SplendidAcylation
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Is pyrrolidine hydrochloride deliquescent, or easy to dry?
Hi,
Just a quick question, I am currently preparing pyrrolidine by the decarboxylation of proline.
So far so good, however I have arrived at the stage where the pyrrolidine is distilled, along with some water, from the crude, basified solution.
I have no particular requirement at this point for pyrrolidine base, and a salt such as the hydrochloride would be much easier to store, and work
with, and less smelly!
So, I intend to deviate from the published procedures, and, instead of adding KOH to dry the distillate, and redistilling to obtain pyrrolidine base,
add hydrochloric acid, and evaporate, yielding pyrrolidine hydrochloride.
That said, I know that some amines, such as methylamine and dimethylamine, form hydrochlorides that cannot be readily obtained in their anhydrous form
by heating alone, whereas others, such as diethylamine, form much less deliquescent salts, which can be dried satisfactorily by simple heating.
I cannot find a lot of information pertaining to pyrrolidine hydrochloride, and I just wondered if anyone has personal experience with it; Is it
impossibly deliquescent, or is it possible to dry it by heating?
Thanks in advance
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Loptr
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I don't believe it should give you too much trouble. There is always a drying chamber or azeotropic distillation.
"Question everything generally thought to be obvious." - Dieter Rams
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SplendidAcylation
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Quote: Originally posted by Loptr  | | I don't believe it should give you too much trouble. There is always a drying chamber or azeotropic distillation. |
Thanks for the reply!
I finally got around to trying it; Pyrrolidine hydrochloride is very deliquescent, it seems!
Once I evaporated off most of the water, I ended up with a syrupy liquid; This was heated at 140c until no more HCl vapour escaped (since an excess of
HCl had been used in neutralizing the amine).
I then allowed it to cool, whereupon a few crystals formed, but it remained quite slushy and liquid.
So I heated it further, until eventually crystals formed in the hot solution, and eventually, I ended up with some reasonably dry crystals, dry enough
that they can be shaken in a sealed container, it kinda has the texture of brown sugar.
In order to obtain 2g of the hydrochloride, it took maybe two or three hours heating at 140c to reach sufficient dryness, moreover, it really is very
deliquescent, the traces left behind on the dish turned into sticky liquid within less than a minute, and i don't think it was a very humid day.
Anyway! Success, i'd say.
Maybe once I've made more I'll dry it more fully in the desiccator.
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Loptr
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I am glad that you tried it and provided that feedback to the community.
I didn't think the salt would be too volatile to easily dehydrate with heating, but then again I have never worked with it.
"Question everything generally thought to be obvious." - Dieter Rams
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Fery
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Interesting findings! I synthesized freebase and it was easily to dehydrate by solid NaOH, b.p. of pyrrolidine was the same as in literature, no other
way to check its purity by me. NaOH also removes carbonates which are formed immediately (visible on walls of glass in few seconds).
If you need its hydrochloride for some following reaction, maybe you can dry it directly in your reactor flask and then add remaining chemicals for
your experiment. Or drying it azeotropically to obtain its dry solution and then add other reactants. Or introduce dry gaseous HCl (dried by bubbling
through conc. H2SO4) into dry pyrrolidine (or into dry solution of pyrrolidine in e.g. dichloromethane or chloroform) - just ideas from my head.
If you plane to store pyrrolidine for long time then it is sitting in a capped flask (plus the gap between glass flask and plastic cap covered with
few turns of PTFE tape) in my lab for a while and it seems to be easy to store. Putting it into an sealed glass ampoule would be even better.
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Lionel Spanner
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In my experience, removing water by vacuum distillation is the easiest way to dry deliquescent salts, as I found out when preparing sodium nitrite and
hydroxylamine bisulphate.
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SplendidAcylation
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I have since repeated the process with another few grams, it was indeed repeatable using a similar procedure, however it does take a few hours to
reach a state of satisfactory dryness when heated at 140c (that is, with the underside of the dish at 140c, held there with the hot-plate + PID
controller).
The compound is very deliquescent, I managed to get it into a sealed jar quickly enough that it remained a free-flowing, although slightly sticky,
solid, however the pieces left behind on the dish were visibly "wet" within about 30 seconds.
Within a few minutes there was no sign of any solids left on the dish, only puddles of solution!
So I'd say its comparable to dimethylamine hydrochloride in its affinity for water, although perhaps more stable to heating and therefore easier to
dry.
| Quote: Originally posted by Fery | Interesting findings! I synthesized freebase and it was easily to dehydrate by solid NaOH, b.p. of pyrrolidine was the same as in literature, no other
way to check its purity by me. NaOH also removes carbonates which are formed immediately (visible on walls of glass in few seconds).
If you need its hydrochloride for some following reaction, maybe you can dry it directly in your reactor flask and then add remaining chemicals for
your experiment. Or drying it azeotropically to obtain its dry solution and then add other reactants. Or introduce dry gaseous HCl (dried by bubbling
through conc. H2SO4) into dry pyrrolidine (or into dry solution of pyrrolidine in e.g. dichloromethane or chloroform) - just ideas from my head.
If you plane to store pyrrolidine for long time then it is sitting in a capped flask (plus the gap between glass flask and plastic cap covered with
few turns of PTFE tape) in my lab for a while and it seems to be easy to store. Putting it into an sealed glass ampoule would be even better.
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Fery, thanks for the suggestions!
I require it for a particular reaction yes, it calls for "pyrrolidine hydrochloride", I haven't really given much thought as to whether it absolutely
must be in the salt form, I didn't anticipate such trickiness in drying it! 
I sometimes find it funny how the literature seldom mentions certain properties of a compound, for example deliquescence, so they just say, for
example, "add 100mmol dimethylamine hydrochloride to a flask" it would be very helpful if they also mentioned that you must expediently
stopper the flask lest the salt will turn to liquid! Haha.
@Lionel Spanner
Interesting!
I have attempted a similar thing:
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
How do you do your vacuum-distillation, to dry deliquescent compounds?
There seemed to be a few problems I could not easily circumvent:
Simply evacuating the apparatus and heating the distilling flask doesn't work, because an equilibrium is eventually set up between the water that has
distilled off, and the dry substance remaining in the flask.
Also the vapour pressure of the water increases the pressure inside the apparatus.
I needed some way to remove the water irreversibly, the most obvious choice being a non-reversible desiccant like phosphorus pentoxide, or sodium, for
example, but another possibility is to continuously run the vacuum pump to remove any vapour produced, or to periodically apply vacuum...
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Lionel Spanner
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I connected a simple distillation apparatus to a vacuum pump, ran ice-cold water through the condenser, and collected the water in a small
round-bottomed flask.
The closest approximation to a vacuum I can pull with that setup and that pump is about 0.06 bar, which is low enough to make water to boil near room
temperature.
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