SuperOxide
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Odd crystals growing in condenser during BnCN reaction reflux
I'm in the middle of making some BnCN at the moment (before anyone asks, I have very good ventilation, wearing full protective gear, im being as safe
as I can. I know how dangerous this is). I'm using the BnCl I made a bit ago (which I had a thread on as well), some rather pure EtOH I got once I
dried and distilled everclear, and some reagent grade NaCN.
The reaction seems to be going just fine. I have the flask temp ~100 °C (measured using an IR thermometer on the outside of the flask), and the
reflux ring was in just the first 2 inches of the condenser (right after the water hits the glass). However, even though the reflux ring was so low, I
saw a little bit of condensation on the hose going outside from the top of the condenser. I assumed this may have been some EtOH, so I lowered the
temp 10 °C.
About ~20 minutes after that, I observed some crystal growth on the inside of the condenser right where the reflux ring was previously at (with the
new reflux ring now just below that). As time goes on, the crystals seem to grow more and more, and now they're slightly changing color (getting
brownish).
Pics:
   
I honestly am not sure what this could be.... The temp of the water coming out of the condensers circulation is just around 22 °C, and the flask is
only at maybe 100-110 °C (just hot enough for a modest reflux rate). I don't see how this can be BnCN crystalizing out as that's not a solid at this
temperature. I don't see how NaCN could have gotten up there (its not splashing at all really and it wouldn't distill up there obviously).
Has anyone had this occur? Other than this oddity, the synthesis is going just fine.
Update: The reaction has finished, and everything in the flask looks A-OK. Once I let the flask cool down and turned off stirring, the crystals in the
condenser darkened a bit, and some brown gunk above it formed (presumably from some of the crystals melting?)
As usual, here's a pic:
[Edited on 18-11-2021 by SuperOxide]
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Tsjerk
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Did you try to add sieves to the not anymore blue BnCl again? Maybe you have an impurity that dissolves in BnCl with the same BP? Which turns blue
when you dehydrate the molecule with sieves?
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SuperOxide
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Quote: Originally posted by Tsjerk  | | Did you try to add sieves to the not anymore blue BnCl again? Maybe you have an impurity that dissolves in BnCl with the same BP? Which turns blue
when you dehydrate the molecule with sieves? |
Nope, I sure didn't. It's the next day now and the solids in the condenser have turned very dark brown:
I have no idea what that is, but it obviously can't be the product I'm going after, so I'll just discard it after handling it with my other CN waste.
But related to that - It's worth noting that it would appear that it wasn't the sieves that caused the color change. Since after I re-purified the
BnCl and added some sieves to a small test of ~5mL, no color change was observed.
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DraconicAcid
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Benzyl chloride plus cyanide give PhCH2CN, right? That could hydrolyze to phenylacetic acid or its amide, I think, and the latter should be a solid.
Not an organic chemist- just guessing.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Tsjerk
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I find it strange you get something apparently volatile to solidify while it doesn't dissolve in ethanol.
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kmno4
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These crystals are surely a sign of impure chemicals 
What can it be - I do not know, maybe some ammonia salt, maybe something completely diffrent.
I made BnCN several times, with several modifications (NaCN, KCN, with/without water, with/without addition of PTC, KI ... etc).
The result was always the same: more or less stinky and brownish post reaction mixture. In many cases unreacted benzyl chloride was detected - it is
better to extend time of reaction, even when larger excess of cyanide was used.
Admixture of isocyanide causes very unpleasent odor of the raw product.
But when purified and distilled, BnCN has rather pleasant odor, somehow reminding me a mixture of benzaldehyde and HCN 
But - in no case such crystalline material was observed in water cooler, at any stage of reaction.
[Edited on 18-11-2021 by kmno4]
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SuperOxide
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| Quote: Originally posted by kmno4 | These crystals are surely a sign of impure chemicals
What can it be - I do not know, maybe some ammonia salt, maybe something completely diffrent.
I made BnCN several times, with several modifications (NaCN, KCN, with/without water, with/without addition of PTC, KI ... etc).
The result was always the same: more or less stinky and brownish post reaction mixture. In many cases unreacted benzyl chloride was detected - it is
better to extend time of reaction, even when larger excess of cyanide was used.
Admixture of isocyanide causes very unpleasent odor of the raw product.
But when purified and distilled, BnCN has rather pleasant odor, somehow reminding me a mixture of benzaldehyde and HCN
But - in no case such crystalline material was observed in water cooler, at any stage of reaction. |
Yeah, I figured. Oh well. I don't think I'll get much crude out of this since it's just a pilot scale. I want to perform a vacuum fractional
distillation to get the pure BnCN, but I think I should get more crude before doing that.
If/when I run this again, I will step up the scale a little bit, and be sure to reflux for longer than several hours. Hopefully that will help bump
the yield up a bit.
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SuperOxide
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Just an update, when cleaning out the condenser, that stuff wouldn't come off with water, soap/water, acetone, alcohol... But came off very easily
with some mildly concentrated NaOH.
Also, when I scraped at it a bit before that, it was a bit gunky. So just some kinda tar... that... looks like crystals early on? lol.
Anyways - crude synthesis over. Pouring it in a bottle for now. And I'm still alive. Always a good day when you end up not dying.
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SuperOxide
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| Quote: Originally posted by kmno4 | ...
But when purified and distilled, BnCN has rather pleasant odor, somehow reminding me a mixture of benzaldehyde and HCN
... |
Speaking of distilling it, do you think fractional distillation is necessary? I have a pretty decent vacuum pump now, so I should be able to pull a
pretty good vacuum. And I have this short path distillation head, and this integrated claisen/vigreux/liebig piece that I could use. Not sure if the integrated set is overkill or not (but it's new so I'm looking for
an excuse to use it, lol)
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Tsjerk
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Definitely go for the second, just because it is better.
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kmno4
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| Quote: | | Speaking of distilling it, do you think fractional distillation is necessary? |
In this particular case, reaction of BnCl and Na(K)CN is rather clean one. Practically, the only product is BnCN (with <1% BnNC), the rest is
unreacted BnCl and brownish tarry material (cyanide condensation/oxidation products) and products of BnCN hydrolysis (rather trace amounts, but
detectable).
According to Vogel, BnNC is removed (HCl aq) before distillation.
I can confirm it - the odor dramatically changes after this treatment. By the way, I think that BnNC is one of the worst smelling and persistant
chemical I had to do.
Raw, dried BnCN is then distilled. About 90% of literature procedures for this operation, recommend vacuum. However, it can be distilled without any
problems at normal pressure and ~240 C.
The raw BnCN can be also distilled off with steam - the product is very clean, but it is long-lasting process, especially on larger scale.
All above is written from my personal experience.
Vacuum/fractional distillation is recommended, but it is rather optional operation, at least I think so.
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SuperOxide
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Great, that's what I figured. Thanks.
| Quote: Originally posted by kmno4 | According to Vogel, BnNC is removed (HCl aq) before distillation.
I can confirm it - the odor dramatically changes after this treatment. By the way, I think that BnNC is one of the worst smelling and persistant
chemical I had to do.
Raw, dried BnCN is then distilled. About 90% of literature procedures for this operation, recommend vacuum. However, it can be distilled without any
problems at normal pressure and ~240 C.
The raw BnCN can be also distilled off with steam - the product is very clean, but it is long-lasting process, especially on larger scale.
All above is written from my personal experience.
Vacuum/fractional distillation is recommended, but it is rather optional operation, at least I think so.
|
I didn't wash it with HCl, but I did wash it with some 50% H2SO4, and that seemed to help.
I have a decent vacuum pump. I don't have a gauge on it (since it broke and I can't get the damn thing off), but I can just slowly ramp it up to as
good a vacuum as it can get.
Thanks for the tips!
Oh, on other notes.. I did run the synthesis again, on a 2x scale. No solids formed in the condenser, even though I refluxed for 4.5 hours instead of
3. But the reaction mixture looks like it's all almost completely black. The two phases are only obvious once I hold a flashlight to the back of it.
Very different than from the first smaller scale synthesis (despite using the same ratio of reactants and same chemicals).
[Edited on 20-11-2021 by SuperOxide]
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Tsjerk
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| Quote: Originally posted by SuperOxide |
Great, that's what I figured. Thanks.
| Quote: Originally posted by kmno4 | According to Vogel, BnNC is removed (HCl aq) before distillation.
I can confirm it - the odor dramatically changes after this treatment. By the way, I think that BnNC is one of the worst smelling and persistant
chemical I had to do.
Raw, dried BnCN is then distilled. About 90% of literature procedures for this operation, recommend vacuum. However, it can be distilled without any
problems at normal pressure and ~240 C.
The raw BnCN can be also distilled off with steam - the product is very clean, but it is long-lasting process, especially on larger scale.
All above is written from my personal experience.
Vacuum/fractional distillation is recommended, but it is rather optional operation, at least I think so.
|
I didn't wash it with HCl, but I did wash it with some 50% H2SO4, and that seemed to help.
I have a decent vacuum pump. I don't have a gauge on it (since it broke and I can't get the damn thing off), but I can just slowly ramp it up to as
good a vacuum as it can get.
Thanks for the tips!
Oh, on other notes.. I did run the synthesis again, on a 2x scale. No solids formed in the condenser, even though I refluxed for 4.5 hours instead of
3. But the reaction mixture looks like it's all almost completely black. The two phases are only obvious once I hold a flashlight to the back of it.
Very different than from the first smaller scale synthesis (despite using the same ratio of reactants and same chemicals).
[Edited on 20-11-2021 by SuperOxide] |
What did you do differently besides the scaling? I don't believe the scaling was the only difference.
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SuperOxide
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| Quote: Originally posted by Tsjerk | | What did you do differently besides the scaling? I don't believe the scaling was the only difference. |
kmno4 mentioned above that maybe an excess of NaCN and refluxing it for longer would help improve the yield, and I used 24.5g of NaCN the first time,
but when I doubled the scale I used 40g (so slightly over 2x, but not by much).
Other than that - It was the same stock of BnCl, same NaCN, same EtOH, everything was the same.
However, remember I said those crystals appeared when I lowered the temperature, thus causing the reflux ring to lower about an inch, and the crystals
appeared right where the ring previously was. I didn't do that this time, but I don't know if that really matters.
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kmno4
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| Quote: | | .....almost completely black |
Post reacton mixture should be more or less brown; in my experiments, in most cases it was pale brown.
In this reaction, quality of metal cyanide has strong effect on end coloration of the post-mixture. Quality = low content of basic materials
(hydroxides, carbonates). These substances decompose cyanide to brown/black polymers. This reaction is greatly accelerated at elevated temperatures
and larger concentrations.
I remember one NaCN preparation. Everything went OK, but moist (EtOH), white cyanide, during drying (< 100 C) turn to brown powder. The cause ? It
was precipitaded from NaOH solution, and remaining (excess) NaOH was not washed completely.
The most possibly your reaction is OK, this black polymeric matter is not harmful, because it is not volatile.
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SuperOxide
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Ladies and gentlemen... I give you, my first ever vacuum fractional distillation!
It mostly came over at 168 °C (which isn't far off from the expected 173 °C). The BnCN fraction is still a bit cloudy. It's probably just a little
wet, or I started collecting a little too soon.
I haven't taken a whiff of it yet, and im not sure I will, lol. But eventually im sure it'll find its way to my nostrils.
| Quote: Originally posted by kmno4 | | Quote: | | .....almost completely black |
Post reacton mixture should be more or less brown; in my experiments, in most cases it was pale brown.
In this reaction, quality of metal cyanide has strong effect on end coloration of the post-mixture. Quality = low content of basic materials
(hydroxides, carbonates). These substances decompose cyanide to brown/black polymers. This reaction is greatly accelerated at elevated temperatures
and larger concentrations.
I remember one NaCN preparation. Everything went OK, but moist (EtOH), white cyanide, during drying (< 100 C) turn to brown powder. The cause ? It
was precipitaded from NaOH solution, and remaining (excess) NaOH was not washed completely.
The most possibly your reaction is OK, this black polymeric matter is not harmful, because it is not volatile.
|
Yeah it did end up being ok. You're probably right about the quality. I mean the NaCN was reagent grade, but the alcohol was dried/distilled
everclear, and the BnCl was made by me as well. And I try to purify my reagents as best I can, but im not a pro at all.
Regardless - It's worked out great! What method would you recommend for drying it? Calcium chloride? Magnesium sulfate? Im not sure I want to put any
sieves in there.
72% yield... Not bad. That was a lot of work, so I wish it was a little higher. But I'll take it!
[Edited on 21-11-2021 by SuperOxide]
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