Download
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Improving chloroform yield with acid.
Hi, mostly new to the forum. I did loads of (in hindsight quite dangerous) chemistry as a kid. I recently got back into it with a goal of synthesising
as many elements as I can that I can't purchase locally. Fireworks too. I've spent far too much money on glassware.
One of the first things I did was make chloroform. I don't immediately have a use for it but it's probably the easiest/cheapest to obtain strong
solvent here in Australia (DCM paint stripper is expensive).
I tried the calcium hypochlorite method first but moved on to NaClO because of the annoying CaOH precipitation. I purchased 12.5L of 125g/L chlorine
NaClO for pools from Bunnings, cooled the whole thing in an ice bath and slowly added acetone. Seeing how easily the reaction can run away, I only
added more acetone once it got below 20C and tried to avoid going over 30C. It took about 4 hours to add the acetone and by that point it was late
enough that I left it until the next morning. Once I separated it the next morning I ended up with only ~750g of chloroform instead of the calculated
1470g.
I suspect some migrated though the HDPE container given the cooling water smelt like chloroform but I'm sure much more was lost to hydroxide
decomposition. I was thinking of adding an acid next time to keep the pH lower.
Weirdly, while looking for info on improving the yield I an across a relatively recent post on this forum from a long time member saying that the
haloform reaction is a net acidifier and they suggested adding sodium hydroxide before hand... This defies what i know about acetone haloform
reactions as I'm pretty certain a hydroxide salt is a reaction product.
Now, I know hydroxide ions are needed for the reaction to take place so it does need to remain basic, but given the reaction produces hydroxide I
think getting very close to 7 pH is probably a good idea in terms of reducing decomposition.
Interesting I found this paper on chloroform decomposition in potassium hydroxide. While they don't explore alkaline decomposition too much, it does show that the
decomposition rates are very high at 7.1*10^-1 mol/L.min at ~12 pH, 2.2*10^-2 mol/L.min at ~10pH and 1.8*10^-4 mol/L.min at ~8 pH. The rates drop to
basically nothing in acidic conditions.
I have to wonder if the net acidifier belief came from the fact that chloroform breaking down consumes hydroxide ions. I'm not sure if posting a link
to the thread is appropriate so I won't.
So has anyone tried improving yield by adding small mounts of acid to control pH? Or maybe I've got this all wrong and it really is a net acidifier.
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Tsjerk
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Welcome to the forum! Always nice to see new people, especially when they think about their experiments.
More references are more better, so I would post a link to the thread.
It would be interesting for a starter to determine the pH of the bleach you have. It is probably a good idea to make a dilution series and measure the
different dilutions. Good chance measurements of highly concentrated solutions not to be too accurate.
The decomposition of chloroform by hydroxides you talk about is described at 100 degrees, does it really happen at low temperatures?
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Amos
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Quote: Originally posted by Tsjerk  |
The decomposition of chloroform by hydroxides you talk about is described at 100 degrees, does it really happen at low temperatures?
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It definitely happens, that's why we leave the leftover reaction mixture in the bleach bottle for a couple days after the synthesis before dumping it
down the drain (as an environmental measure). A short time afterward no smell of chloroform can be noticed. In my experience yield also greatly
suffers if I come back to separate the layers more than 12 hours after the preparation.
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Tsjerk
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| Quote: Originally posted by Amos |
It definitely happens, that's why we leave the leftover reaction mixture in the bleach bottle for a couple days after the synthesis before dumping it
down the drain (as an environmental measure). A short time afterward no smell of chloroform can be noticed. In my experience yield also greatly
suffers if I come back to separate the layers more than 12 hours after the preparation. |
Then maybe add acid while adding the acetone? I thought about using a buffer salt like HPO42-, but the concentrations are way
too high to be buffered. Adding acid to acetone will probably make the acetone polymerize. Then possibly adding acid to react with the hydroxide could
improve yield.
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Amos
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| Quote: Originally posted by Tsjerk | | Quote: Originally posted by Amos |
It definitely happens, that's why we leave the leftover reaction mixture in the bleach bottle for a couple days after the synthesis before dumping it
down the drain (as an environmental measure). A short time afterward no smell of chloroform can be noticed. In my experience yield also greatly
suffers if I come back to separate the layers more than 12 hours after the preparation. |
Then maybe add acid while adding the acetone? I thought about using a buffer salt like HPO42-, but the concentrations are way
too high to be buffered. Adding acid to acetone will probably make the acetone polymerize. Then possibly adding acid to react with the hydroxide could
improve yield. |
My approach has always been "chloroform is already expensive to make as a solvent, I'm not spending more on additives to increase yield by another
5%". As long as you carefully control heat (freezing your bleach is a must) and process the chloroform in about 8-12 hours you'll consistently get
70-80%.
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Texium
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If you look at the first step of the haloform reaction mechanism, you’ll see that base is required to deprotonate the ketone to form its enolate.
This will not happen to an appreciable extent at pH 7... otherwise we’d see mixtures of acetone and water spontaneously polymerize. Yes, the base
will degrade the chloroform, but it’s also necessary for its formation. As Amos said, the key to success with the haloform reaction is temperature
control and proper timing.
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Download
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It still sounds like an opportunity to increase yield by a bit more than 5%. Once the reaction is done you could neutralise the base with HCl and then
distil the remaining product dissolved in the water with minimal degredation. Assuming the numbers on Wikipedia are correct, chloroform's solubility
is 10g/L at 0C. In my case of 12.5L NaClO that's immediately 125g more chloroform or a yield increase of ~15%. HCl isn't expensive.
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Mush
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| Quote: Originally posted by Download | | It still sounds like an opportunity to increase yield by a bit more than 5%. Once the reaction is done you could neutralise the base with HCl and then
distil the remaining product dissolved in the water with minimal degredation. Assuming the numbers on Wikipedia are correct, chloroform's solubility
is 10g/L at 0C. In my case of 12.5L NaClO that's immediately 125g more chloroform or a yield increase of ~15%. HCl isn't expensive.
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I am sure the trapped CHCl3 can be pushed out from the water phase (aka salting out) with saturated NaCl solution as well.
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draculic acid69
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Or distilled off.
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Fyndium
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https://www.youtube.com/watch?v=heqMdYqqMGs
They appear to be getting high yield from this process?
They make a slurry from calcium hypochlorite and water, mix in acetone and distill off chloroform, add dilute NaOH (to neutralize?) and CaCl2 (to dry?
no layer separation showed) and then redistill the chloroform.
Considering that calcium hypochlorite is a lot cheaper per unit than solution of bleach, this should be better method by all means.
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XeonTheMGPony
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| Quote: Originally posted by Fyndium | https://www.youtube.com/watch?v=heqMdYqqMGs
They appear to be getting high yield from this process?
They make a slurry from calcium hypochlorite and water, mix in acetone and distill off chloroform, add dilute NaOH (to neutralize?) and CaCl2 (to dry?
no layer separation showed) and then redistill the chloroform.
Considering that calcium hypochlorite is a lot cheaper per unit than solution of bleach, this should be better method by all means.
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That's the method I prefer, clean simple and effective.
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Amos
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Since the topic of using calcium hypochlorite is fresh in the thread, are there any US members that know of a consumer product that is nearly pure
calcium hypochlorite? I've seen EU and Australian members claim that they find it as a single chemical, but all of the pool shock products I've seen
here have a maximum of around 57%, with impurities of calcium chloride and calcium hydroxide.
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mr_bovinejony
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Dry tec shock claims 68%
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Panache
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| Quote: Originally posted by XeonTheMGPony | | Quote: Originally posted by Fyndium | https://www.youtube.com/watch?v=heqMdYqqMGs
They appear to be getting high yield from this process?
They make a slurry from calcium hypochlorite and water, mix in acetone and distill off chloroform, add dilute NaOH (to neutralize?) and CaCl2 (to dry?
no layer separation showed) and then redistill the chloroform.
Considering that calcium hypochlorite is a lot cheaper per unit than solution of bleach, this should be better method by all means.
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That's the method I prefer, clean simple and effective. |
Yeah...thats one weird video, love the piano.
Pharmacists certainly ain't chemists
Xeon if you use this method have you abbreviated it in any way. What i find odd is that they begin distilling off chloroform immediately, however
wouldn't acetone come over initially, without at least sometime to reflux/react??
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RustyShackleford
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| Quote: Originally posted by Amos | | Since the topic of using calcium hypochlorite is fresh in the thread, are there any US members that know of a consumer product that is nearly pure
calcium hypochlorite? I've seen EU and Australian members claim that they find it as a single chemical, but all of the pool shock products I've seen
here have a maximum of around 57%, with impurities of calcium chloride and calcium hydroxide. |
I titrated my "pool shock" in northern europe to 82%
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