The classic way to perform the reduction of an aromatic nitro group to an amine is using

tin and hydrochloric acid. We've done this in the previous video so check it out for more information.

But we've been reading a lot about some other potential methods.

So today we're going to try them out on a qualitative scale and see which work and which don't.

Okay, let's get right into reaction number one.

Here's our setup for our first reduction reaction.

We've got a 100 ml flask sitting on a hot plate.

And equipped with magnetic stirrin.

And we've got the smallie big condenser with cool water running through it ready.

Here's our first reducing agent contestant.

This is 15 grams of tin to chloride.

Otherwise known as stannous chloride.

It's what gets formed in the reaction between tin and hydrochloric acid.

But we've read several reports that it's a viable reducing agent in its own right for aromatic nitro compounds

when dissolved.

This is the dihydrate salt, which is how this compound is normally available.

It's a white crystalline solid as you can see.

Okay, here's our solvent for the first reaction.

Here we've measured out 25 ml of absolute ethanol.

Given the salt is the dihydrate we're guessing you could use regular azeotropic ethanol as well.

Let's get this into the flask.

And now we'll add our tin to chloride to this.

Here we are all added.

And we allow this to stir for a few minutes.

Pretty soon we've got a nice clear solution formed.

The salt is pretty soluble in ethanol.

Now for our aromatic nitro compound.

We're going to use ortho nitro toluene.

Because this has a strong aroma, a bit like floor polish and almonds,

it's going to be easy to figure out if the reaction has taken place.

The reduction product, ortho toluene, also has a strong aroma,

a bit like horse manure.

So again, it's going to help us find out what's happening.

We're going to add just one ml of the nitro toluene so that the reducing agent is in massive excess.

The mixture rapidly turns a yellow color.

More so than can be explained by the color of the small amount of nitro toluene we added.

We've attached the condenser and now we're going to bring the mixture to a reflux.

Now it's not our imagination, but the mixture does seem to be turning a more intense color.

It becomes almost orange in color.

And then after about 15 minutes or so of refluxing,

it becomes paler again.

Here we are after 30 minutes.

According to the sources we read the reaction is complete at this point.

However we checked and there's still the strong nitro toluene aroma in the flask.

So we're going to continue refluxing.

We continued for 2 hours in total.

And then stopped the stirring and allowed the mixture to cool down.

Well this is already interesting

because on opening the apparatus,

we can tell immediately that the nitro toluene aroma has disappeared.

And we've got that unpleasant horsey stables manure type smell there.

Which is exactly what Tardine smells like.

So it's not smelling good,

but it's looking good as far as a successful and completed nitro reduction goes.

Here's about 150 ml of cold water.

Let's pour in the reaction mixture and see what happens.

First we'll add a ml or two of hydrochloric acid

in order to stabilize any residual tin to chloride.

This is beautiful.

Any residual nitro toluene would create a cloudiness in the solution.

And this looks completely clear.

Any Tardine present is going to exist as the hydrochloride salt.

In order to liberate it we would need to make the mixture alkaline.

This is a strong sodium hydroxide solution.

We'll add this to the mixture now and see what happens.

A thick precipitate of tin hydroxide forms.

So the reduction works, but there's a catch.

If we check the pH now,

you can see that the solution is still strongly acid due to the tin chloride forming hydrochloric acid in solution.

There's no way to get the Tardine out using an organic solvent until you've neutralized all of this.

And adding more sodium hydroxide generates an even thicker precipitate.

So you're going to have to add a lot.

And then steam distill the Tardine out of the mixture

and it's going to work with that amount of precipitate.

So the verdict.

Tin to chloride works great but you need to reflux for a long time.

And use 5 to 6 times molar excess.

There are however no shortcuts to the workup though

and you'll need to steam distill to get a product.

One final note on this reagent.

We did the reaction again using methanol as the solvent

and the reduction was much much slower.

So it looks good.

Looks like you do have to use ethanol.

Okay now it's time for reaction test number 2.

And our second reducing agent contestant is iron to sulfate.

We picked up a Chinese undergraduate lab book

and found a procedure for aromatic nitro group reduction using iron to sulfate and aqueous ammonia.

It sounded too crazy to be true but it is right there in the student practical guide.

So we had to give this a go.

Here's 10 grams of iron to sulfate heptahydrate crystals.

We're going to dissolve these in 20 mils of warm water.

There's a slight orange color and we've probably got some slight iron oxidation in our starting material.

But not too much.

Okay here's our starting aqueous ammonia.

Here's 15 mils of 25% ammonia solution in water.

We're using the same flask as we used for the previous reaction with magnetic stirrin and on a hot plate.

Let's get the ammonia solution in.

And as before, we'll add 1 mil of our ortho nitro toluene.

It doesn't dissolve but with vigorous stirrin there's a good suspension.

Okay now we're going to add the iron to sulfate solution to the flask.

The dark blue color and a thick precipitate first form.

Which becomes more liquid again after a few seconds.

And an orange color appearing on the rim of the liquid.

Well the instructions say to warm this now for 30 minutes.

So we're going to switch on the heat and get this up to about 50 degrees C.

Not a lot happens.

Although some fumes of ammonia are produced.

And at one point in the reaction we're sure there is an aroma a bit like methylamine gas coming off.

After 15 minutes we add a further 15 mils of 25% aqueous.

To the flask.

And we continue warming and stirrin the mixture for a further hour.

Just to be sure that any reaction can take place.

After an hour is up, we switch off the heating and stirrin and allow the mixture to cool down.

Then we chill it down in the fridge.

You can see there's a heavy black precipitate and a green colored supernatant liquid.

Well we can't smell anything due to the ammonia fumes.

So we'll have to do some work with this.

First we set up to filter the mixture.

Filtration is pretty slow.

Not just because our filter pump is pretty dodgy at the moment.

The black precipitate is magnetic and as you can see it forms a clump around the stir bar.

We then wash the precipitate with 20 mils of dichloromethane.

Okay here's our filtrate and in the bottom there's an orange colored DCM layer.

Since our solution is already alkaline due to the ammonia.

The product should be in this layer.

So we simply pipette off the DCM.

And then dry it with a little magnesium sulfate.

And then evaporate it down to remove the solvent.

The result is an orange colored oil.

But there's only a faintest aroma of tildine.

The overwhelming smell is of nitrotoluene.

So this method looks like it hasn't worked.

And our pore center was a nightmare to clean.

In the end it took aqua regia before we got the black color out.

So our verdict is a no.

We don't know what they're teaching over in China.

Okay now it's time quickly for our final contestant.

Sodium diphenate.

We're going to make up a fresh solution of this for the reaction.

We're starting now.

Starting with 25 grams of sodium bisulfite.

And we're going to dissolve this in 25 ml of warm water.

And here we've got 10 grams of powdered zinc metal.

We've done another video separately showing you how to make sodium diphenate.

So check it out for more detailed instructions.

We've got our lukewarm solution in an ice bath.

And now we add the zinc metal.

After about 5 minutes this has reacted and the mixture is thick in texture and paler in color.

So set up to filter this.

And use a little cold water to extract the solids in the filter.

And here we go.

A freshly prepared solution of sodium diphenate.

Which we can use to test the nitro group reduction.

Okay our setup is exactly the same as for the first reaction we tried.

We're also going to use 20 ml of methanol as a co-solvent for the nitro toluene.

Let's add this to the diphenate solution in the flask.

Some precipitate forms.

But we'll just leave this as it is.

Now we add 1 ml of nitro toluene to the flask.

We've got a pale yellow mixture.

To work diphenate uses alkali conditions.

And a normal way to achieve this is by using sodium carbonate in solution.

So we've measured out about 5 grams of sodium carbonate here.

And a small amount of water.

Add this carefully just in case it bubbles.

Okay now set up for reflux and heat the mixture gently for about 30 minutes.

At the end of this time allow to cool.

There's a pale yellow precipitate present in the bottom of the flask.

We pour the reaction mixture into cold water.

And it soon settles forming a clear liquid with a yellow solid.

And the aroma of nitro toluene is completely gone.

Replaced with the characteristic horcetalardine smell.

So this is a positive test.

It looks like this is a really fast, efficient and OTC way to reduce aromatic nitro carbonate.

To work this up you simply filter off the yellow solid and collect the clear filtrate.

We're not too sure what the solid is.

Perhaps it's sulfur but it looks slightly different and more crystalline.

Check that the solution is strongly alkaline.

And then extract using your favorite organic solvent.

This is just a test so we're not going to work this up.

But the verdict is that this looks great.

And unlike many reduction techniques the conditions are mild.

Perhaps mild enough for acid and base sensitive compounds such as esters to survive.

We have a winner.

Thanks for watching.