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Author: Subject: Avoiding Phosgene with Hexavalent Chromium and DCM
JJay
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[*] posted on 8-1-2018 at 07:53
Avoiding Phosgene with Hexavalent Chromium and DCM


I have a bottle containing some DCM and some likely benzyl alcohol from a months ago experiment involving oxidizing benzyl alcohol to benzoic acid (there is probably a low concentration of benzaldehyde in it though it was barely detectable by scent so I assume it is very low, and the reaction conditions were too hot and too wet to produce much benzaldehyde, and the chromic acid was purposefully used in excess). The solution is bright red. It was used to wash a basified solution of impure benzoic acid to remove organic impurities. I'd like to try to recover the DCM and possibly the benzyl alcohol, but I'm not sure exactly what the red color is or why some of the benzyl alcohol didn't react (my guess is that the untitrated drain cleaner acid I used just wasn't concentrated enough or that it had impurities that interfered with the reaction). I'd like to finish cleaning up from this experiment since I want to use the bottle that is holding the DCM extracts.

I'm having a hard time explaining exactly what the red color is... it seems to me that it just shouldn't be there. It is conceivable that it is some sort of addition product between the drain cleaner inhibitor and chromic acid.... But whatever it is, it is there, and I have to finish the cleanup eventually.

I have seen references to older syntheses of phosgene from dichromate oxidation of DCM. This is something I would like to avoid. Aside from opening the bottle in a fume hood, are there any special precautions I should take? Also, the presence of unreacted benzyl alcohol should greatly limit the concentration of phosgene, right?

My plan is to distill off the DCM and try to clean it up and recover it and then perhaps vacuum distill the benzyl alcohol, although I'll probably just dispose of it. Would it be a good idea to try to remove the red color with a reducing agent or perhaps more acid before trying the distillation or should I not bother?




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[*] posted on 8-1-2018 at 08:08


I would not worry about phosgene in this setup. The red color is due to dissolved CrO3. This dissolves in many solvents to some extent, it is a covalent compound.

If you want to recover the DCM, then I would shake it with a dilute solution of NaOH. This extracts the CrO3, which then dissolves in the solution of NaOH, forming Na2CrO4. This is an ionic compound and certainly will not dissolve in the DCM. Next you can separate the DCM from the aqueous layer and distill it from a drying agent to get dry DCM. Probably, shaking it with CaCl2 or MgSO4 will do the job. The DCM has a very low boiling point, so it should be easy to distill it from the solid material, even if some clumped mess of the solid is obtained.




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JJay
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[*] posted on 8-1-2018 at 08:33


I will try NaOH to remove the red color... previously I used Na2CO3, which did not remove it....



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[*] posted on 8-1-2018 at 08:48


It could be that the colour is due to some organic by-products.
random speculation on my part would suggest some quinoid structure

Phosgene is not stable in the presence of water. If there's an aqueous layer in the bottle then there's probably no phosgene.
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[*] posted on 8-1-2018 at 09:00


You'll smell the wet-straw smell of phosgene, too.



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NEMO-Chemistry
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[*] posted on 8-1-2018 at 10:01


The one and only time i got drain quality sukphuric acid that was dark red! looked like someone had over did the dye thing!!

Is good to recover what you can, i hate it when people just throw things down the sink!

[Edited on 8-1-2018 by NEMO-Chemistry]
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JJay
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[*] posted on 8-1-2018 at 15:23


I'll add some water to the bottle in the fume hood and keep it capped overnight just in case it does contain some phosgene. I didn't dry it after the extraction, so it probably does contain traces of water, but adding some can't hurt....



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[*] posted on 8-1-2018 at 16:26


Quote: Originally posted by JJay  
I'll add some water to the bottle in the fume hood and keep it capped overnight just in case it does contain some phosgene. I didn't dry it after the extraction, so it probably does contain traces of water, but adding some can't hurt....

Good idea, i cant remember the other stuff that is supposed to kill off phosgene, green crystals and an iron compound used to clear moss i think???

Dont quote me FFS lol.

Lets face it phosgene isnt something to take chances with.
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