In this video, we're going to be preparing an interesting heterocyclic compound called
parabenic acid. You can see the structure here on the screen. It's a five-membered ring with two
nitrogens and three carboxyl ketone groups. You can't think of it as a cyclic urea derivative,
and it will be no big surprise that urea is one of our starting reagents. So let's look at the two
key reagents we need. First up is crystalline urea, and we've weighed out 6.5 grams here. It's
powder, and we've used it straight out of the bottle with no extra drying. And our other key
reagent is thymethyloxylate. We prepared this in a video not so long ago, so if you want more
details of how we prepared this, check it out at the link shown on the screen. We're using 12.8
grams of this white crystalline solid, which is an equal molar amount to the urea.
Let's look at the setup. For the reaction, we're going to use a 250ml flask. We've got this on a
hot plate, but we're only going to be using this for stirring. We've got a good stir bar in here,
so we can stir vigorously. And we've got the condenser attached, which we'll need for the
first part of the reaction. Having nice cold water is also helpful. Okay,
we're ready for action now. For the two reagents to react and form our product, we're going to need
to create a very strongly basic reaction mixture. To do this, we'll make up a solution of sodium
methoxide. So here's 70ml of dry methanol, which we'll use to do this, and as the solvent for the
reaction. Let's first get this into our reaction flask.
And here's 5 grams of sodium metal. This is a 2 times molar ratio to each of our main reactants.
As per usual, wipe the mineral oil away carefully using a paper towel. You also don't want these
into small lumps, otherwise the initial reaction will be very vigorous. So let's get this going.
Let's get the condenser on, and now we'll let this react. The reaction of sodium with methanol
is much more vigorous than with ethanol, and the temperature of the mixture rises
quite quickly, and soon the methanol starts to reflux quite rapidly.
We leave this reacting for about 30 minutes until there's no trace of the sodium metal left.
Then we leave stirring on a hot plate for another 30 minutes until the temperature has dropped back
down to room temperature again. And now we're going to add the urea to the reaction flask.
It doesn't dissolve immediately, so once it's added we leave the mixture stirring for about 5 minutes.
And we end up with a clear solution in the flask.
Okay now we're going to add the dimethoxylate. Since this is a solid we first need to get it
into solution, so to do this we'll use methanol as a solvent. We'll start with 40 mils.
As the solid dissolves it cools the mixture, so you'll need to stir and potentially warm the
mixture very slightly to get it all to dissolve in the solvent. After a few minutes of stirring,
here we go. So now we're going to add this slowly to the well stirred reaction mixture.
The cloudiness forms, and then a white precipitate which temporarily makes stirring difficult.
We were expecting an exo-firm, but the reaction mixture only warms very slightly.
Okay now we've added the dimethoxylate.
We're just going to leave the mixture stirring at room temperature now for the next hour in order
for the reaction to complete. Here we go an hour later. The stirring has splattered a bit of the
white precipitate around the flask, but this is okay. Our product in here is the sodium salt of
parabenic acid.
The first thing we are going to do in order to work up this mixture is convert it to the freassid
by acidifying the mixture.
To do this we'll use 20 mils of concentrated hydrochloric acid.
This is going to neutralize the SOBO
ethoxide present in the solution and convert our product to the acid form.
The reaction with the alkaline is going to produce some heat though, so we'll need to add this slowly.
Use the pipette to add the acid in one milk portions.
We do this over about 15 minutes and the temperature becomes warm, but not too much.
Let's test the pH.
And the mixture is strongly acid so we're done.
We're going to leave this to cool down now.
As the mixture settles the white precipitate sinks to the bottom.
This is mostly sodium chloride.
Our product, the parabenic acid, is soluble in methyl and so still in solution.
So let's filter.
Here we go.
A white precipitate which is mostly sodium chloride salt which is insoluble in the solution
which consists of mostly mephanol as the solvent with only a small amount of water.
From the acid we added.
So now we transfer the filtrate into a flask and we're going to distill this to remove
the mephanol solvent.
Not much point in using a thermometer.
And to assist we're going to use a vacuum pump to make the distillation go a bit faster.
So let's rock.
Let's go.
The first two thirds of the mephanol that comes off we retain and we'll recycle
this.
You can see solids starting to form in the flask.
We need to keep going though until there's no mephanol left at all.
Okay this is more like it.
Now we've just got some water left.
And as this cools down our product crystallizes out from the aqueous solution as a solid precipitate.
To purify this we can do a fairly simple recrystallization from warm water.
So let's get the crude product into a beaker.
And we'll wash out the flask with a little water.
Okay let's get this heating up until it's nice and warm.
We'll add a little more water if we need to in order to get the solids into solution.
Nearly dissolved now.
And now all the solids are in.
The solution is slightly cloudy but there appear to be no solids there.
So now we'll take this off the heat, cover it up and allow to cool.
Then we let the mixture chill overnight in the fridge so that crystallization could complete.
Here's the result 12 hours later.
A massive nice crystal in the beaker.
Let's filter these off.
You'll need to leave on the pump for some time in order for them to dry completely.
Here's our dried recrystallized product.
Exactly 9 grams of parabenic acid.
And this is a 73% yield on our starting urea or dimethyloxylate, which isn't bad at all.
Parabenic acid is interesting because it's a cyclic amide otherwise known as a lactam.
The compound exists has an equilibrium between two forms.
As you can see here.
The ethyl form of the compound is able to form salts with alkalis such as sodium hydroxide.
So it's a bit similar to barbituric acid.
Unlike barbiturates though, parabenic acid derivatives don't seem to have any physiological effects or medicinal use.
But it's a nice reaction and an interesting product.
Thanks for watching and stay tuned for more reactions.
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