In the previous video, we prepared nitrobenzoic acid by oxidizing nitrotoluene using dichromate and sulfuric acid.

Here's the pyronitrobenzoic acid we prepared.

In this video we're going to do a few more reactions with this and transform it into something very interesting.

The first thing we're going to do is convert this carboxylic acid into its ethyl ester.

Here we're starting with 5 grams of pyronitrobenzoic acid, which was the product obtained from the reaction in the previous video.

We're going to use a 250 ml flask on a hot plate, and equipped with magnetic stirring for the reaction.

The flask looks wet, but it's not.

We rinsed it thoroughly with absolute ethanol in order to make sure there was no water present.

Here's 40 ml of absolute ethanol that we'll use for the reaction.

Let's first get this into the flask.

And now we'll add the nitrobenzoic acid to the flask as well.

Let's get it all in and stirring.

Okay now for our acid catalyst.

Here we've measured out 5 ml of concentrated sulfuric acid.

Add this carefully.

The addition gives off quite a bit of heat as you can see.

Okay now we're going to attach a reflux condenser and start heating the mixture.

We want to get a good strong reflux going and keep it there for a few hours in order for the reaction to stand a chance of going to completion.

After a few minutes of strong heating we're getting there.

It also looks as though the solid starting material is starting to dissolve.

And after about 10 minutes of refluxing we've got a nice clear yellow mixture formed.

We're going to leave this refluxing now for the next 3 hours in order for the reaction to take place.

Acid-catalyzed esterification such as this do take a while to complete,

because the reaction itself is in equilibrium, rarely give 100% yields.

Let's take a closer look at what's happening.

First of all the strong acid catalyst protonates the much weaker carboxylic acid.

The resulting structure is stabilized but effectively leaves the carbon with an electron deficiency.

This then leaves it open to nucleophilic attack by the alcohol,

creating an intermedia which is then able to lose a molecule.

Leaving our ester.

Okay here we are 3 hours later.

We've removed the condenser and we're going to boil off a little of the ethanol now.

We did this for another 30 minutes, after which probably about 10 mls of the ethanol had evaporated off.

Then we stopped stirring and allowed the mixture to cool down a bit.

Here's about 75 mls of cold water.

We've added an ice cube just to make sure it's nice and cold.

And now we pour in our warm reaction mixture.

Immediately a solid precipitate forms in the mixture.

Let this settle for a few minutes, and then we'll set up to filter this off.

The product takes a lot of drying, so leave it on the filter until very dry and fluffy.

Here's our crude intermediate ethyl ester of nitro-benzoic acid.

After completely drying as much as we can, we've got 5.9 grams here.

This is over 100% of the expected yield, so it's obviously not completely dry.

We also expect that there's some of the starting material in here as well.

However it has a different texture to the nitro-benzoic acid,

and has an interesting subtle aroma of floor polish and almonds,

but with sweetness and candy too.

Okay.

Today we're going to treat you and do the reactions in one video.

We've now made the ethyl ester of the nitro-benzoic acid.

What we're going to do next is reduce the nitro group to an aromatic amine.

In a video we did recently we checked out a few different ways to do this reduction

and we're going to use the sodium diphenate method.

First we're going to make the diphenate solution.

This is sodium bisulfite and we're using 50 grams,

50 mils of water,

and finally 19 grams of zinc metal powder.

We'll cover this next part very quickly because you can check out our previous video for the full details here.

We get the sodium bisulfite on a magnetic stirrer and put in a stirred bar.

Then we add the 50 mils of water and get it stirring to dissolve.

Then we add the 50 mils of water and get it stirring to dissolve.

Add a little more water if you need.

Then when dissolved, transfer into a cool water bath.

And add the zinc metal.

Keep stirring until it forms a thick slurry of gray-colored zinc hydroxide.

Then filter.

And here we go.

A solution of sodium diphenate, which we can use for our reduction.

Let's check the pH.

It's slightly acid.

Diphenonite requires an alkalized solution to reduce an aromatic nitro group however.

So here's 8 grams of anhydrous sodium carbonate.

This should increase the pH enough that the reduction can take place,

hopefully not enough to hydrolyze our ethyl ester.

We'll dissolve this directly in the diphenonite solution.

The mixture goes a bit cloudy, but nearly all of the sodium carbonate seems to dissolve.

Let's test the pH.

It's now at about 10.

Possibly on the low side.

But we can't get any more carbonate to dissolve.

So we'll give this a try.

We're going to use a 250 ml flask equipped with a stir bar for the reaction.

Let's get the alkaline diphenonite solution in.

And now for our starting material.

We're going to use 4 grams of the esters.

And dissolve.

Using 20 ml of ethanol.

With some gentle warming the ester goes into solution.

Let's get our reaction mixture stirring.

And we're warming the mixture slightly using the hot plate.

Now let's slowly add the ester solution to this.

As the ethanol solution is added there's a transient pink color formed.

Which soon disappears.

And by the time we've completely added the solution, there has been quite a temperature increase.

And there's some more.

Some refluxing inside the flask.

We'll leave this heating now at nearly boiling point for the next hour and a half.

Instead of using a condenser we're going to allow the ethanol present to boil off.

As it does we see a white solid depositing on the sides of the flask.

Okay here we are an hour and a half later.

We allow the mixture to chill down.

You can see what looks like a separate top layer and a white precipitate in the bottom.

This is interesting because there's nothing in here that should form a separate layer of this size.

So it's possible that it's actually ethanol which has separated somehow.

We'll extract our product from the mixture.

Using 40 ml of total ethanol.

Dichloromethane.

And with shaking and then allowing to settle.

We've now got two layers.

The top aqueous and the bottom DCM.

And some white precipitate remaining.

First we'll filter to remove the precipitate.

And here we go.

All two layers.

This separates pretty quickly.

Now we'll make up a little sodium hydroxide solution using an ice cube and 3 grams of sodium hydroxide.

The idea is to make the mixture strongly alkaline.

Our product will be guaranteed to be in its freebase form and so soluble in the DCM layer.

Let's add this to our mixture.

Give it a good stir.

And now we transfer to a separating funnel.

We give it a good shake.

Allow to separate.

And then drain off the bottom DCM layer.

Here we go.

Just to make sure we've got all the product.

We have another 20 ml of DCM to the separating funnel.

Shake well again.

Allow to settle.

And then drain off the bottom DCM layer.

Hey, raise our combined DCM extract.

In order to extract our amine product, we're now going to make up an acid solution that will be able to turn it into a water-soluble salt.

Here's 15 ml of concentrated hydrochloric acid and 100 ml of water.

And now we place the DCM solution back into the separating funnel and add the dilute hydrochloric acid to it.

We shake well for a few minutes and then allow to separate.

In theory our product should now have formed a hydrochloride salt and transferred into the aqueous layer, leaving behind any starting materials.

So first we drain off the DCM, and then extract the aqueous layer.

And now to precipitate our product.

We adsorb the acid.

Eventually a persistent precipitate forms when the acid is all neutralized.

We'll chill this down for a while.

And then filter our product.

And here we go.

1.1 grams of FLP amino benzoate.

Otherwise known as benzocaine, a local anesthetic.

The yield is not great at just 32% from our starting nitrobenzoic acid.

But it was an experimental procedure so we're okay with that.

There are a few syntheses of this compound on YouTube but we think this exact one is a first.

And it certainly numbs the tongue.

Thanks for watching.

We'll see you next time.