So, I am going to separate a mixture that contains an organic acid, an organic base,
and a neutral compound using acid-base extraction.
Specifically, I will dissolve a mixture that contains benzoic acid, p-bromoaniline, and
phenanthrene in dichloromethane, and then throw it into a separatory funnel.
Then I will add a solution of hydrochloric acid, which will convert the p-bromoaniline
into it's hydrochloride salt, which can then be removed in the aqueous layer.
Once that's removed, I will throw in a solution of sodium hydroxide, which will convert the
benzoic acid into a sodium salt, which can then also be removed in the aqueous layer.
Once the three compounds are separated, I will convert them all back to their neutral
solids.
Benzoic acid is corrosive.
Phenanthrene is an irritant.
p-Bromoaniline is an irritant.
Dichloromethane is toxic.
Here I have a one to one to one by mass mixture of benzoic acid, phenanthrene, and p-bromoaniline.
I going to dissolve the mixture in ten milliliters of DCM or dichloromethane.
I am going to add the solution to a separator funnel.
Next, I am going to add approximately five milliliters of three molar hydrochloric acid.
Two layers are noticeable.
I am going to cap the sep funnel and shake it to mix the two layers so the p-bromoaniline
that is in the DCM layer will come in contact with the HCl and cause protonation.
The p-bromoaniline HCl salt will then migrate to the aqueous layer.
I will vent it so the pressure does not build up too much.
Shake some more, vent again, shake one more time, and vent one more time.
To be safe, I am going to add a few more drops of DCM and watch where they go so I know which
layer is the DCM layer and which layer is the aqueous layer.
You can see the drops fall to the bottom.
This makes sense because DCM has a greater density than water and is usually the bottom
layer.
So I am going to label this bottom flask with an "N", which will be the neutral, organic,
dichloromethane layer.
And now I will just drain off the bottom layer being careful not to get any of the top layer.
I am going to label a second flask "A.E.", which will be the acidic extract or aqueous
layer containing the HCl and the p-bromoaniline salt.
I will drain the solution into the flask, close the sep funnel, and add back the neutral
DCM layer so I can extract it one more time to make sure I get all the p-bromoaniline
out of it.
I will add approximately another five milliliters of three molar HCl, cap the sep funnel, shake
it, vent it, shake it, vent it, drain off the bottom, neutral DCM layer, and then collect
the top aqueous acidic extract layer.
I will pour back the neutral layer that should no longer contain p-bromoaniline, but still
contains benzoic acid and phenanthrene.
To remove the benzoic acid, I will add approximately five milliliters of three molar sodium hydroxide.
I will shake the sep funnel to facilitate the deprotonation of benzoic acid by sodium
hydroxide, forming ionic sodium benzoate which will migrate to the aqueous layer.
And as before, just to be safe, I will add a few drops of DCM.
The DCM as expected is the bottom layer.
I will label a new flask "B.E."
for the base extracted aqueous layer.
First though, I will drain off the bottom, neutral DCM layer, and then collect the base
extracted aqueous layer.
I will close the sep funnel and pour back in the neutral DCM layer, then extract it
one more time with approximately five milliliters of the three molar sodium hydroxide solution.
You can sort of see two layers on the sides of the sep funnel, but not in the middle.
I'm going to add some more three molar sodium hydroxide to see if I can get the layers to
be more noticeable.
Now I will drain off the bottom, neutral DCM layer and then collect the second portion
of the base extracted aqueous layer.
So now all three compounds should be separated.
The acidic extract should have the p-bromoaniline hydrochloride salt.
The basic extract should have the sodium benzoate.
And the neutral DCM layer should still have the phenanthrene.
So now I need to turn them all back into the neutral solids they once were.
I'm going to add anhydrous sodium sulfate to the neutral DCM layer to absorb any water
that may be present.
You can tell enough has been added when you can swirl it around and its not all completely
clumped together.
I will set this aside for now.
Next, I will add six molar HCl solution to the basic extract, which will protonate the
sodium benzoate, converting it back to the benzoic acid.
I will monitor the pH of the solution to ensure the protonation occurs.
Right now the basic extract has a pH of about 10 and the six molar HCl has a pH of about
2.
I will know the protonation is complete when the basic extract also has a pH of 2.
I will grab some ice to cool the basic extract before the addition of the HCl solution since
some sodium hydroxide is also present and to minimize the solubility of the benzoic
acid.
For similar reasons, I will also start cooling the acid extract and some distilled water
to help during transfers.
I will add some of the HCl solution, and then take the flask out of the ice-water bath so
it can be seen easier.
Formation of benzoic acid as a white precipitate is noticeable.
After adding approximately five milliliters of the HCl solution, I will now test the pH,
which is the same as the HCl solution and confirmation that all of the sodium benzoate
has been converted to benzoic acid.
I will collect the solid using vacuum filtration.
I will wet the filter and attempt to transfer as much of the benzoic acid as possible.
I will then rinse the flask a few more times and then leave the solid to dry on the vacuum
for awhile.
When it's dry, I will collect it on a watch glass and record its mass.
Similar steps are used to convert the p-bromoaniline HCl salt back to the free base form.
In this case, six molar sodium hydroxide is used instead of the six molar HCl and when
addition of the sodium hydroxide solution is complete, the acidic extract should have
a pH of about 10.
After at least ten minutes of drying with sodium sulfate, the neutral DCM extract, which
should still contain phenanthrene, can be transferred to a beaker, leaving behind the
solid sodium sulfate and the excess water it absorbed.
The DCM may uncontrollably drip out of the pipe.
To prevent this from happening during transfer, it can be sucked up a few times to equilibrate
the inside of the pipet before attempting to transfer it.
To prevent any of the sodium sulfate from entering the pipet, positive pressure can
be applied to the pipet bulb until it reaches the very bottom of the flask.
To ensure I did not leave behind any of the phenanthrene, I will rinse the flask with
a few more milliliters of additional DCM and then transfer that as well.
Next, I will evaporate the DCM solution on a hotplate in a sand bath.
Instead of a boiling chip, I will add a wood applicator stick.
DCM has a low boiling point at forty degrees Celsius.
To prevent burning the solution when there is only a little bit of it left, I will make
sure it is no longer in direct contact with the sand.
I will also let the very last of it evaporate on its own at room temperature and increase
its surface area by rolling it around a few times in a beaker.
I can then scrape it from the beaker to a watch glass and when its completely dry, record
its mass.
I started with 520 milligrams of the mixture.
I ended up with 100 milligrams of p-bromoaniline, 107 milligrams of benzoic acid, and 145 milligrams
of phenanthrene.
So the percent recovery was 68 percent.
This value makes sense because I lost some of it during transfers.
The highest recovery for phenanthrene may be due to its greater tendency to always stay
within the dichloromethane layer and also it was recovered by evaporation of that layer,
as opposed to collecting by filtration using the Buchner funnel.