In our previous video we prepared propiathanone via a Friedel-Crafts reaction using benzene
and propionyl chloride.
We have discovered a different way to perform the reaction using much more OTC propionic
acid.
You do however need phosphorus pentoxide, but this is more accessible than the powerful
chlorinating agents.
We'll show you how we did it.
First of all set up a 250 ml flask equipped with a strong stir bar and on a hot plate.
Measure out 50 ml of benzene.
This is an excess of what we need for the reaction and will also add as a solvent.
Add this to the flask.
Now measure out 32 grams of anhydrous aluminium chloride.
You can check out our other video for details of how to make this using zinc chloride.
Keep this tightly wrapped up because it will fume on exposure to moisture.
Now add the rest of this to the flask and stir in the air.
With stirring on vigorously, carefully add the aluminium chloride to the benzene in the
flask.
If your benzene is dry you won't get any fireworks.
It won't dissolve but it will form a suspension.
Now measure out 19 grams of phosphorus pentoxide.
Again, be sure to keep this tightly wrapped as it will rapidly absorb moisture.
With stirring continuing, add this to the flask.
If your benzene isn't dry then you will have a big surprise at this point.
The slurry of solid is quite thick but still able to be stirred.
Set up the flask with a Leibig condenser attached with cold water circulating.
Set up a calcium chloride drying tube attached to the top of this to protect the reaction
from moisture.
Now weigh out 50 grams of hydrogen chloride.
Add 15 grams of propionic acid.
And get the dropper ready so that you can drip it into the reaction.
Remove the drying tube and start strong ventilation.
Now begin a very slow addition of the propionic acid through the top of the condenser.
As you add the temperature will increase rapidly, and you'll see clouds of HCl gas star to
form.
Keep the addition drop wise to begin with otherwise the mixture could boil violently.
Soon the reaction starts to boil on addition of the acid.
And you'll see a lot of HCl fumes escaping.
Here we are on complete addition.
Now heat the mixture to keep it at reflux.
The mixture will foam slightly.
So be careful with this.
Remember to reattach the drying tube once addition is complete.
On refluxing, foam will increase temporarily, and what you can see here is as bad as it
got for us.
Now leave the mixture stir in and refluxing for 2 hours.
During this time you'll see some changes in the consistency and color of the reaction
mixture.
The mixture will turn brown and then finally very dark brown.
You can see it becoming brown, but not very dark.
Now heat the mixture to keep it at reflux.
It will also turn quite solid and stirring will become impossible.
We detached the condenser temporarily after an hour and stirred the mixture by hand using
a metal spatula for a few seconds.
After 2 hours, remove the hot plate and allow to cool right down.
The mixture is too thick to pour so we experimented with a few drops of water, to see how vigorous
the quenching reaction was.
The water boils after a few seconds but not violently.
So we measured out 50 mils of ice cold water and added this to the flask.
When the liquid started to boil we added the further 50 mils of ice cold water.
The reaction never got uncontrollable.
But we recommend being very careful.
Once this died down we were left with an organic layer and an aqueous layer containing a precipitate.
Measure out 10 mils of concentrated hydrochloric acid and add this carefully to the flask.
Some further heat is generated.
Allow to cool, and then pour into a separating funnel.
This is the final result.
You'll end up with two layers.
The top one containing bar product.
Separate off the bottom aqueous layer.
Remove as much water as you can.
And then separate the top dark layer into a 250 mil flask.
Now set up for distillation.
The benzene comes off pretty quickly and we recovered around 15 mils of this.
The benzene comes off pretty quickly and we recovered around 15 mils of this.
The benzene comes off pretty quickly and we recovered around 15 mils of this.
We then used some tin foil on the still head to recover the higher boiling products.
We then used some tin foil on the still head to recover the higher boiling products.
We then used some tin foil on the still head to recover the higher boiling products.
We then used some tin foil on the still head to recover the higher boiling products.
We then used some tin foil on the still head to recover the higher boiling products.
At around 140 degrees C, a small amount of propionic acid came over it.
At around 140 degrees C, a small amount of propionic acid came over it.
We then prepared a weighted flask and collected the distillate coming over it
This was a yellow color as you can
The still head contained some dark residue at the end of distillation.
Here's the product.
8.1 grams of propiothinone.
This represents a 30% yield on starting propionic acid.
It's a slight yellow color but relatively pure.
So yields from this reaction are much lower than from a regular Friedel-Crafts ethylation,
but we're impressed that it's possible at all and we think that yields can probably
be improved with better quenching and workup, and maybe a slightly different reaction setup.