Pentaborane
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Unexpected Side Reactions with Hazardous Products
I figured I'd share my most recent mishap as my first "real" post here. This of course happened in a professional setting, but it could easily have
happened during one of my own projects (and possibly in a much more hazardous setting).
I was running the typical reaction of converting a carboxylic acid dissolved in dioxane to an acyl chloride with SOCl2. Needless to say, this was done
in a good fume hood with the off-gas being handled by an NaOH scrubber. Having run this reaction multiple times before, it didn't seem very special,
so I went to focus on other things (like vacuum pump oil being fouled by some kind of unidentifiable solids).
The surprise came later when it was found that the reaction mixture was full of bis(chloroethyl) ether. The significant toxicity/carcinogenicity of
this product made this notable. Of course, this was not in any of the other runs which used dioxane from another (now empty) 4 L carboy. It was later
found that the almost a third of the dioxane wasn't dioxane at all, it was diethylene glycol!
The morals of this story are as follows:
1. Always check your reagents to be sure they are what you think they are, even "inert" solvents. Needless to say, we will now be doing QC on most new
solvents.
2. Be prepared for unexpected side reactions, even if this is your fifth run.
In this case, bis(chloroethyl) ether is quite toxic and carcinogenic and was handled accordingly once discovered. It can be thought of as the oxygen
analog of sulfur mustard (although it isn't quite as reactive).
[Edited on 20-7-2023 by Pentaborane]
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Loptr
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I have never seen bis(chloroethyl) ether. How did you identify it as a product? Did you have to work it up?
"Question everything generally thought to be obvious." - Dieter Rams
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Pentaborane
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It was a high-boiler during solvent recovery and was identified with GC-MS. The very poor yield was also a good indication that something wasn't
right.
It is now sitting in the stock room after being distilled again (about 250 mL). No point in letting it go to waste at the significant expense of
disposal, I may try using it to make crown ethers later.
There are worse things as far as toxicity, but it still isn't something you want to make inadvertently without being prepared for it.
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BromicAcid
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When I was in college (way back in the early 2000's) my final project was the study of the template effect using various combinations of chloro ethers
and alcohols to make 18-crown-6. Of course that includes the traditional synthesis using bis(chloroethyl) ether. I'd read the cautions and
approached my supervisor since I was running these on the multi-liter scale. He shrugged it off, "Everything we work with causes cancer, we just
don't know it yet."
Not being blase to the situation, it's in interesting side reaction. More than anything it just points to making sure you wear proper PPE at all
times and don't take chances with chemical exposures.
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Pentaborane
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I'm sure the product is a carcinogen by current standards, along with most of the reagents used in its synthesis. The final product will actually be
electrofluorinated to something that probably easily falls into "PFAS derivative" territory.
The bis(chloroethyl) ether itself doesn't concern me that much, we work with much nastier things regularly (note the electrofluorination step above).
After all, I was perfectly comfortable saving it for later. My main issue was that it popped up unexpectedly, especially in a relatively routine
synthesis where people can easily get complacent.
I do think we need to be a little more cautious of impurities that can cause hazardous side reactions, even from reputable suppliers with a COA. We
get a bit complacent here as well. We had some issues with tributyl phosphate recently as well.
[Edited on 21-7-2023 by Pentaborane]
[Edited on 21-7-2023 by Pentaborane]
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