Warning: This experiment generates toxic gases and handles corrosive acids.
Wear gloves and work outside or in a fume hood.
Greetings fellow nerds.
In this video we’re going to make propionic acid.
It’s a carboxylic acid like the much more well-known acetic acid but has one additional carbon in its backbone.
It’s used to make solvents, fragrances, plastics, herbicides and other chemicals.
I want to use it to make an antiprotozoal drug.
Now we already started making propionic acid in a previous video
where we used the haloform reaction to produce chloroform from methyl ethyl ketone.
A byproduct of this reaction is the formation of propionic acid which we will recover and purify in this video.
So here is the aqueous solution of the various products
from the reaction of a full one gallon bottle of 8% sodium hypochlorite bleach and 93 mL methyl ethyl ketone.
The yellow color is from the various reaction side products
as i used excess methyl ethyl ketone rather than perfectly titrate the bleach to determine how much i needed.
Nonetheless this will still work and the propionic acid is still recoverable.
Now the haloform reaction creates an alkaline solution so the propionic acid is actually in the form of sodium propionate.
We’ll need to convert it to the acid form to recover it.
Now i’ve placed the solution in the electrolysis box i made in a previous video.
I’m not doing electrolysis but i’m instead taking advantage of its ability to remove toxic gases
by creating a negative relative pressure in the box.
I could also use a fume hood but i need to use mine for other experiments.
Now to convert the sodium propionate to propionic acid we add 600 mL of 31% hydrochloric acid.
This is a huge excess of hydrochloric acid and will ensure the reactions proceeds forward.
Now I put the lid on the electrolysis box and sealed it, turning on the diaphragm pump to remove gases.
What the gases consist of is small amounts of toxic chlorine from unreacted bleach, and chlorine dioxide.
This comes from the reaction of sodium chlorate with hydrochloric acid.
Now you’re probably wondering where that sodium chlorate came from.
Remember in my previous haloform video where i said my bleach was less concentrated than what the label said?
Well one decay pathway is the creation of oxygen,
but another pathway is the creation of sodium chlorate.
Decayed bleach contains small amounts of it that is now reacting with the hydrochloric acid.
This is why you have to do this reaction in a fume hood, outside or in a gas removal box like my electrolysis box.
Alternatively you can also add in a reductant like sodium metabisulfite or sulfamic acid but that’s an additional cost.
There is also the problem of not knowing how much to add exactly so you’ll need to titrate it.
Anyway. I left this going overnight to remove as much of chlorine and chlorine dioxides as possible.
Unfortunately chlorine and chlorine dioxide are somewhat soluble in water so be aware of that when you handle the solution later.
It will continually emit small amounts of the gases.
Now to recover the propionic acid from the salt water we use fractional distillation.
Pure propionic acid has a boiling point of 141 degrees Celsius.
Unfortunately propionic acid forms an azeotrope with water consisting of 17% propionic acid by mass at 99.96 degrees celsius.
Now even a professional distillation setup is going to have a difficult time resolving a difference of 0.04 degrees
so we’re not going to bother.
We’re just going to collect everything at 100 degrees Celsius.
A lot of that should be the propionic acid azeotrope.
You’re probably wondering what that white powder in the receiver is.
That’s sodium bicarbonate, as the distillation starts what comes off first is the dissolved chlorine and chlorine dioxide.
The alkaline sodium bicarbonate helps to absorb the gases
so there is less emitted into my gas scrubbers and ultimately into my fume hood.
You can also use sodium hydroxide or other caustic chemicals.
Scrubbing the gas like this optional if you don’t mind releasing it into the environment.
Now once the chlorine and chlorine dioxide gases are boiled out
some of the leftover organic chemicals will start distilling.
This includes unreacted methyl ethyl ketone as well as unrecovered chloroform.
So discard the first 1% of the solution volume which contains most of the volatile organics.
Now i’m distilling 500mL at a time since my apparatus can only handle that much
so i’m going to discard the first 5 mL of liquid in this run.
Once that is distilled over we now change out the receiver and collect our mixture of propionic acid and water.
Now a lot of liquid will distill over, more than half our original volume, so be patient.
Propionic acid can also be recovered by liquid-liquid extraction using a non-aqueous organic solvent like diethyl ether.
While it is likely much faster I didn’t do this since I had a gallon of solution to process
and didn’t want to deal with a proportionally large amount of organic solvent.
Anyway, keep distilling everything that boils around 100 degrees.
Since we used an excess of hydrochloric acid it will start distilling after we have exhausted most of the water.
When the temperature of the distillate starts rising beyond 100 degrees we are beginning our transition.
I stopped my distillations at around 103 degrees.
Now distilling 500 mL at a time of a one gallon solution took me a couple of weeks of work in between other projects.
So i’ll skip way ahead to when i was finished.
And here we are with over 2 liters of distillate containing water and propionic acid
as well as around two liters of residue that contains mostly hydrochloric acid, less volatile organics and salt.
That salt was what was originally present in the bottle of bleach and has now crystallized out.
The sodium hypochlorite only amounts to tens of grams but the salt it was made from is on the order of several hundreds of grams.
This is what the manufacturers used to make the bleach originally.
It’s too costly to remove it from the bleach so the manufacturers just leave it in.
Something to think about the next you use bleach.
Anyway, we now have to separate the water from the propionic acid.
We can’t just distill it since it forms an azeotrope.
So we’re going to go a roundabout way and add sodium bicarbonate to react with the propionic acid
and convert it back into sodium propionate.
Since we don’t know exactly how much there is we’ll have to crudely titrate by adding small amounts of sodium bicarbonate
and stopping when it no longer fizzes.
At this point all of the acids have reacted.
Now we remove the water by simply boiling it off.
You can boil it off in an open container but i’m going to try something different and distill it off under a weak vacuum.
The color of the solution is from the lingering organics darkening due to the alkaline conditions and high temperatures.
It looks bad but really the contamination is very minor.
You’ll notice I connected a Claisen adapter so i can add in more solution as i need to.
The distillation apparatus is completely sealed and the hose on the right is connected to a 6 dollar diaphragm pump
I demonstrated this pump in my electrolysis box video.
This will provide the weak vacuum.
Being just 6 dollars i don’t expect amazing vacuum but i’m interested in seeing what happens.
And here we go. We’re boiling water at around 90 celsius,
ten degrees below the usual boiling point at atmospheric pressure.
That’s a pretty good improvement for just a 6 dollar pump i got off ebay.
By reducing the pressure and thus the boiling point of water, i can speed up the distillation somewhat.
Professional labs use a machine called a rotovap that essentially does the same thing but much better and faster.
Anyway, continue removing water and adding more solution until finished.
Once all the water is gone you’ll have a mass of sodium propionate.
Get its weight so we know how much sulfuric acid to add later.
I previously weighed the flask and stir bar to 333.1g so our sodium propionate mass is 58g.
Now we liberate our propionic acid by adding concentrated 98% sulfuric acid.
About 1mL for every gram.
You can already see the fumes of propionic acid forming.
Some of these fumes are also from hydrogen chloride as inevitably there was some sodium chloride
in the dry sodium propionate from small amounts of hydrochloric acid that was distilled over.
As usual this must be done outside or in a fume hood.
Now distill off the propionic acid.
I’m not using a vacuum but it looks like we’re not reaching the boiling point of propionic acid at around 141 celsius.
There might still be some lingering impurities like water and organics.
You’re probably wondering why we didn’t just dry the original sodium propionate solution
and add sulfuric acid to that to distill out the propionic acid.
This is because as we shown there is a tremendous amount of salt in it so we’d need a huge amount of sulfuric acid to react it out.
If sulfuric acid is dirt cheap to you then you can go that route if you want.
Just remember that hydrogen chloride will also be released and you'll need to deal with that.
Anyway keep distilling until nothing more distills over.
Looks like i blundered here and didn’t adequately mix the sodium propionate and sulfuric acid beforehand.
So there are a few chunks of unreacted sodium propionate and the mixture is too thick to mix in the remaining sulfuric acid.
To solve this i just added a lot more sulfuric acid to ensure everything is dissolved.
If you’re stingy about using sulfuric acid then be sure to crush up and thoroughly mix the sulfuric acid and sodium propionate beforehand.
Anyway, once the distillation is finished we now have a crude sample of propionic acid.
Now we can not dry it using molecular sieves since those decompose in acids like this.
So instead we dry it with anhydrous sodium sulfate and then fractionally distill it.
Now we get our proper boiling point of around 141 degrees.
And there is our purified propionic acid.
My final purified yield was 42g or about 44% based on methyl ethyl ketone added.
I think the poor yield was primarily due to the bad bleach I got.
If you get a factory fresh bottle I think you’ll do better.
Now the primary commercial use of propionic acid is a food preservative.
Humans and animals can safely ingest small diluted quantities of it but some types of mold and bacteria cannot.
Grains, bread and cheese are particular well preserved by it.
But I do not recommend you taste this stuff though,
this is a highly concentrated and purified acid that will chemically burn you.
Anyway, I want to use it as a precursor to try and make an antiprotozoal drug.
Not sure if i’ll be successful but we’ll see what happens.
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