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Author: Subject: Somewhat Unusual Properties of Ammonium Benzoate
hodges
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[*] posted on 18-3-2022 at 18:12
Somewhat Unusual Properties of Ammonium Benzoate


I was doing some experiments involving polar / non-polar solubility. I started with sodium benzoate, which is soluble in water. I added HCl to precipitate benzoic acid, which of course has very little solubility in water. After washing and drying the precipitate, I dissolved it in acetone and re-crystalized. Then I slowly added ammonium hydroxide, until all the precipitate dissolved (because it turned into ammonium benzoate, which is polar).

I decided to crystalize and keep the ammonium benzoate. And that's where I noticed a couple of somewhat unusual properties.

When enough water had evaporated for crystals to begin forming, they formed on top of the solution, not at the bottom of the beaker. In fact, the crystals got so thick near the top of the solution that they were preventing water from continuing to evaporate, so I had to stir. Every time I would stir, the crystals would fall - but new crystals soon covered the top of the water. I looked up the density of ammonium benzoate, and it is only 1.25. This is one of the lowest densities I have seen for a salt, and may explain why the crystals do not readily sink.

Then the crystals began climbing the side of the beaker. I had to scrape them down daily to get them to fall back into the solution, because they were climbing up the side of the beaker fast enough that they appeared likely to escape with time. I'm guessing this is probably because they form on the surface, and leave a thin layer of liquid below them, which is then carried up by capillary action, with new liquid constantly being exposed at the top.

Then there is the smell of the crystals. They smell like a cross between a cheap fruity soft drink and the kind of plastic/rubber used to make pool rafts (you can smell it when you blow them up).

Anyway, the crystals are nearly dry now. Guess I will keep them, though I don't really know of much use for them. Apparently if you heat them they decompose to form benzamide (saw this on YouTube), but have not seen much else about this substance.
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SplendidAcylation
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[*] posted on 20-3-2022 at 06:05


That's interesting!
Is there any particular reason you recrystallized the benzoic acid from acetone?
I've always had good results using boiling water.

I have also made this compound myself, for the purposes of making benzamide (more on this below), I don't remember if it floated on the top of the solution, although I suspect it did...

I'm pretty sure it didn't have any smell, though, maybe a slight smell resembling benzoic acid?
Would you say the smell is substantially different from that of benzoic acid?

I would urge caution regarding the ammonium benzoate to benzamide idea.
Unlike the analogous reaction using ammonium acetate to form acetamide, this reaction doesn't seem to be feasible in an open vessel, as the loss of ammonia is favoured over the loss of water, so you just end up with benzoic acid, all the ammonia having escaped...

So because of this, I tried doing the reaction in a sealed tube; I put about 10g of dry ammonium benzoate in a test-tube and sealed it shut with a propane torch.
The tube was then buried in a sand-bath and heated to around 200c IIRC.
Unfortunately, I didn't pay close enough attention to the temperature, and (thankfully when I was out of the room, the tube exploded with a very loud bang, sending sand and broken glass everywhere, and filling the room with a cloud of vapour which fell back to the ground as sparkly crystals...

I suspect the reaction had worked, as the vapour, although smelling of benzoic acid, didn't have the same cough-inducing properties as benzoic acid (if you've ever heated benzoic acid you'll know what I mean)

Not really the best test for benzamide, but there we go...

Still, none was recovered, as it all evaporated.

The explosion was pretty violent so it could have been quite injurious especially to the eyes.
It may be possible to do this reaction as atmospheric pressure by heating the ammonium benzoate in a stream of dry ammonia, I'm not sure.
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[*] posted on 20-3-2022 at 12:35


Quote: Originally posted by SplendidAcylation  


Is there any particular reason you recrystallized the benzoic acid from acetone?
I've always had good results using boiling water.

I'm pretty sure it didn't have any smell, though, maybe a slight smell resembling benzoic acid?
Would you say the smell is substantially different from that of benzoic acid?

I would urge caution regarding the ammonium benzoate to benzamide idea.
Unlike the analogous reaction using ammonium acetate to form acetamide, this reaction doesn't seem to be feasible in an open vessel, as the loss of ammonia is favoured over the loss of water, so you just end up with benzoic acid, all the ammonia having escaped...


I was testing solubility of polar vs. non-polar compounds. Sodium benzoate and ammonium benzoate are polar, so they dissolve in water but not in acetone, whereas benzoic acid is non-polar so it dissolves in acetone but not in water. I wanted to see this for myself. I also thought that if there was any trapped sodium chloride in the benzoic acid precipitate, it would not dissolve in the acetone and I could filter it out. I did filter the acetone, but didn't see anything left behind so apparently it was already fairly pure just from multiple washings with water.

I note almost no smell from sodium benzoate. Benzoic acid to me smells just like some sort of soda I used to drink years ago as a child. It must have been loaded with benzoic acid as a preservative. I only noted the rubbery smell in addition with ammonium benzoate, not benzoic acid. It is possible that this may be due to impurities in the acetone though. The acetone container says it contains denatonium benzoate, apparently to prevent consumption. I'm sure the amounts are very small, but may be enough to change the smell.

Here is one of the few YouTube videos I have found that mention ammonium benzoate.
https://www.youtube.com/watch?v=lzw2qbmYivg
The author heats a test tube containing ammonium benzoate and a thick vapor escapes. Author concludes (not sure why) that benzamide was formed based on this.
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[*] posted on 20-3-2022 at 19:05


Quote: Originally posted by hodges  
When enough water had evaporated for crystals to begin forming, they formed on top of the solution, not at the bottom of the beaker. In fact, the crystals got so thick near the top of the solution that they were preventing water from continuing to evaporate, so I had to stir. Every time I would stir, the crystals would fall - but new crystals soon covered the top of the water. I looked up the density of ammonium benzoate, and it is only 1.25. This is one of the lowest densities I have seen for a salt, and may explain why the crystals do not readily sink.

This isn't too unusual, epsom salt and zinc acetate do this all the time and they're quite a bit denser. Happens especially with small evaporating dishes. Also happens with sodium and copper sulphate, and sodium chloride occasionally.

Quote: Originally posted by hodges  
Then the crystals began climbing the side of the beaker. I had to scrape them down daily to get them to fall back into the solution, because they were climbing up the side of the beaker fast enough that they appeared likely to escape with time. I'm guessing this is probably because they form on the surface, and leave a thin layer of liquid below them, which is then carried up by capillary action, with new liquid constantly being exposed at the top.

I said this exact thing! I called the phenomenon "salt creep" because it's especially bad with sodium chloride.

I attached a picture of it growing entire damn structures outside the bowl. The shell stays wet just beneath the surface, indefinitely. Such a mess to scrape off.

Anyway, it's definitely capillary pumping. Side tangent: I read that Mars' water cycle might have worked by capillary pumping due to the pressure differential between the lowlands and highlands. Might even still be active today as a really thick frost-resistant saltwater brine.


unknown.png - 145kB
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SplendidAcylation
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[*] posted on 22-3-2022 at 03:10


Quote: Originally posted by hodges  
Quote: Originally posted by SplendidAcylation  


Is there any particular reason you recrystallized the benzoic acid from acetone?
I've always had good results using boiling water.

I'm pretty sure it didn't have any smell, though, maybe a slight smell resembling benzoic acid?
Would you say the smell is substantially different from that of benzoic acid?

I would urge caution regarding the ammonium benzoate to benzamide idea.
Unlike the analogous reaction using ammonium acetate to form acetamide, this reaction doesn't seem to be feasible in an open vessel, as the loss of ammonia is favoured over the loss of water, so you just end up with benzoic acid, all the ammonia having escaped...


I was testing solubility of polar vs. non-polar compounds. Sodium benzoate and ammonium benzoate are polar, so they dissolve in water but not in acetone, whereas benzoic acid is non-polar so it dissolves in acetone but not in water. I wanted to see this for myself. I also thought that if there was any trapped sodium chloride in the benzoic acid precipitate, it would not dissolve in the acetone and I could filter it out. I did filter the acetone, but didn't see anything left behind so apparently it was already fairly pure just from multiple washings with water.

I note almost no smell from sodium benzoate. Benzoic acid to me smells just like some sort of soda I used to drink years ago as a child. It must have been loaded with benzoic acid as a preservative. I only noted the rubbery smell in addition with ammonium benzoate, not benzoic acid. It is possible that this may be due to impurities in the acetone though. The acetone container says it contains denatonium benzoate, apparently to prevent consumption. I'm sure the amounts are very small, but may be enough to change the smell.

Here is one of the few YouTube videos I have found that mention ammonium benzoate.
https://www.youtube.com/watch?v=lzw2qbmYivg
The author heats a test tube containing ammonium benzoate and a thick vapor escapes. Author concludes (not sure why) that benzamide was formed based on this.



Ahhh.

Yes, I recall looking at that video; You'd certainly get some benzamide by heating in an open vessel, but not much, the clue is at 2 mins in the video:
https://youtu.be/lzw2qbmYivg?t=121

He says ammonium benzoate will lose ammonia on heating so it can't be evaporated to dryness; So strong heating will certainly drive off ammonia one would suspect.
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[*] posted on 29-3-2022 at 07:01


I noticed that when I was doing the development of this product for my company.
What was incredible was when we scaled-up this product to the manufacturing plant, using a reactor of about 4,000L, and the top of the solution was completely covered by this thin layer of product.
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[*] posted on 29-3-2022 at 14:25


Ammonium benzoate is quite an effective corrosion inhibitor for ferrous metals. It is used commercially in fairly large quantities for this purpose. It’s also listed as such in a number of patents.
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