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Author: Subject: Fractionally distilled benzyl chloride turns blue overnight
SuperOxide
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[*] posted on 5-11-2021 at 13:41
Fractionally distilled benzyl chloride turns blue overnight


I just finished making some benzyl chloride yesterday. The stuff is pretty vile, and I wanted to make a decent amount so that I hopefully wouldn't have to make it for a while again (hopefully ever). I did a couple crude synthesis, then combined the crude products after they were dried, and fractionally distilled it all. The crude product was yellow, and after distillation the distillate was perfectly colorless (and the mixture in the distillation pot turned green, oddly enough).
I put some 3A molecular sieves in the container with the BnCl so it would be totally anhydrous (even though I had some calcium chloride in the receiving flask, which seemed to work well.

Note: This may look slightly discolored, but I think it's just because of the refractive index of BnCl and the fact the sieves are in there. When I held this up to the light, it was absolutely colorless.


Then I put it on the shelf and go to sleep. The next morning when I go to check it out... it turned a slight shade of blue..


I am pretty doubtful that it was the 3A molecular sieves, but just to be safe, I moved them to a new container after filtering off the sieves.



Has anyone had this happen? Pretty upsetting. I made a bunch of BnCl so I wouldn't have to do it again, purified/dried it as best I could, and it somehow went blue over night?! >_<
I have a couple things planned for this (denatonium, benzyl cyanide), this should still probably work out alright though. I might try re-distilling it using a short path distillation head, but I'm not certain how volatile the blue colorant is, and if it will come over or not.

I guess it could be worst.... it could go yellow. But still, lol.

[Edited on 5-11-2021 by SuperOxide]




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[*] posted on 5-11-2021 at 13:51


I don't know what it is, but I think you can best wait until someone here does before distilling again. The distillation might not work and then you did it for nothing. I guess it is some impurity you can get rid of by treating it somehow, if necessary. I think it is something you dehydrated with the sieves, or something dehydrated which reacted with the chloride.

Edit: my message looked like j_sum's :D

[Edited on 5-11-2021 by Tsjerk]
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S.C. Wack
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[*] posted on 5-11-2021 at 15:59


Who recommends treatment of this with sieves?

BTW this should not be distilled without vacuum, even though it's been done (e.g. Vogel).

I'd wash with water in solvent and see what the pH is...maybe it's too late...might fix the color.




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[*] posted on 5-11-2021 at 17:11


How did you make your BnCl? More details are needed to assess your problem. I have made BnCl many times from BnOH + conc HCl and never seen any coloration. I posted my results of this synthesis procedure somewhere here. Wack is correct - distillation must be under vacuum.

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[*] posted on 5-11-2021 at 17:26


Quote: Originally posted by S.C. Wack  
Who recommends treatment of this with sieves?

I haven't seen it in any of the preps that I read, but in NileReds video, he uses them, and I don't recall the BnCl being blue when he used them later, lol.
Granted, even though my sieves are new, they are probably a different material.

Quote: Originally posted by S.C. Wack  
BTW this should not be distilled without vacuum, even though it's been done (e.g. Vogel).
Why not? It seemed to work alright - Just took a lot of heat to get it up the fraction column. I do have a decent vacuum pump though, so I could re-do it if needed.

Quote: Originally posted by S.C. Wack  
I'd wash with water in solvent and see what the pH is...maybe it's too late...might fix the color.
Good idea. I'll give it a shot after I look into it more.

Quote: Originally posted by AvBaeyer  
How did you make your BnCl? More details are needed to assess your problem. I have made BnCl many times from BnOH + conc HCl and never seen any coloration. I posted my results of this synthesis procedure somewhere here. Wack is correct - distillation must be under vacuum.

AvB

I used USP grade benzyl chloride, and an excess of ~34% HCl, refluxed for ~15-20 mins, neutralize with sodium carbonate, dry with saturated salt then calcium chloride, then fractional distillation and re-dried with calcium chloride... Then the sieves :-(

[Edited on 6-11-2021 by SuperOxide]




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[*] posted on 5-11-2021 at 18:23


Could it be contaminants introduced by the cap, or pigments from the cap itself? In the lab I used to work at this happened with a couple of chlorinated solvents, but only when they had blue or purple caps; nothing happened to those with the classic yellow-orange ones.

[Edited on 11-6-2021 by Amos]
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[*] posted on 5-11-2021 at 19:19


The stuff left over in the pot was green?

That's a combination of blue and the yellow.

Sounds like whatever's going on in the product might have also happened, but faster, in the boiling flask.







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[*] posted on 5-11-2021 at 20:03


Quote: Originally posted by Amos  
Could it be contaminants introduced by the cap, or pigments from the cap itself? In the lab I used to work at this happened with a couple of chlorinated solvents, but only when they had blue or purple caps; nothing happened to those with the class yellow-orange ones.

Lol... I was actually thinking that. This purple cap is Pyrex, but it think it's pretty old. It scratches pretty easily... I actually threw it out after I moved the BnCl to a new bottle.
You may very well be right.

Quote: Originally posted by SWIM  
The stuff left over in the pot was green?

That's a combination of blue and the yellow.

Sounds like whatever's going on in the product might have also happened, but faster, in the boiling flask.


So it was yellow at first as the distillation was under way, then when it was over, it was orange. I let it cool off for an hour or so and by the time I went to clean it up it was a dark green (pic) - But the distilled BnCl was still crystal clear at this point. I get why you would think this is the case, but I think its more likely either the fucke dup old Pyrex GL45 cap, and/or the 3A sieves.




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[*] posted on 5-11-2021 at 20:41


Benzyl chlorides and esp. bromides are not perfectly thermally stable. (nor inert, much like molecular sieves) This is worse with certain impurities and better with others. IIRC there was a post at the-hive where someone spoke of redistilling their chloride in nice clean glass only to find HCl pouring out upon heating and condensation products, no one could figure this out; no problems the first time.

I'd go with what Armarego and Perrin says over NileRed. (not so much the P2O5 or CaH2 part)

[Edited on 7-11-2021 by S.C. Wack]




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[*] posted on 5-11-2021 at 21:14


Quote: Originally posted by S.C. Wack  
Benzyl chlorides and esp. bromides are not perfectly thermally stable. (nor inert, much like molecular sieves) This is worse with certain impurities and better with others. IIRC there was a post at the-hive where someone spoke of redistilling their chloride in nice clean glass only to find HCl pouring out upon heating and condensation products, no one could figure this out; no problems the first time.

I'd go with what Armarego and Perrin says over NileRed. (not so much the P2O5 or CaH2 part)


Hm, interesting... I don't recall having any of those issues. Ill see if it gets even darker over night, if not, then it was probably the sieves and/or cap.

I really hope I don't have to redo the whole synthesis... I only got like 77% yield, which isn't terrible I suppose. It's honestly not even that it's a lot of work, it just sucks dealing with the smell. lol.

I actually was doing just fine until my dumbass decided to use a plastic keck clip on the still head connection to the condenser (all others were metal before that), and it melted enough that the condenser came loose just a bit. Suddenly my eyes started to tear up a bit and I fixed it pretty quick. It also didn't last very long - Still though, what a vile substance.




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[*] posted on 6-11-2021 at 04:13


I wouldn't redo the synthesis before a vacuum distillation and storage in something that isn't blue or purple. The sieves are probably fine, but you could store without sieves, see if the color is fine, and add them later to check for the color again.
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[*] posted on 6-11-2021 at 04:17


A single bead of "indicating molecular sieve" would add enough cobalt to explain the colour.
You say the residue is green.
Please add a little water to it and see if it changes.
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[*] posted on 6-11-2021 at 07:44


Quote: Originally posted by Tsjerk  
I wouldn't redo the synthesis before a vacuum distillation and storage in something that isn't blue or purple. The sieves are probably fine, but you could store without sieves, see if the color is fine, and add them later to check for the color again.

Ok, ill give that a shot today. I might do it on a smaller scale (test tube size) to see if it helps, before processing >200mL of this crap.

Quote: Originally posted by unionised  
A single bead of "indicating molecular sieve" would add enough cobalt to explain the colour.
But these weren't the "indicating" type. Just typical 3A molecular sieves.


Quote: Originally posted by unionised  
You say the residue is green.
Please add a little water to it and see if it changes.

The residual reaction mixture left in the pot was green. I already neutralized that and threw it out. I don't recall it turning blue when i added the NaOH solution in there though.

[Edited on 6-11-2021 by SuperOxide]




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[*] posted on 6-11-2021 at 18:46


Quote: Originally posted by SWIM  
The stuff left over in the pot was green?

That's a combination of blue and the yellow.

Sounds like whatever's going on in the product might have also happened, but faster, in the boiling flask.


You sir... may be right.

I haven't had time to do the water washings today, but I just took a quick peek to see what was up, and.... The fuckin thing has gone green..




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[*] posted on 6-11-2021 at 19:01


Actually, I have the time, I just didn't want to deal with the smell tonight, lol.

So here it is, I did a quick water wash - No help at all. The aqueous layer remained clear/colorless, and the bottom layer was still green (but turbid):


And I checked the pH of the water layer as well, it seems pretty neutral (looks like it's maybe slightly yellowish/6, but it looked damn near neutral to me IRL):


Next time I have a weekend to do nothing but chemistry again, I will probably re-distill it under vacuum.




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[*] posted on 6-11-2021 at 20:36


Assuming that these sieves don't add color to other organics, it would be interesting to see if the same thing happens to perfectly pure chloride. I assume that the color of the sieves is due to the clay binder? Reddish brown clay contains what...

..on that note, if one makes the chloride from straight hardware store acid, it might be wise to wash (perhaps starting with dilute base) and filter the product well (after drying with a neutral drying agent) before distilling.




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[*] posted on 6-11-2021 at 20:54


Quote: Originally posted by S.C. Wack  
..on that note, if one makes the chloride from straight hardware store acid, it might be wise to wash (perhaps starting with dilute base) and filter the product well (after drying with a neutral drying agent) before distilling.

Yep, I did all the above.




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[*] posted on 6-11-2021 at 22:17


Another update... Turns out, I just wasn't patient enough. Letting it sit with some water for a couple hours seems to have taken care of the color.



I would expect that if the color was pulled out, then it would have changed the color of the aqueous layer... but it doesn't look like it has. Interesting.

[Edited on 7-11-2021 by SuperOxide]




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[*] posted on 7-11-2021 at 08:26


Could be a nitrogen-containing dye, that is reacting with the HCl produced by the BnCl hydrolysis, becoming a colorless salt that is water-soluble compared to the freebase in the BnCl. You might want to try distilling out of an HCl solution



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[*] posted on 7-11-2021 at 09:15


Quote: Originally posted by Triflic Acid  
Could be a nitrogen-containing dye, that is reacting with the HCl produced by the BnCl hydrolysis, becoming a colorless salt that is water-soluble compared to the freebase in the BnCl. You might want to try distilling out of an HCl solution

How would the nitrogen get there? The HCl was mostly reagent grade 37% HCl, but since I was running low on it, I did have to use a small amount of the Klean Strip Muriatic Acid that I had, but not very much. I've used the klean strip HCl for quite a few things (whenever dilute HCl is needed as to not waste my 37% stuff), and I haven't ever had anything like this happen. Im just not sure where any nitrogen dye/contaminant would have come from.

Quote: Originally posted by Triflic Acid  
You might want to try distilling out of an HCl solution

The BnOH+HCl reaction was done with an excess of HCl. So I'm not entirely sure that this would help.

Im going to separate the BnCl from the aqueous layer in the small test bottle, I'll then dry it with some calcium chloride and store it separately to see if it turns blue/green again over night.




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[*] posted on 7-11-2021 at 12:51


So I removed the water layer, neutralized it with sodium carbonate (though I don't think it needed it, no fizzing happened either), dried it with saturated NaCl then with calcium chloride, and here's the result:



I'm going to keep this separate to see what happens. But I think whatever impurity/reaction caused the BnCl to turn blue/green is probably still there, since the only thing that was done was a water wash (which was already done before).




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[*] posted on 8-11-2021 at 06:11


So it's the next morning - It hasn't changed color at all. Still clear. hmmm.... Very odd.



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[*] posted on 8-11-2021 at 06:36


What happens when you add sieves again? It could still be something that is in there and turns blue when dehydrated.
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[*] posted on 8-11-2021 at 07:13


Sieves are not inert, and actually slightly basic, so it could be that the blue color is a product of something in the sieves reacting with traces of HCl from your product, or with the benzyl chloride itself. Storing acidic or acid-producing compounds over sieves is generally not recommended.



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[*] posted on 8-11-2021 at 07:35


Quote: Originally posted by Tsjerk  
What happens when you add sieves again? It could still be something that is in there and turns blue when dehydrated.
Yeah, that would be odd though because it was pretty dry before I added the sieves (saturated NaCl sol. then anhydrous CaCl2), and it continued to change color even when I removed the sieves (went from a light blue to a darker green, as seen in the pics above).
But regardless, I have put a small sample of the re-processed BnCl into a test tube with some of the same (new and recently dehydrated) 3A molecular sieves, lets see what happens.


Quote: Originally posted by Texium  
Sieves are not inert, and actually slightly basic, so it could be that the blue color is a product of something in the sieves reacting with traces of HCl from your product, or with the benzyl chloride itself. Storing acidic or acid-producing compounds over sieves is generally not recommended.

Right, I did know that you can't use sieves to dry acids, but I was pretty sure I removed any acid from the BnCl with several Na2CO3 washes, and the pH didn't read acidic.




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