We had read that it wasn't possible to produce acetic anhydride via the dehydration of acetic

acid using phosphorus pentoxide.

But we wanted to test this so we got a small amount of phosphorus pentoxide in a beaker.

And we added a few drops of glacial acetic acid.

Nothing happens initially but the temperature in there is rising rapidly.

Some sort of uncontrolled dehydration is obviously taking place.

Okay, so using pure reactants isn't going to give us any useful product here.

Here's some more phosphorus pentoxide.

And this time we're using propionic acid.

The reaction is slower.

But soon the temperature rises.

This is a good sign.

And the process accelerates.

We're not sure what is actually happening here or if any anhydride is being generated.

So we tried a different approach.

We measured out 80 mils of dry dichloromethane as a solvent.

We placed this into a 250 mil flask sitting in an ice bath and equipped with strong magnetic stirrin.

We then weighed out 18 grams of phosphorus pentoxide.

This is wrapped up carefully to protect from atmospheric moisture.

We added this to the stirred dichloromethane.

There was no reaction but a suspension of powder in the solvent was created which was easily stirred.

We then set up a reflux condenser on top of the flask.

We then set up a reflux condenser on top of the flask.

And made sure that there were no oil residues.

After the water running through was ice cold, we measured out 20 ml of glacial acetic acid

which is approximately 21 grams.

We placed this into a container so that we could very slowly add it through the top of

the condenser.

And we slowly began an addition of the acetic acid into the suspended phosphorus pentoxide.

The reaction didn't appear to be vigorous and a steady drop wise addition didn't cause

the solvent to boil.

Pretty soon however the pentoxide glued together into a sticky mass and this prevented stirring.

We resorted to shaking the flask from side to side.

Enough heat was generated to create a very slow reflux of the solvent.

Complete addition took 15-20 minutes.

you know.

We then removed the water bath and heated the mixture until it reached a gentle reflux for an hour.

The viscous solids in the flask changed color and became a yellow and then brown color.

After an hour the flask was allowed to cool down and then left at room temperature for a further hour.

This is the result.

You can see that the phosphorus trioxide or phosphoric acid lump has turned a dark brown color.

We set up for simple distillation and the dichloromethane first came off very rapidly around 40 degrees C.

We got all of our starting solvent back as you can see.

The temperature of the distillation then increased and starting around 80 degrees C and going up to 120

we got a small amount of acetic acid.

We collected around 10 ml of this.

But beyond this no more distillate was produced and the contents of the boiling flask turned to a carbonous black.

This experiment was a failure.

So we decided to try something different.

In a previous reaction we had created propiothenone using propionic acid in benzene containing valuminium chloride and phosphorus pentoxide.

So we mixed a small amount of valuminium chloride and phosphorus pentoxide together.

And then added acetic acid.

The reaction definitely occurs and fumes of HCl are produced.

So we decided to scale up and see if there was any acetyl chloride generated which could be isolated.

First we weighed out 18 grams of phosphorus pentoxide.

And then using our process starting from zinc chloride and valuminium powder.

We prepared 26 grams of fresh anhydrous valuminium chloride.

We wanted to use a solvent which was high boiling but wouldn't react.

So we used 50 ml of mineral oil.

The color is slightly yellow but we tested it with the reactants to prove it would be neutral.

On stirring and heating with a small amount of valuminium chloride and phosphorus pentoxide there was no obvious reaction.

We placed the oil into a 250 ml flask equipped with magnetic stirring and on a water bath.

The water bath would initially keep the mixture cool.

And later be able to be heated.

First we added the phosphorus pentoxide to the flask.

On complete addition this was still just about stirable.

Then we added the aluminium chloride.

The mixture became very thick and difficult to stir.

And we had to use a glass rod to thoroughly mix the contents.

Next we measured out 15 ml of glacial acetic acid.

This is a slight excess to what we need.

We set up the flask with a condenser and a dropping funnel on top of the still head.

And lots of ice to ensure the cooling water was very cold.

If anything interesting gets produced, we'll catch it.

We allowed the acetic acid into the flask a little at a time.

There was a definite reaction although not vigorous.

The contents of the flask warmed and started to bubble.

Complete addition took around 15 minutes.

And at this point we started to gently warm the flask on the water bath.

No distillate collected.

But huge amounts of HCl gas started to be produced.

And we needed to use a gas vent tube.

The HCl was definitely the main product of the reaction.

Even on stronger heating no distillate collected.

This experiment was also a failure.

This leaves us with a complete puzzle as to how our previous experiment which created propiothenone using propionic acid actually worked.

It doesn't seem as though acetic anhydride can be made using phosphorus pentoxide and acetic acid.

And with aluminium chloride present it is not possible to isolate acetochloride.

Maybe aluminium acetate is active.

Let's keep experimenting and see if we can come up with any answers.

For the moment though, this particular reaction is myth busted.

Thanks for watching.

See you in the next video.