contents of the funnel.
We will do this by inverting it and lightly swirling a few times, holding the stopper
down with one finger as we do so to ensure that nothing spills out.
Mix for about 10 seconds each time.
After each round of mixing, hold the funnel upside down and open the stopcock like this,
to vent any gases that accumulate from the reaction.
Close it, and then continue mixing.
When gases are vented, you will hear a brief hissing sound.
When the sound no longer occurs, no more gas is being produced, and we can be confident
that mixing is complete.
After mixing, it is time to separate.
Return your separatory funnel to the ring support and allow the contents to settle.
You should see the two layers separate clearly.
You can now open the stopcock to let the bottom layer drop into a clean flask.
This is where you have to be careful.
Watch the aqueous layer drip slowly so that you can stop it right before any of the top
layer goes through.
When the line separating the layers reaches the stopcock, close it.
You have now separated the two layers.
The top layer can be removed by pouring it through the top of the funnel into a separate
flask.
Be sure to pour it out the top and not through the stopcock, as pouring out the bottom would
contaminate it.
With most extractions, we will need to do more than one separation so as to maximize
your yield, as not all of the benzoic acid made it out of the organic layer.
So let’s put this first aqueous extract to the side, return the organic layer to the
separatory funnel, and make sure everything is secured.
We can then add more aqueous base to the organic layer and mix just as we did before.
Allow the layers to separate, and we can now collect the bottom layer again, adding it
to the original extract.
So now the extraction itself is complete.
We have some aqueous sodium benzoate, and then we have 2-methoxynapthalene still in
the organic solvent.
Let’s isolate this first.
The organic layer will likely contain residual water from the multiple washes that we performed.
How do we get it out?
We use a drying agent.
A drying agent is a compound like sodium sulfate that removes water when added to a solution.
The water should bind to the drying agent and thus join the solid phase.
Keep adding the drying agent until you see a clear solution with all of the solid clumped
at the bottom.
You will know its dry when lightly shaking the flask does not cause any solid to move
around the solution.
To separate the drying agent from the liquid, we’ll need to do a gravity filtration.
Since we discussed this at great length in the previous tutorial, we won’t go over
the details here, but set up your filtration, and pour your dried organic layer in.
The drying agent will remain on the filter paper and the target molecule will move through
with the solvent.
Make sure to rinse the flask with more solvent so that everything is transferred.
Then just evaporate the solvent on a hot plate, as ether is rather volatile, and we will be
left with a residue of pure 2-methoxynaphthalene.
The aqueous layer will be a bit trickier, as we wanted to isolate benzoic acid, but
here we have sodium benzoate.
To get benzoic acid we will need to do another acid-base reaction to protonate the benzoate.
Let’s add about five milliliters of 6 molar hydrochloric acid.
You may need to add a little more than this, just monitor the solution with litmus paper
and add it until the solution starts to turn acidic.
A pH of around 2 means that the acid is no longer neutralizing base but is turning the
solution acidic, which means the acid is in excess and the reaction is complete.
As benzoic acid forms, it will begin to precipitate.
If this is happening very slowly, the flask can be placed in an ice bath or cold water
bath to encourage solid formation.
Once precipitation of benzoic acid is complete, it can be collected by vacuum filtration.
Again, we have described this technique in the previous tutorial, so we will not go into
detail here, but collect your solid on the Buchner funnel and allow it to dry on the
bench for some time, then moving it to an oven if further drying is necessary.
And with that, we have completed a liquid-liquid extraction.
We had two organic molecules mixed together in an organic solvent, and we separated them
based on their chemical properties, namely their propensity to undergo an acid-base reaction.
This is an important tool that is used constantly in organic chemistry.
It is also widely used in industry for the extraction of natural products, ranging from
caffeine extraction from leaves to metal extraction from minerals.
With this now understood, let’s move forward and check out some other techniques.