SplendidAcylation
Hazard to Others
Posts: 209
Registered: 28-10-2018
Location: Starving in some deep mystery
Member Is Offline
Mood: No one I think is in my tree.
|
|
Removing dissolved ammonia from acetonitrile?
Hi,
After a (too long) unplanned break from chemistry, I have recently attempted the preparation of acetonitrile by the dehydration of acetamide using
ammonium sulfamate.
The reaction appears to have been quite successful, I have obtained around 50mL of cloudy acetonitrile, however the nitrile seems to contain large
quantities of dissolved ammonia (as evidenced by the smell, and the stopper popping out of the flask when it warms up even slightly).
Needless to say, I need to remove the ammonia, but I'm not sure what would be the best approach.
The most obvious method would be some sort of acid-wash, however acetonitrile is miscible with water and will likely mix with even concentrated acids.
It occurred to me that I might use a solid acid, added to the acetonitrile, however I think most solid, non-volatile acids (sulfamic acid, benzoic
acid, etc) would probably dissolve to some extent in the nitrile.
I think calcium chloride forms a complex with ammonia, so perhaps I could prepare a saturated solution of calcium chloride (which should be immiscible
with the nitrile), and wash the nitrile with it, removing both ammonia, and any water present.
Another option might be to dissolve calcium chloride in hydrochloric acid, and wash the nitrile with this, then use a strong solution of potassium
carbonate to wash the nitrile to remove traces of acid.
Unfortunately I have no potassium carbonate!
It would also be possible to add a solid acid (such as sulfamic acid) to neutralize the ammonia, then distil the nitrile, however the distillate might
contain traces of acid, so an alkaline wash, followed by a second distillation, would probably be required.
I'd rather avoid having to do two distillations if I can help it, since I'm dealing with relatively small quantities.
Anyway, I feel like there might be an easier, less convoluted way to do this, so I thought I might ask you geniuses first before I proceed to lose all
of my precious nitrile by doing something stupid.
Thanks in advance
[Edited on 4-3-2025 by SplendidAcylation]
|
|
|
unionised
International Hazard
Posts: 5135
Registered: 1-11-2003
Location: UK
Member Is Offline
Mood: No Mood
|
|
Wash the nitrile with a saturated solution of CaCl2, then distill.
|
|
|
DraconicAcid
International Hazard
Posts: 4412
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
If you want a solid, non-volatile acid, use NaHSO4 or NaH2PO4. They may dissolve in the acetonitrile, but they won't let the ammonia go during
distillation.
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.
|
|
|
Deathunter88
National Hazard
Posts: 544
Registered: 20-2-2015
Location: Beijing, China
Member Is Offline
Mood: No Mood
|
|
I think the easiest way to achieve "good enough" removal in your case would be to simply reflux the acetonitrile for a while.
|
|
|
SplendidAcylation
Hazard to Others
Posts: 209
Registered: 28-10-2018
Location: Starving in some deep mystery
Member Is Offline
Mood: No one I think is in my tree.
|
|
Thanks for the replies!
This was my initial plan, I was just a little worried that acetonitrile might complex with the calcium ion. Thanks!
Quote: Originally posted by DraconicAcid  | | If you want a solid, non-volatile acid, use NaHSO4 or NaH2PO4. They may dissolve in the acetonitrile, but they won't let the ammonia go during
distillation. |
Yes. I do have sodium bisulfate, this is a good idea. I would have to avoid using calcium chloride to dry it afterwards to avoid the formation of
hydrogen chloride. I suppose I could add a mixture of NaHSO4 and anhydrous MgSO4, decant, then distill.
| Quote: Originally posted by Deathunter88 | | I think the easiest way to achieve "good enough" removal in your case would be to simply reflux the acetonitrile for a while. |
This hadn't even occurred to me at all! interesting idea. I guess a good condenser would be necessary to avoid any of the nitrile getting carried over
in the ammonia gas escaping.
I will try one, or more of these suggestions and let you know how I get on 
|
|
|
SplendidAcylation
Hazard to Others
Posts: 209
Registered: 28-10-2018
Location: Starving in some deep mystery
Member Is Offline
Mood: No one I think is in my tree.
|
|
Oops
Well, that didn't go too well...
I decided to opt for the calcium chloride washing method.
I dissolved ~34g calcium chloride (which I incorrectly assumed to be anhydrous) granules in 50mL water, then cooled it in the freezer.
I did one 10mL wash with this, and the aqueous phase readily separated, sinking to the bottom.
The aqueous phase, when run off, smelled strongly of ammonia.
So far, so good... But then again maybe it shouldn't smell of ammonia if it is complexing with it.
Anyway, I noticed the volume of nitrile remaining had dropped a little, but this is to be expected, since we are removing impurities after all.
A second wash also smelled of ammonia and caused a reduction in nitrile volume.
At this point it occurred to me "perhaps my calcium chloride solution is insufficiently concentrated" and perhaps it was dissolving some acetonitrile.
After all, it should be saturated.
So I dissolved more calcium chloride in the remainder of the solution, until no more dissolved.
This resulted in a white, cloudy, viscous, heavy solution that behaved a bit like mercury.
I did a wash with 10mL of this, expecting the nitrile's volume to remain roughly the same... This time the aqueous phase when run off did not smell of
ammonia, but some of the nitrile was again missing.
I did two more washes (probably too many!) and each time a considerable proportion (say 5%) of the nitrile's volume disappeared into the aqueous
phase.
After this, I ran off the nitrile into a flask, added 10% w/w calcium chloride granules, and I'll let it stand and then decant and distill later.
So I probably achieved the desired outcome, but with rather too much loss.
Why did the acetonitrile diminish in volume so much?
It is possible that it did contain a lot of ammonia and water, and this was being removed, however this became increasing unlikely by the fifth wash.
A more likely explanation, I think, is that the nitrile complexes with the calcium ion, since it is a Lewis base and could act as a ligand.
This would also explain why the more concentrated solution of calcium chloride seemed to absorb equal or greater quantities, not less, of the nitrile.
|
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
