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Author: Subject: Need help identifying a mysterious product.
CaptainMolo
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[*] posted on 25-2-2018 at 17:27
Need help identifying a mysterious product.


Okay so I have a perplexing mystery on my hands that I am hoping that one of you may be able to shed some light on. I have been attempting to produce Glacial Acetic Acid from Sodium Acetate and Sulfuric Acid.

During my work, I used Dichloromethane to extract the concentrated Acetic Acid from the distillate after the main reaction between the Sodium Acetate and Sulfuric Acid. I had read that this was an easy way to remove the bulk of the water before moving on to other techniques. I then dried this mixture with Magnesium Sulfate and then distilled off the Dichloromethane.

After I recovered the Dichloromethane, I attempted to dry the Dichloromethane with Sodium Hydroxide, under the impression that the Chlorines on the molecule wouldn't want to react, but any residual Acetic Acid would react to form Sodium Acetate. Following this I planned to redistill the Dichloromethane so that I would have pure and dry DCM for future work. In hindsight, I should have tried a metal carbonate first, as they are more mild bases and sufficiently hygroscopic to remove the small amount of water that should have been present.

After I added the solid Sodium Hydroxide, some heat was generated. I initially attributed this to the heat generated from Sodium Hydroxide absorbing water from the DCM, but it quickly became more intense to the point where the DCM began to boil and reflux in the flask.

Shortly after this happened, I started to see an immiscible liquid forming on the surface of the DCM. I thought this was simply a solution of Sodium Hydroxide and water, but needle shaped crystals started to form in the liquid.

I quickly ran the mixture through a filter to get it off of the NaOH and stop this unknown reaction. I was worried it might go to thermal runaway or any number of things considering I had no idea what was happening.

A small mass of needle shaped crystals were caught in the filter, but a larger portion of crystals formed in the filtrate in the beaker.

I began trying to characterize the compound that was formed, it appears to have a relatively high melting point, I tried burning a small amount with a lighter, and it was not flammable, and only melted after a few seconds of direct heat.

It appears to be fairly soluble in water, has a very faint odor, which is reminds me of something I can't quite put my finger on. It's ethereal and somewhat sweet.

Another odd property is that whilst trying to figure out what it was, I got some on my gloves, and it appears to be able to pass through the nitrile and affect my fingers. It caused a sensation I can only describe as that which one gets from the "Icy Hot" product used for muscle pain.

One thing that should be mentioned is that my DCM was distilled from a paint stripper product. I took great care during the distillation, and the only other volatile compound listed in the MSDS was Methanol. It's possible that there's some compound that came over during the distillation that caused this reaction, but I don't know what it would be.

I am planning on running a test reaction to produce more of this compound to figure out what is going on here. When I do I'll be using rigorously purified Acetic Acid, DCM and Sodium Hydroxide, to try to rule out an unknown impurity as the source of the reaction.

Needless to say I'm stumped. I'm not the most well versed chemist out there, so I may be overlooking something obvious, but if anyone has any ideas of what this compound is, I'd love to hear some theories. Thank you for taking the time to read this and give it some thought.




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[*] posted on 26-2-2018 at 03:38


@BaFuxa
The reverse is true. Methanol is miscible in DCM. The two also form an azeotrope at around 7% methanol with a BP a couple of degrees lower than DCM's.
Around here the standard paint strippers are labelled as 930g/L of DCM and 70g/L methanol plus dyes, polymer gels, and who knows what else junk. This means they are basically azeotropic. When you distil the methanol comes off with the DCM. This is not usually a problem for most applications but is worth knowing.




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Boffis
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[*] posted on 26-2-2018 at 04:12


The tuff passing through your gloves is almost certainly residual DCM, its notorious for this. As to the reaction of sodium hydroxide with DCM, the initial product should be formaldehyde but the reality is probably that the very reactive carbo cations produced transiently will react with anything that is around and from your comments we might expect at least methanol, methyl acetate, acetic acid, Na salts etc to be present. My money I think would go on it being sodium acetate contaminated with DCM as a first hypothesis. And the ethereal smell DCM+ methyl acetate.
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[*] posted on 26-2-2018 at 13:39


I am not sure that DCM can be hydrolyzed so quickly by NaOH (to NaCl and formaldehyde), but formaldehyde, if it is formed, quickly reacts with excess NaOH to give sodium formate HCOONa and methanol (Canizzaro reaction).
So, your needles can be both NaCl or HCOONa.

[Edited on 26-2-2018 by Outer]
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[*] posted on 26-2-2018 at 14:32


"nother odd property is that whilst trying to figure out what it was, I got some on my gloves, and it appears to be able to pass through the nitrile and affect my fingers. It caused a sensation I can only describe as that which one gets from the "Icy Hot" product used for muscle pain. "

DCM will do that.

You might get DCM to react with a strong base
https://en.wikipedia.org/wiki/Carbene

And I wonder if the needle shaped crystals are hydrated NaOH, or even NaOMe
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CaptainMolo
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[*] posted on 27-2-2018 at 07:29


These are some great suggestions, I will look up some of the properties of these compounds and run some tests, possibly this evening if I have enough time. I think a worthwhile starting point would probably be solubility in a couple of solvents, melting point and it's reactions with acids. I will post my results when I have them.

Thinking about it, I think the most likely candidate at the moment would be sodium acetate with a DCM impurity, maybe trapped inside from crystallizing rapidly. I looked up the crystal structure of sodium acetate, and it's listed as monoclinic on Wikipedia, which when small could very much look like needles.

If this is the case, then a non-trivial amount of Acetic Acid must have distilled over with the DCM, which is surprising considering their vast difference in boiling points. It might be that they form an azeotrope of which I was unaware. I will do some searching and see if anyone has written about that specifically.

Sodium Methoxide is listed as hexagonal crystals, so I don't think it's that, however I did notice a smell when it dissolves in water, which I was unsure if it's related to the impurities in the crystals, or some kind of hydrolysis reaction, so I'm not going to rule it out entirely as a possible product.

Thank you everyone for your input so far, at least one mystery is solved, the "Icy Hot" sensation sounds like it definitely came from the DCM. That narrows the field considerably now that I don't have that to consider.







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