Sciencemadness Discussion Board

Fractionally distilled benzyl chloride turns blue overnight

SuperOxide - 5-11-2021 at 13:41

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]

Tsjerk - 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]

S.C. Wack - 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.

AvBaeyer - 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.

AvB

SuperOxide - 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]

Amos - 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]

SWIM - 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.



SuperOxide - 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.

S.C. Wack - 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]

SuperOxide - 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.

Tsjerk - 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.

unionised - 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.

SuperOxide - 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]

SuperOxide - 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..

SuperOxide - 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.

S.C. Wack - 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.

SuperOxide - 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.

SuperOxide - 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]

Triflic Acid - 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

SuperOxide - 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.

SuperOxide - 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).

SuperOxide - 8-11-2021 at 06:11

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

Tsjerk - 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.

Texium - 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.

SuperOxide - 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.

Tsjerk - 8-11-2021 at 08:47

What I meant with dehydration was dehydration of a molecule, so making a new molecule with in total two hydrogen and an oxygen less, which is a known feature when working with sieves. Look at a acetone and sieves.

The above reaction could have been reversed when you added water, which removed the blue color with some time given.

Storing benzyl chloride over sieves shouldn't be a problem as the acid is only produced after hydrolysis, which won't happen without water. Drying the compound with sieves could be a problem as the water will be present in the sieves after drying, but that is not what you did.


[Edited on 8-11-2021 by Tsjerk]

S.C. Wack - 8-11-2021 at 14:57

Aldrich says their sieves have a pH of 10.5.

The color of these here is something, probably inorganic, probably bad for the chloride. My sieves certainly don't have this color, and IDK that any sieve intended for drying anything other than air does.

Find one reference of substance where someone dries a benzyl halide with sieves and bad things don't happen...shouldn't be too hard should it, there must be many thousand purifications of them out there.

SuperOxide - 8-11-2021 at 17:16

Quote: Originally posted by S.C. Wack  
The color of these here is something, probably inorganic, probably bad for the chloride. My sieves certainly don't have this color, and IDK that any sieve intended for drying anything other than air does.
I thought the same thing... I got them from here (you will notice that the sieves in the picture aren't the same color/size, lol).
When I got them I was just as suspicious, but I tested them out on alcohols and chlorinated solvents and they seemed to work just fine.
They also don't really have almost any sieve dust. I have some other higher grade 3A sieves that are more typical looking, but they do have some dust. So when I plan on storing something over sieves, I tend to use these ones. I should probably just buy more higher quality sieves, maybe use these ones in the desiccator or something.

Triflic Acid - 8-11-2021 at 21:19

Superoxide, I was thinking that it might have been leached out of the cap, the nitrogen-containing dye that is. So maybe try a new bottle?

SuperOxide - 9-11-2021 at 06:46

Quote: Originally posted by Triflic Acid  
Superoxide, I was thinking that it might have been leached out of the cap, the nitrogen-containing dye that is. So maybe try a new bottle?

yeah, that could have been it. It did continue to darken even when moved to a different bottle/cap though.

Also, neither the sample in the test tube or the 20mL sample in the smaller container has turned color. Oh well, I suppose I'll repeat the washing for the entire thing.

Edit: I still have that shitty purple cap, I'll put it on a 100ml bottle with some of the clear BnCl and see what happens.

[Edited on 9-11-2021 by SuperOxide]

SuperOxide - 11-11-2021 at 09:53

So, knowing that it wasn't the cap, it wasn't the sieves, and the color somehow went away after just redoing the workup (water wash, neutralization with sat. Na2CO3, dry with sat NaCl, dry further with anhydrous CaCl2), I figured it was ok to re-process the rest.
At first, I thought I could get by doing it quickly, and I only did a half assed water washing, and 1 Na2CO3 wash, and... it came out just slightly less green, but still green.
I went ahead and actually did a proper workup, and it seems to have worked thus far:


You can kinda tell that it has a slightly yellow color to it, and I'm honestly not sure if that's just because of the light in the picture, because it's much less noticeable in person.

Overall, thus sucks, lol. I lost ~70.2g worth of BnCl, which I worked my ass off to make, just because of some stupid problem that I'm not even sure is actually solved or will go away (the impurity may still be lurking in the BnCl).

But, regardless, I'm calling it quits. If I ever make BnCl again in the future, I will pay more attention to the boiling point of the distilling liquid and the storage container. I know the BP of BnCl is 179°C, but I kept collecting to like ~3 °C past that because the liquid distilling over still looked crystal clear (and I only trust my thermocouple to a few degrees of accuracy). Perhaps something came over that shouldn't have.

Thanks for the input guys. Im not sure I would have tried the water wash right off the bat. I may have jumped to a needless distillation. So that was helpful.